Karl W Flessa

Interests

Research

Paleontology, conservation biology, science and po9licy, Colorado River

Teaching

Paleontology, Conservation Biology

Courses

Scholarly Contributions

Books

• Dietl, G., & Flessa, K. W. (2017). Conservation Paleobiology: Science and Practice. University of Chicago Press.

Chapters

• Boyer, A., Brenner, M., Burney, D., Pandolfi, J., Savarese, M., Dietl, G., & Flessa, K. W. (2017). Conservation paleobiology roundtable: From promise to application. In Conservation Paleobiology: Science and Practice. University of Chicago Press.
• Rosalind, B. H., Cathy, R. J., Sue, J. E., & Flessa, K. W. (2017). The co-construction of environmental (instream)flows and associated cultural ecosystem benefits. In Social Science and Sustainability(pp 131-144). Clayton South, Victoria, Australia: CSIRO Press.
• Flessa, K. W. (2014). Research networks and large-landscape conservation and restoration: The case of the Colorado River Delta. In Conservation Catalysts. The Aacademy as Nature's Agent(pp 47-59). Cambridge, MA: Lincoln Institute of Land Policy.

Journals/Publications

• Bianchi, T., Butman, D., Raymond, P., Ward, N., Kates, R., Flessa, K. W., Zamora, H., Arellano, A., Ramirez, J., & Rodriguez, E. (2017). The experimental flow to the Colorado River Delta: Effects on carbon mobilization in a dry watercourse. Journal of Geophysical Research Biogeosciences.
• Flessa, K. W., Kendy, E., Schlatter, K., de la Parra, C., Hinojosa, O., Carrillo, Y., & Guillen, E. (2017). Leveraging environmental flows to reform water management policy: Lessons learned from the 2014 Colorado River delta pulse flow. Ecological Engineering.
• Shafroth, P., Schlatter, K., Gomez-Sapiens, M., Lundgren, E., Grabau, M., Ramirez, J., Rodriguez, E., & Flessa, K. W. (2017). A large-scale environmental flow experiment for riparian restoration in the Colorado River delta. Ecological Engineering.
• Bark, R. H., Robinson, C. J., & Flessa, K. W. (2016). Tracking cultural ecosystem services: water chasing the Colorado River restoration pulse flow. ECOLOGICAL ECONOMICS, 127, 165-172.
• Smith, J. A., Auerbach, D. A., Flessa, K. W., Flecker, A. S., & Dietl, G. P. (2016). Fossil clam shells reveal unintended carbon cycling consequences of Colorado River management. Royal Society Open Science, 3(9).
• Dietl, G., Kidwell, S., Brenner, M., Burney, D., Flessa, K. W., Jackson, S., & Koch, P. (2015). Conservation Paleobiology: Leveraging knowledge of the past to inform conservation and restoration. Annual Reviews of Earth and Planetary Sciences, 43, 79-103.
• Lomeli, M. A., Ramirez-Hernandez, J., Glenn, E. P., Zamora-Arroyo, F., & Flessa, K. W. (2015). Monthly Water Balance of an Iconic Coastal Desert Wetland Under Reduced Flows and Increased Salinities; Implications for Management. WETLANDS, 35(4), 783-793.
• Lomeli, M., Ramirez-Hernandez, J., Glenn, E. P., Francisco, Z., & Flessa, K. W. (2015). Monthly water balance of an iconic coastal desert wetland under reduced flows and increased salinities: Implications for management. Wetlands, 35, 783-793.
• Stokes, P. J., Levine, R., & Flessa, K. W. (2015). . Choosing the geoscience major: Important factors, race, ethnicity and gender.. Journal of Geoscience Education, 63:, 250=263.
• Bark, R., Frisvold, G., & Flessa, K. W. (2014). The role of economics in transboundary restoration water management in the Colorado River Delta. Water Resources and Economics, 8(doi:10.1016/j.wre.2014.10.006), 43-56.
• Stokes, P. J., Levine, R., & Flessa, K. W. (2014). Why are there so few hispanic students in geoscience?. GSA Today, 24(1), 52-53.
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  Abstract: The article discusses why the number of Hispanic students is lower in geoscience in the US universities. If the student reported that the incident was worthwhile or supported their decision to major in geoscience, it was classified as a positive outcome. If the student reported that the incident was frustrating, created a hurdle to persistence, or detracted from their confidence about choice of major, it was classified as a negative outcome. There was no significant difference in the average numbers of total critical incidents reported by Hispanic students and white students and no significant difference in the number of positive critical incidents between white and Hispanic students. More informal outdoor experiences for Hispanic youth could result in more Hispanic undergraduate geoscience majors. Familial factors are important to Hispanic students considering an undergraduate major in geosciences, and Hispanic students encounter more resistance from their families than do white students.
• Carrillo-Guerrero, Y. K., Flessa, K., Hinojosa-Huerta, O., & López-Hoffman, L. (2013). From accident to management: The Cienega de Santa Clara ecosystem. Ecological Engineering, 59, 84-42.
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  Abstract: The 1977 creation of the Cienega de Santa Clara, an 18,000. ha wetland (6500. ha cattail marsh with 11,500. ha of open water lagoons and mudflats) in Sonora, Mexico, was the unintended consequence of the solution to reduce the salinity of the U.S. water deliveries to Mexico. Under the 1944 Water Treaty, the U.S. was obliged to deliver water to Mexico. But the water that was delivered from the U.S. harmed crops in Mexico because it included brackish agricultural runoff from fields in the Lower Colorado River basin. In 1972, Minute 242 to the Treaty called for a reduction in the salinity of U.S. deliveries. As a result, the U.S. diverted the agricultural runoff to a desiccated Colorado Delta floodplain in Sonora, Mexico, inadvertently creating the Cienega de Santa Clara, a wetland that now provides habitat for protected species (Desert Pupfish and the Yuma Clapper Rail), and wintering grounds for more than 200,000 migratory waterbirds. In June 1993, the wetland became part of Mexico's Upper Gulf of California and Colorado River Delta Biosphere Reserve. The Yuma Desalting Plant, completed in 1992, was constructed by the U.S. to desalinate the agricultural runoff that flows to the Cienega. The high cost of operation and abundant Colorado River flow have kept the plant idle. However, the past decade of drought in the Colorado Basin has now made the agricultural runoff a valuable resource to meet growing U.S. water demands, but operating the plant could risk damage to the Cienega wetlands. In response, a binational collaboration of government agencies, environmental groups and university scientists began working together to protect and study this wetland. Replacement flows and environmental monitoring during the 2010-2011 trial run of the Yuma Desalting Plant are prompting a shift from passive to active management of this binational ecosystem. The purpose of this paper is to identify management practices that could help maximize the ecological benefits of water flows. The suggested management practices are based on historical variations in quantity, timing and quality of inflows, occasional dredgings and wildfires. The most important management recommendation is a formal allocation of water of adequate volume and quality. The agreement reached in 2010s Minute 316 to the Treaty is a precedent for successful efforts to protect shared ecosystems along the U.S.-Mexico border. © 2013 Elsevier B.V.
• Flessa, K. W., Glenn, E. P., Hinojosa-Huerta, O., A., C., Ramírez-Hernández, J., Schmidt, J. C., & Zamora-Arroyo, F. A. (2013). Flooding the Colorado river delta: A landscape-scale experiment. Eos, 94(50), 485-486.
• Garcia-Hernandez, J., Flessa, K., Santiago-Serrano, E., Romero-Hernandez, S., Zamora-Arroyo, F., & Ramirez-Hernandez, J. (2013). Salinity responses to inflow alterations in a 6500 ha Typha wetland. Ecological Engineering, 59, 18-29.
• García-Hernández, J., Flessa, K., Santiago-Serrano, E., Romero-Hernández, S., Zamora-Arroyo, F., & Ramírez-Hernández, J. (2013). Reprint of: Salinity responses to inflow alterations in a 6500ha Typha wetland. Ecological Engineering, 59, 18-29.
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  Abstract: Salinity has affected the Colorado River for more than five decades: agriculture, mining and urban sprawl have exacerbated the problem, with the most severe impacts occurring in its southern basin and Mexico. However, moderate salinities (2.6gL-1 Total Dissolved Solids) and constant average flows (4m3s-1) from a U.S. agriculture drain, have allowed the establishment of a 6500ha cattail (Typha domingensis) dominated wetland in the mudflats of the upper Gulf of California in Mexico, called "Cienega de Santa Clara" (Cienega). This wetland could now be threatened due to the operation of the Yuma Desalting Plant (YDP). Operations will result in reduced flows and salinity increases in the canal that feeds the wetland. From 2006 to 2011 we monitored water quality conditions at the Cienega as part of a binational effort. Monthly monitoring of field parameters was made at the inflows and at 28 stations inside the wetland. In 2010, 4 automatic loggers were installed inside the wetland to take half an hour readings of temperature and salinity. At the center of the Cienega from 70 to 80% of salinity variations depended on salinity from the main inflow; this response was observed with both monthly data and daily data, indicating a rapid response from salinity inflows. The YDP demonstration run at 10% capacity caused a mean salinity increase from 3.0 to 3.3gL-1 TDS at the center of the wetland and from 3.8 to 5.2gL-1 TDS salinity increase at the periphery. During the YDP pilot run at one-third capacity, mean salinity increased from 3.2 to 4.3gL-1 TDS at the center of the wetland and from 4.3 to 8.3gL-1 TDS at the periphery. There was no evidence of upper Gulf of California tidal intrusion into the wetland. Salinity concentrations at the main inflow should not exceed 3.0gL-1 TDS in order to prevent a reduction in cattail growth inside the wetland. Nevertheless, our recommendation is to maintain salinities at 2.6±0.2gL-1 TDS at the main inflow, in order to preserve vigorous cattail stands. Operation of the YDP plant at 10%, one-third, two-thirds and full capacity without replacement water, will have negative effects on the vegetation. Future efforts should be oriented to compensate for possible salinity increases. © 2013 Elsevier B.V.
• García-Hernández, J., Flessa, K., Santiago-Serrano, E., Romero-Hernández, S., Zamora-Arroyo, F., & Ramírez-Hernández, J. (2013). Salinity responses to inflow alterations in a 6500ha Typha wetland. Ecological Engineering, 52, 191-202.
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  Abstract: Salinity has affected the Colorado River for more than five decades: agriculture, mining and urban sprawl have exacerbated the problem, with the most severe impacts occurring in its southern basin and Mexico. However, moderate salinities (2.6gL-1 Total Dissolved Solids) and constant average flows (4m3s-1) from a U.S. agriculture drain, have allowed the establishment of a 6500ha cattail (Typha domingensis) dominated wetland in the mudflats of the upper Gulf of California in Mexico, called " Cienega de Santa Clara" (Cienega). This wetland could now be threatened due to the operation of the Yuma Desalting Plant (YDP). Operations will result in reduced flows and salinity increases in the canal that feeds the wetland. From 2006 to 2011 we monitored water quality conditions at the Cienega as part of a binational effort. Monthly monitoring of field parameters was made at the inflows and at 28 stations inside the wetland. In 2010, 4 automatic loggers were installed inside the wetland to take half an hour readings of temperature and salinity. At the center of the Cienega from 70 to 80% of salinity variations depended on salinity from the main inflow; this response was observed with both monthly data and daily data, indicating a rapid response from salinity inflows. The YDP demonstration run at 10% capacity caused a mean salinity increase from 3.0 to 3.3gL-1 TDS at the center of the wetland and from 3.8 to 5.2gL-1 TDS salinity increase at the periphery. During the YDP pilot run at one-third capacity, mean salinity increased from 3.2 to 4.3gL-1 TDS at the center of the wetland and from 4.3 to 8.3gL-1 TDS at the periphery. There was no evidence of upper Gulf of California tidal intrusion into the wetland. Salinity concentrations at the main inflow should not exceed 3.0gL-1 TDS in order to prevent a reduction in cattail growth inside the wetland. Nevertheless, our recommendation is to maintain salinities at 2.6±0.2gL-1 TDS at the main inflow, in order to preserve vigorous cattail stands. Operation of the YDP plant at 10%, one-third, two-thirds and full capacity without replacement water, will have negative effects on the vegetation. Future efforts should be oriented to compensate for possible salinity increases. © 2013 Elsevier B.V.
• García-Hernández, J., Glenn, E. P., & Flessa, K. (2013). Identification of chemicals of potential concern (COPECs) in anthropogenic wetlands of the Colorado River delta. Ecological Engineering, 59, 52-60.
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  Abstract: An early step in an Ecological Risk Assessment (ERA) process is the screening of chemicals to identify those that constitute a potential risk by comparing concentrations to levels that are believed to be nonhazardous or "benchmarks". In the present study we analyzed concentrations of metals and metalloids (mercury, arsenic, selenium, and copper) as well as organochlorine pesticides in samples of water, sediment, and fish from the Cienega de Santa Clara wetland, Rio Hardy wetland, and the Upper Gulf of California-estuary. These wetlands are located in northwestern Mexico in the Colorado River delta. All contaminants analyzed were retained as Chemicals of Potential Ecological Concern (COPECs). Mercury was identified as a priority COPEC due to its potential negative effects in freshwater biota, benthic invertebrates and fish eating birds in all three study areas. Copper posed a potential high risk to Rio Hardy aquatic biota. Arsenic represented a risk to aquatic plants and to benthic invertebrates of the three areas with special concern at the Upper Gulf of California-estuary. Selenium was found at levels that could cause a slight (5.6%) reduction in avian reproductive success at the Cienega de Santa Clara and Rio Hardy, similar to the effects found at the Salton Sea. For organochlorine pesticides, DDT represented a potential high risk for aquatic biota at the Cienega de Santa Clara and Rio Hardy, DDD posed a risk for fish eating birds at the Cienega de Santa Clara, and DDE represented a high risk for fish eating birds at the Rio Hardy. In addition to these contaminants, other published studies in the area have identified chromium, lead, boron, PCBs, organophosphorous pesticides and contaminants of emergent concern (CECs) as toxic or potentially toxic for aquatic biota, benthic invertebrates, and wildlife. We recommend to follow-up with an ERA process where the actual effects of COPECs in species and plants of concern (i.e. endangered, rare, keystone species) are evaluated. © 2013 Elsevier B.V.
• Glenn, E. P., Flessa, K. W., & Pitt, J. (2013). Restoration potential of the aquatic ecosystems of the Colorado River Delta, Mexico: Introduction to special issue on "Wetlands of the Colorado River Delta". Ecological Engineering, 59, 1-6.
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  Abstract: The delta of the Colorado River in Mexico supports about a million hectares of riparian, marsh and estuarine habitats of international importance. Some of these habitats depend on flows of fresh and brackish water from the U.S. and Mexico. Up to now, these flows were the incidental result of water management actions taken to provide water for agriculture and municipal use, protect against flooding, and dispose of saline agricultural return flows. This paper briefly describes the wetlands and documents recent bi-national efforts to provide environmental flows to the delta, codified in Minutes 306, 316 and 319 of the water treaty between the U.S. and Mexico for the utilization of Colorado River water. Providing water for environmental uses in this watershed will be a daunting task given the many competing uses for water and expected diminished flows due to climate change. The paper serves as an introduction to a special issue of Ecological Engineering, Wetlands of the Colorado River Delta, which contributes 17 new research articles to the science based on these diverse aquatic habitats. We hope these studies will be useful to those developing management strategies to preserve and enhance these habitats for the future. © 2013 Elsevier B.V.
• Mexicano, L., Glenn, E. P., Hinojosa-Huerta, O., Garcia-Hernandez, J., Flessa, K., & Hinojosa-Corona, A. (2013). Long-term sustainability of the hydrology and vegetation of Cienega de Santa Clara, an anthropogenic wetland created by disposal of agricultural drain water in the delta of the Colorado River, Mexico. Ecological Engineering, 59, 111-120.
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  Abstract: The Ciénega of Santa Clara is a valuable coastal wetland sustained almost entirely by discharge of brackish agricultural drain water from the U.S. and Mexico. In other locations, agricultural drain water has been problematic in supporting wetlands due to problems of salinity buildup, toxic substances and undesirable plant succession processes. We studied the development of the Cienega de Santa Clara from its creation in 1977 to the present to determine if it is on a sustainable trajectory in terms of vegetation, hydrology and habitat value. We used Landsat NDVI imagery from 1975 to 2011 to determine the area and intensity of vegetation and to estimate evapotranspiration (ET) to construct a water balance. Remote sensing data were combined with hydrological data, site surveys and other sources of information on the Cienega. The vegetated area increased from 1978 to 1995 and has been constant at about 4200ha since then. The dominant vegetation type is Typha domingensis (southern cattail), and peak summer NDVI since 1995 has been stable at 0.379 (SD=0.016), about half of NDVIMax. Flows into the marsh have been stable both month-to-month and year-to-year, with a mean annual value of 4.74m3s-1 (SD=1.03). Salinity has been stable with a mean value of 2.09gL-1 TDS (SD=0.13). About 37% of the inflow water is consumed in ET, with the remainder exiting the Cienega as outflow water, mainly during winter months when T. domingensis is dormant. The sustainability of the Cienega is attributed to: stable inflow rates; salinities within the tolerance limit of the dominant vegetation; and tidal flushing which maintains the wetland as an open system. © 2013 Elsevier B.V.
• Zamora, H. A., Nelson, S. M., Flessa, K. W., & Nomura, R. (2013). Post-dam sediment dynamics and processes in the Colorado River estuary: Implications for habitat restoration. Ecological Engineering, 59, 134-143.
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  Abstract: River-sea connectivity is essential for restoring ecosystem services in the Colorado River delta. The mixing of river water and seawater sustains biodiversity and provides brackish-water nursery grounds for both commercially important and endangered marine species. The Colorado River no longer reaches the sea except during particularly high tides and anomalously wet years. The river's relict channel is now obstructed by an accumulation of sediments deposited during flood tides; ebb flows are not strong enough to keep the channel open. Landsat 5-TM and Landsat-7 scenes from the Colorado River delta and tide prediction tables were used to reconstruct river-sea connectivity and geomorphic processes after 50 years of extensive human manipulation of the Colorado River. Historical documentation, previous topographic surveys and sediment cores were used to estimate sedimentation rates in the lower river channel. Satellite images and tide charts show that currently the river reaches the sea or the sea reaches the river about 12 days per year, unlike 10 years ago when a year-round connection existed. Reduction in connectivity results from the evolution of a tidal sandbar located within the bedload convergence zone, about 35. km upstream from the river's mouth. Historical documentation and sediment core analyses suggest sedimentation rates in the range of 10-21. cm per year. With the current conditions prevailing, active management - dredging - is required and needs to occur once every 5-10 years to reconnect the remaining riparian wetlands in the Colorado River to the Gulf of California. © 2012 Elsevier B.V.
• Baeza, K., Lopez Hoffman, L. -., Glenn, E. P., Flessa, K. W., & Garcia-Hernandez, J. (2012). Salinity limits of vegetation in the Cienega de Santa Clara, an oligotrophic marsh in the delta of the Colorado River, Mexico: Implications for a salinity increase.. Ecological Engineering., http://dx.doi.org/10.1016/j.ecoleng.2012.08.019.
• Baeza, K., Lopez-Hoffman, L., Glenn, E. P., Flessa, K., & Garcia-Hernandez, J. (2012). Salinity limits of vegetation in Cienega de Santa Clara, an oligotrophic marsh in the delta of the Colorado River, Mexico: Implications for an increase in salinity. Ecological Engineering, 59, 157-166.
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  Abstract: Greenhouse and field trials were conducted to determine the salinity limits for main vegetation types in Cienega de Santa Clara, an oligohaline marsh in the delta of the Colorado River in Mexico. The Cienega is the largest brackish marsh in the Sonoran Desert and supports numerous bird, mammal and invertebrate species, including threatened or endangered marsh birds. It is supported by brackish agricultural return flows from the USA and Mexico, and operation of the Yuma Desalting Plant (YDP) could reduce the volume and increase the salinity of inflows. Current inflows average 4m3s-1 at 2.8gL-1 Total Dissolved Solids (TDS), and the dominant vegetation type is Typha domingensis, with subdominant stands of Schoenoplectus americanus and other emergent species distributed amidst the Typha stands. A greenhouse experiment showed that under flooded-soil conditions, T. domingensis had a linear reduction in relative growth rate based on biomass production (RGRBiomass) with salinity, with zero RGRBiomass at 8.3gL-1 TDS and a 50% reduction at 4.0gL-1 TDS. S. americanus was about twice as salt-tolerant, with zero RGRBiomass at 12.5gL-1 TDS and 50% RGRBiomass at 9gL-1 TDS. The results are consistent with other studies that show a mean reduction in RGRBiomass of 13.3% per g L-1 TDS for Typha spp. and 4.3% per g L-1 TDS for Schoenoplectus spp. Field surveys showed that T. domingensis stands were restricted to salinities of 6.5gL-1 TDS or less, and that annual biomass production was reduced by 85% in a stand at 5.9gL-1 TDS compared to a stand at 3.0gL-1 TDS. Normalized Difference Vegetation Index (NDVI) values of T. domingensis stands from satellite imagery showed a linear decline with increasing salinity in the marsh. It was concluded that 6gL-1 TDS is the approximate upper limit for vigorous stands of T. domingensis, and that replacement by S. americanus is a possibility if salinities increase. Implications for marsh vegetation structure and habitat value are discussed under different possible operating scenarios for the YDP. © 2012 Elsevier B.V.
• Cintra-Buenrostro, C. E., Flessa, K. W., & Dettman, D. L. (2012). Restoration flows for the Colorado River estuary, México: Estimates from oxygen isotopes in the bivalve mollusk Mulinia coloradoensis (Mactridae: Bivalvia). Wetlands Ecology and Management, 20(4), 313-327.
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  Abstract: Because of competing demands for freshwater, restoration of estuaries requires estimates of inflows to sustain key species. In this study we estimated the pre-dam salinities of the Colorado River estuary by using oxygen isotopes in subfossil shells of the bivalve mollusk Mulinia coloradoensis. Since the construction of upstream dams and water diversions, average salinity in the estuary has increased to 38 practical salinity units (psu) and the population of M. coloradoensis has decreased by ~90%. In the pre-dam estuary, specimens grew when salinity ranged from 22 to 33 psu at the mouth of the river while populations 40 km distant grew at salinities from 30 to 38 psu. The river flow needed to reduce salinities at the mouth of the river to those recorded in the most distant localities (40 km from river's mouth) ranges from 120 to 290 m 3 s -1. If these flows were sustained for a year, they would total 7-16 % of the river's annual average historical flow (~1.8 × 10 10 m 3). © 2012 Springer Science+Business Media B.V.
• Nelson, S. M., Fielding, E. J., Zamora-Arroyo, F., & Flessa, K. (2012). Delta dynamics: Effects of a major earthquake, tides, and river flows on Ciénega de Santa Clara and the Colorado River Delta, Mexico. Ecological Engineering, 59, 144-156.
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  Abstract: The intertidal portion of Mexico's Colorado River Delta is a dynamic environment subject to complex interactions of tectonic, fluvial, and tidal forces at the head of the Gulf of California. We review the historical interactions of these forces, use sequential satellite images, overflights, ground observations, and interferometric synthetic aperture radar (InSAR) data to study the effects of the 2010 Mw 7.2 El Mayor-Cucapah Earthquake on changing patterns of tidal inundation within the Delta, and assess effects of these changes to the fluvial/hydrological regime of the Colorado River estuary and nearby Ciénega de Santa Clara wetland. The objectives of this study are to highlight for environmental scientists, land managers, and ecological engineers the contribution of tectonic forces in shaping the intertidal Delta environment and to provide information on the effects of the 2010 earthquake which will be of practical value in planning and designing management measures and restoration projects for the estuary and Ciénega.The Colorado River estuary is at present blocked by a tidal sand bar which restricts access by marine species to the upper estuary and obstructs the flow of fresh water into the lower estuary. Located 13. km east of the estuary, the Ciénega is a 6000. ha wetland supported by agricultural drain water from Arizona and Mexico. South of the Ciénega is the Santa Clara Slough, an unvegetated 26,000. ha basin subject to periodic inundation from the northern Gulf's high amplitude tides, which have historically reached the margins of the Ciénega several times each year.The El Mayor-Cucapah earthquake ruptured the previously unknown Indiviso Fault which extends into the intertidal zone just west of the Ciénega. The Ciénega experienced only minor surface deformation having no direct effects to the wetland. Most of the significant ground movement and surface deformation occurred west of the Indiviso Fault adjacent to the estuary, where portions of the intertidal flats underwent extensive liquefaction, northward coseismic displacement and post-seismic subsidence. These surface deformations changed the pattern of tidal inundation, triggering development of a new system of natural tidal channels and creating conditions favorable for installation of projects to restore connectivity between the upper and lower estuary. The changed pattern of tidal inundation may also have contributed to an observed reduction in the occurrence of tidal flooding along the southwestern margin of the Ciénega following the earthquake. © 2012 Elsevier B.V.
• Dietl, G. P., & Flessa, K. W. (2011). Conservation paleobiology: Putting the dead to work. Trends in Ecology and Evolution, 26(1), 30-37.
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  PMID: 21035892;Abstract: Geohistorical data and analyses are playing an increasingly important role in conservation biology practice and policy. In this review, we discuss examples of how the near-time and deep-time fossil record can be used to understand the ecological and evolutionary responses of species to changes in their environment. We show that beyond providing crucial baseline data, the conservation paleobiology perspective helps us to identify which species will be most vulnerable and what kinds of responses will be most common. We stress that inclusion of geohistorical data in our decision-making process provides a more scientifically robust basis for conservation policies than those dependent on short-term observations alone. © 2010 Elsevier Ltd.
• Swift, C. C., Findley, L. T., Ellingson, R. A., Flessa, K. W., & Jacobs, D. K. (2011). The delta mudsucker, gillichthys detrusus, a valid species (Teleostei: Gobiidae) endemic to the colorado river delta, Northernmost gulf of California, Mexico. Copeia, 93-102.
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  Abstract: Substantial genetic and subtle morphological characters document that the Delta Mudsucker or chupalodo delta, Gillichthys detrusus Gilbert and Scofield, 1898, family Gobiidae, is a valid species separate from its widespread sister species, the Longjaw Mudsucker, G. mirabilis Cooper, 1864. This species was erroneously placed in the synonymy of G. mirabilis in 1907 and has since remained unrecognized until this study. The Delta Mudsucker is restricted to a narrow zone of tidally influenced channels of the lowermost Colorado River and adjacent to the mouth of the river within its delta. It is the second fish species endemic to the river's delta in Mexico's Reserva de la Biósfera del Alto Golfo de California y Delta del RÃ-o Colorado (Upper Gulf of California and Colorado River Delta Biosphere Reserve). This study underscores the importance of continued reassessment of baseline and cryptic biodiversity, especially in habitats where initial assessment was scant prior to extensive anthropogenic influence. © 2011 by the American Society of Ichthyologists and Herpetologists.
• López-Hoffman, L., Varady, R. G., Flessa, K. W., & Balvanera, P. (2010). Ecosystem services across borders: A framework for transboundary conservation policy. Frontiers in Ecology and the Environment, 8(2), 84-91.
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  Abstract: International political borders rarely coincide with natural ecological boundaries. Because neighboring countries often share ecosystems and species, they also share ecosystem services. For example, the United States and Mexico share the provisioning service of groundwater provided by the All-American Canal in California; the regulating service of agave crop pollination by long-nosed bats; and the aesthetic value of the North American monarch butterfly, a cultural service. We use the Millennium Ecosystem Assessment (MA) to elucidate how drivers in one country can affect ecosystem services and human well-being in other countries. We suggest that the concept of ecosystem services, as articulated by the MA, could be used as an organizing principle for transboundary conservation, because it meets many of the criteria for successful transboundary policy. It would frame conservation in terms of mutual interests between countries, consider a diversity of stakeholders, and provide a means for Unking multiple services and assessing tradeoffs between uses of services. © The Ecological Society of America.
• Rowell, K., Flessa, K. W., Dettman, D. L., Román, M. J., Gerber, L. R., & Findley, L. T. (2008). Diverting the Colorado River leads to a dramatic life history shift in an endangered marine fish. Biological Conservation, 141(4), 1138-1148.
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  Abstract: Diversion of river water has diminished freshwater flow into many estuaries worldwide, yet the effects of these diversions on marine fisheries, many of which depend on estuaries, are largely unexplored. We document the impact of diverting Colorado River flow from the Gulf of California on the life history of a now-endangered marine fish (Totoaba macdonaldi, Sciaenidae). Growth increments in prehistoric (1000-5000 ybp) otoliths document that pre-dam juveniles grew twice as fast and matured 1-5 years earlier than post-dam fish. Oxygen isotopes link these changes to elimination of estuarine habitat. This study provides evidence that river diversion can have a dramatic effect on life history of marine fishes by slowing growth during the juvenile stage, thus delaying maturation. These findings also provide valuable insight into the relative influence of habitat alteration versus fishing pressure on marine fishes. © 2008 Elsevier Ltd. All rights reserved.
• Rowell, K., True, C., Flessa, K. W., & Dettman, D. L. (2008). Fish without water: Validation and application of δ¹⁸O in Totoaba macdonaldi otoliths. Ciencias Marinas, 34(1), 55-68.
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  Abstract: The geochemistry of fish otoliths is a valuable tool for reconstructing environmental conditions, migrations and life histories. In this study we validate the relationship between temperature, the water oxygen isotope composition (δ18O) and otolith δ 18O for the endangered Totoaba macdonaldi, raised under controlled aquaculture conditions and in the field. This type of validation is instrumental for habitat reconstruction. By comparing δ18O values in the natal portion of totoaba otoliths from modern and pre-dam specimens ∼1000-4500 yr BP, we test the hypothesis that the totoaba used the Colorado River estuary as a nursery site before the river was over-allocated. We found that otolith δ18O could be predicted in a laboratory setting as well as in the wild. Totoaba otoliths from before river diversion had drastically lower natal δ18O values than predicted values, indicating that these differences in δ18O values are the result of a change in the water δ18O, a consequence of diverting the isotopically negative Colorado River flow from the totoaba's nursery grounds. We conclude that the Colorado River flow was a major component of the totoaba's nursery habitat before river diversions. These results are pertinent to ongoing research on this endangered fish, using otoliths to piece together important ecological and life history information.
• Glenn, E. P., Flessa, K. W., Cohen, M. J., Nagler, P. L., Rowell, K., & Zamora-Arroyo, F. (2007). Just add water and the Colorado river still reaches the sea. Environmental Management, 40(1), 1-6.
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  PMID: 17546520;Abstract: A recent article in Environmental Management by All argued that flood flows in North America's Colorado River do not reach the Gulf of California because they are captured and evaporated in Laguna Salada, a below sea-level lakebed near the mouth of the river. We refute this hypothesis by showing that (1) due to its limited area, the Laguna Salada could have evaporated less than 10% of the flood flows that have occurred since 1989; (2) low flow volumes preferentially flow to the Gulf rather than Laguna Salada; (3) All's method for detecting water surface area in the Laguna Salada appears to be flawed because Landsat Thematic Mapper images of the lakebed show it to be dry when All's analyses said it was flooded; (4) direct measurements of salinity at the mouth of the river and in the Upper Gulf of California during flood flows in 1993 and 1998 confirm that flood waters reach the sea; and (5) stable oxygen isotope signatures in clam shells and fish otoliths recorded the dilution of seawater with fresh water during the 1993 and 1998 flows. Furthermore, All's conclusion that freshwater flows do not benefit the ecology of the marine zone is incorrect because the peer-reviewed literature shows that postlarval larval shrimp populations increase during floods, and the subsequent year's shrimp harvest increases. Furthermore, freshwater flows increase the nursery area for Gulf corvina (Cynoscion othonopterus), an important commercial fish that requires estuarine habitats with salinities in the range of 26-38‰ during its natal stages. Although flood flows are now much diminished compared to the pre-dam era, they are still important to the remnant wetland and riparian habitats of the Colorado River delta and to organisms in the intertidal and marine zone. Only a small fraction of the flood flows are evaporated in Laguna Salada. © 2007 Springer Science+Business Media, LLC.
• Liebig, P. M., Flessa, K. W., & Taylor, T. A. (2007). Taphonomic variation despite catastrophic mortality: Analysis of a mass stranding of false killer whales (Pseudorca crassidens), gulf of California, Mexico. Palaios, 22(4), 384-391.
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  Abstract: A concentration of Pseudorca crassidens remains resulting from a mass stranding on the tidal flats of the Colorado River Delta, Baja California, Mexico, was analyzed to determine how bone and individual density and variation in taphonomic condition differs from a time-averaged assemblage of marine mammals. Five hundred and thirty seven skeletal elements, including 26 whole skulls, were found among 204 bone sites in a 13,000 m2 area. Skulls provide the best estimate of minimum number of individuals; all other skeletal elements are underrepresented. Twenty bones per individual, one bone per 26 m2, and one individual per 536 m2 characterize this the mass-stranding locality. Bone density and individual density are greater at this locality than in a previously studied time-averaged assemblage from the Colorado Delta. Although the lack of variation in taphonomic condition is sometimes used as one criterion for a mass-death assemblage, the condition of the remains in this mass stranding varies both within and among skeletal elements. Teeth tend to be in good condition, earbones in fair condition, and vertebrae in poor condition. The taphonomic differences are a result of variation in the density and size of the skeletal element, variation in associated sediment (sand or mud), and variation in exposure (surface or buried). Despite the fact that all the individuals died at the same time, the taphonomic condition of their skeletal elements varies greatly. Copyright © 2007, SEPM (Society for Sedimentary Geology).
• Avila-Serrano, G., Flessa, K. W., Téllez-Duarte, M., & Cintra-Buenrostro, C. (2006). Distribution of the intertidal macrofauna of the Colorado River Delta, northern Gulf of California, Mexico. Ciencias Marinas, 32(4), 649-661.
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  Abstract: Surveys were conducted in 1999 and 2000 to document the composition and distribution of the shelly (mollusk, echinoderm and brachiopod) fauna of the intertidal zone of the Colorado River Delta, northern Gulf of California, Mexico. Samples of the benthic fauna were taken along ten high-to-low intertidal transects, during two seasons to assess seasonal variation in the fauna. Twenty-six species were identified among 1954 individuals in 112 samples. The fauna was dominated by the epifaunal gastropod Nassarius moestus, infaunal bivalves, infaunal echinoids and the lingulid brachiopod Glottidia palmeri. Faunal density was generally greater toward the lower part of the intertidal zone. Species richness did not vary significantly among transects, although Shannon diversity and equitability were greater in the southern transects, those distant from the river mouth. The distinctive species composition, diversity and equitability of the southern transects may be the result of regional variation in salinity and substrate. Current faunal densities of 3-7 ind m-2 are much lower than the estimates before upstream dams and water diversions affected the habitats of the Colorado River Delta. The post-dam decrease in density is largely the result of the decline in the population of the mactrid bivalve Mulinia coloradoensis.
• Hunda, B. R., Hughes, N. C., & Flessa, K. W. (2006). Trilobite taphonomy and temporal resolution in the Mt. Orab shale bed (Upper Ordovician, Ohio, U.S.A.). Palaios, 21(1), 26-45.
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  Abstract: Clay-rich units, locally termed "butter shales," contain the best-preserved trilobites in the richly fossiliferous Cincinnatian Series and likely provide the highest temporal resolution available within these rocks. Sedimentological and taphonomic evidence indicates that the 0.46-m-thick Mt. Orab "butter shale" bed of the Arnheim Formation is composed of a series of stacked event beds, each representing rapid deposition from a flow bearing fine-grained sediment, most likely associated with distal storm processes below storm-wave base. It contains sedimentary structures similar to those of distal mud turbidites, and comprises a total of at least seven, and possibly many more, alternating silt and clay couplets. These clay and silt layers are interpreted to represent the products of different energetic regimes in a series of discrete depositional events accumulated within a common depositional regime. Trilobites within individual clay beds represent census assemblages of animals alive at the same time, and evidence from sedimentology, taphonomy, and stratigraphic architecture are consistent with accumulation of the whole bed within a period from 101 to 103 years. Silt layers of the Mt. Orab events beds are interpreted to represent parautochthonous assemblages, while clay layers, although displaying reorientation of specimens, are interpreted as autochthonous assemblages. Both layers are deposited in a shallower-water environment than the comparable "granulosa" trilobite cluster of the Kope Formation, which represents an autochthonous assemblage with in-situ burial of trilobites. Copyright © 2006, SEPM (Society for Sedimentary Geology).
• Schöne, B. R., Rodland, D. L., Fiebig, J., Oschmann, W., Goodwin, D., Flessa, K. W., & Dettman, D. (2006). Reliability of multitaxon, multiproxy reconstructions of environmental conditions from accretionary biogenic skeletons. Journal of Geology, 114(3), 267-285.
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  Abstract: Evaluation and quantification of climate change require data on subseasonal to daily environmental extremes from those periods before instrumental records were available. This study employs a high-resolution, multitaxon, multiproxy approach and analyzes how faithfully accretionary biogenic skeletons record environmental extremes. Six specimens of two bivalve mollusks (Chione fluctifraga, Mytella guyanensi) and one barnacle species (Chthamalus fissus) from a single habitat (northern Gulf of California, Mexico) were collected. Contemporaneous shell portions from these specimens were analyzed for shell growth rates (sclerochronology) and stable isotopes (δ18O, δ13C) and were compared to instrumental records. The results of these analyses included some significant observations. First, shell δ18O values overestimate winter temperatures and underestimate summer temperatures. Second, the actual diurnal temperature range is not recorded in the biogenic skeletons. Third, skeletal growth is biased toward a species-specific optimum growth temperature (24°-30.9°C. Therefore, higher sampling resolution will not necessarily capture actual environmental extremes. Despite measured temperature extremes of 37.8° and 4.5°C, none of the studied species recorded temperatures above 30.9° or below 12.2°C. Duration and timing of the annual growing period is species specific as well. Faster shell growth occurred at higher temperatures. Up to 58% (C. fissus) of the variability in shell growth can be explained by water temperature during growth. Contemporaneous trends in shell δ13C show a weak correlation with pigment concentration (R2 = 0.17). Higher levels of chlorophyll appear to increase shell production. rates. Our study highlights the difficulties inherent in using biogenic skeletons for the reconstruction of paleoenvironmental extremes and demonstrates the power and utility of multiproxy and multitaxon approaches. © 2006 by The University of Chicago. All rights reserved.
• Cintra-Buenrostro, C. E., Flessa, K. W., & Avila-Serrano, G. (2005). Who cares about a vanishing clam? Trophic importance of Mulinia coloradoensis inferred from predatory damage. Palaios, 20(3), 296-302.
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  Abstract: Analyses ofpredatory damage on subfossil hard parts can be used to document the trophic role played by species that were abundant prior to human impact even when pre-impact surveys are lacking or when predators have not left skeletal remains. Before upstream dams and water diversions, the bivalve Mulinia coloradoensis was the most common mollusk inhabiting the Colorado River Estuary. Today, only a small population has survived the environmental changes caused by reduction in the river flow. When abundant, this species was a major source of food for predatory gastropods and crabs, as shown by the characteristic damage inflicted by these predators. Boreholes made by predatory gastropods were found in 23% of the 600 individuals sampled from shell accumulations that date from the era prior to upstream dams and diversions. Marginal shell damage characteristic of predatory portunid crabs was found in 27% of the individuals. Thirty-four percent of the individuals had damage from earlier attacks. Shells were tumbled to provide criteria to distinguish ante-mortem biological damage to the shell margin from post-mortem, physical damage. Decline in the population of this prey species likely caused a decline in the populations of its predators and species higher in the food chain, prey-switching, or both effects. Restoration of river flow would increase populations of M. coloradoensis and species that depend on it for food, including commercially important crabs. Copyright © 2005, SEPM (Society for Sedimentary Geology).
• Flessa, K. W., & Jackson, S. T. (2005). Forging a common agenda for ecology and paleoecology. BioScience, 55(12), 1030-1031.
• Rowell, K., Flessa, K. W., Dettman, D. L., & Román, M. (2005). The importance of Colorado River flow to nursery habitats of the Gulf corvina (Cynoscion othonopterus). Canadian Journal of Fisheries and Aquatic Sciences, 62(12), 2874-2885.
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  Abstract: We test the hypothesis that Colorado River flow is important in providing nursery habitat for the Gulf corvina (Cynoscion othonopterus), a commercially valuable and endemic fish in the upper Gulf of California. We use oxygen isotopes in otoliths to determine when these fish inhabit isotopically different bodies of water (Gulf of California versus the Colorado Estuary). The δ18O values in the natal otoliths of C. othonopterus, significantly more negative than can be predicted by temperature alone, provide evidence that this species uses the brackish habitats created by flow of the Colorado River. A significant log-linear relationship between the natal δ18O values and the cumulative flow of the Colorado River during natal development confirms use of brackish habitat in years that the Colorado River water reached the Gulf. Natal δ18O values indicate that C. othonopterus seek out estuarine habitats with salinities between 26‰ and 38‰. Reduction in Colorado River flow since the construction of upstream dams has reduced the size of nursery habitat for C. othonopterus. Our results support the hypothesis that declines in commercial landings can be at least partially attributed to reduced river flow. Increased flow would increase nursery habitat and likely benefit recruitment. © 2005 NRC.
• Dettman, D. L., Flessa, K. W., Roopnarine, P. D., Schöne, B. R., & Goodwin, D. H. (2004). The use of oxygen isotope variation in shells of estuarine mollusks as a quantitative record of seasonal and annual Colorado River discharge. Geochimica et Cosmochimica Acta, 68(6), 1253-1263.
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  Abstract: We describe a new method for the calculation of river flow that uses the oxygen isotope composition of bivalve mollusk shells that grew in the river-water/seawater mixing zone of the Colorado River estuary. Sclerochronological techniques are used to identify tidally-induced, fortnight-scale bundles of daily growth increments within shell cross-sections. These fortnightly markers are used to establish a chronology for samples taken for δ18O analysis. A composite seasonal δ18O profile derived from five shells that grew in the absence of river-water flow is used as a baseline against which profiles of river-influenced shells are compared. Because this comparison is between matched fortnights within a year, the temperature of shell growth is likely to be very similar. The difference in δ18O between the river-influenced shell and the "no-flow" composite shell therefore represents the change in the δ18O of the water due to the presence of river water in the mixing zone. The river water end-member is also determined within a fortnightly context so that the change in the δ18O of mixing-zone water can be used to calculate the relative proportions of seawater and fresh-water. The fresh-water end-member is calculated from the δ18O of bivalves alive prior to the emplacement of dams and water diversions on the Colorado River. The marine end-member is based on direct measurements of the δ18O of northern Gulf of California water during times of no Colorado River flow. The system has been calibrated to absolute flow amounts using recent releases of known volume and rate. © 2004 Elsevier Ltd.
• Goodwin, D. H., Flessa, K. W., Téllez-Duarte, M. A., Dettman, D. L., Schöne, B. R., & Avila-Serrano, G. A. (2004). Detecting time-averaging and spatial mixing using oxygen isotope variation: A case study. Palaeogeography, Palaeoclimatology, Palaeoecology, 205(1-2), 1-21.
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  Abstract: In principle, bivalve mollusks living at the same time and in the same place will experience the same temperature and salinity regimes and will have identical annual oxygen isotope (δ18O) profiles. Bivalve mollusks living at different times or in different places are more likely to have different annual δ18O profiles. Thus, differences in annual δ18O profiles can be used to detect temporal or spatial mixing. We devised eight metrics to quantitatively compare sclerochronologically calibrated annual δ18O profiles from different shells: difference in maximum value, difference in minimum value, difference in amplitude, the number of non-contemporaneous isotopic enrichment events (NNEE), the average fortnightly difference (AD), the standard deviation of the average fortnightly differences (SDD), the maximum fortnightly difference (MaxD) and the number of fortnights separating the minimum values. These metrics vary among northern Gulf of California shells from four temporal and spatial categories: (1) same time and same place; (2) same time and different place; (3) different time and same place; and (4) different time and different place. Different time/different place comparisons include comparisons of live-collected shells with shells alive during times of Colorado River flow and shells from a Pleistocene interglacial deposit. The same time/same place comparison has the most similar metric values, whereas comparisons among the different time/different place shells are usually the least similar. Between-shell oxygen isotope differences can reveal temporal or spatial mixing of shells that would be undetectable with radiocarbon or amino-acid racemization dating. Application of the technique to a Holocene deposit with shells in life position reveals that the bivalves were alive at different times, despite indistinguishable radiocarbon ages. Two adjacent but disarticulated Pleistocene shells appear to be both temporally and spatially mixed. The method can detect temporal or spatial mixing in any shell material unaffected by diagenesis, regardless of the age of the specimens. © 2004 Published by Elsevier B.V.
• Liebig, P. M., Taylor, T. A., & Flessa, K. W. (2003). Bones on the beach: Marine mammal taphonomy of the Colorado delta, Mexico. Palaios, 18(2), 168-175.
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  Abstract: How well does a death assemblage of marine mammal bones reflect the diversity, species composition, and proportion of bone types in the living fauna? Marine mammal remains were surveyed along the beaches of the Colorado River delta, Baja California, Mexico. Three carcasses and 470 bones were found among 112 localities along 4.0 km of shoreline. The location of each site was recorded and each bone was identified, photographed, and measured and its taphonomic condition was noted. The proportion of bone types found was compared to the proportions known in living marine mammals. The list of species found as bones was compared to the list of species known to live in the northern Gulf of California. The maximum skeletal ratio of skull:vertebrae:ribs:phalanges: girdles/limbs in a typical Gulf of California marine mammal is 1:74:30:56:16. The 28 skulls and 442 post-cranial bones found provided a skeletal ratio of 1:12:3:1:1. Although vertebrae are the most common bones in the bone assemblage, only 316 were found, not the ∼2,000 predicted by the 28 skulls, indicating that vertebrae are under-represented. Therefore, skulls provide the best estimate of the minimum number of individuals. Smaller bones appear to be more easily destroyed, buried, or transported away. Most vertebrae were in good condition, suggesting that most bones arrived on the beach recently. Remains of 8 of the 18 species recorded in the northern Gulf were found: Zalophus californianus (California Sea Lion, 8 skulls), Delphinus delphis (Common Dolphin, 7 skulls), Tursiops truncatus (Bottlenose Dolphin, 6 skulls), Phocoena sinus (Vaquita, four skulls), Pseudorca crassidens (False Killer Whale, one skull), Kogia breviceps (Pygmy Sperm Whale, one skull), and a possible Mesoplodon sp. (Beaked Whale, one skull). One Physeter macrocephalus (Sperm Whale) was identified by its large vertebrae. Differences in population size, habitat use, and behavior among species may affect species composition and abundance within the bone assemblage. Migrants and rare species are not as abundant as residents in the bone assemblage. Coastal species are more common than oceanic ones. Marine mammal remains are common within the 3% of Colorado Delta shoreline surveyed, and provide a remarkably good sample of the living fauna. Surveys of mammal remains may be a valuable and cost-effective supplement to aerial and nautical surveys of the live fauna.
• Rodland, D. L., Kowalewski, M., Dettman, D. L., Flessa, K. W., Atudorei, V., & Sharp, Z. D. (2003). High-resolution analysis of δ¹⁸O in the biogenic phosphate of modern and fossil lingulid brachiopods. Journal of Geology, 111(4), 441-453.
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  Abstract: Laser ablation and silver phosphate procedures were used to measure the oxygen isotope composition in organophosphatic shells of lingulid brachiopods at a variety of scales: within valves, between valves from the same individual, between individuals collected at the same location at the same time, between localities, and between modern and fossil specimens. Specimens included modern lingulids from several patches in the northern Gulf of California (Mexico) and the Gulf of Nicoya (Costa Rica), and fossil specimens from the lower Triassic Dinwoody Formation. All specimens display a high degree of intrashell variability, frequently exceeding 4‰. This variability is not symmetrical within the shell, does not appear to reflect growth bands, and is not consistent with a published lingulid phosphate-oxygen isotope thermometer. Shells analyzed using silver phosphate preparations have variability similar (>3.5‰) to that of shells analyzed using the laser ablation technique, ruling out the influence of organic carbon contamination. We interpret this variation as primary, representing a vital effect, possibly the result of enzymatic fractionation near mantle canals and muscle scars or in vivo mineralogical changes in shell composition. In contrast, oxygen isotope analysis of carbonate from these shells is repeatable and appears to represent equilibrium values. Although oxygen isotope analyses from laser ablation and silver phosphate methods indicate that the phosphate in lingulid valves is an unreliable recorder of oxygen isotope ratios in seawater, it may be possible to derive paleoclimate data from the carbonate fraction.
• Schöne, B., Flessa, K. W., Dettman, D. L., & Goodwin, D. H. (2003). Upstream dams and downstream clams: Growth rates of bivalve mollusks unveil impact of river management on estuarine ecosystems (Colorado River Delta, Mexico). Estuarine, Coastal and Shelf Science, 58(4), 715-726.
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  Abstract: We studied how the extensive diversion of Colorado River water, induced by dams and agricultural activities of the last 70 years, affected the growth rates of two abundant bivalve mollusk species (Chione cortezi and Chione fluctifraga) in the northern Gulf of California. Shells alive on the delta today ('Post-dam' shells) grow 5.8-27.9% faster than shells alive prior to the construction of dams ('Pre-dam' shells). This increase in annual shell production is linked to the currently sharply reduced freshwater influx to the Colorado River estuary. Before the upstream river management, lower salinity retarded growth rates in these bivalves. Intra-annual growth rates were 50% lower during spring and early summer, when river flow was at its maximum. Growth rates in Chione today are largely controlled by temperature and nutrients; prior to the construction of dams and the diversion of the Colorado River flow, seasonal changes in salinity played an important role in regulating calcification rates. Our study employs sclerochronological (growth increment analysis) and geochemical techniques to assess the impact of reduced freshwater influx on bivalve growth rates in the Colorado River estuary. A combination of both techniques provides an excellent tool to evaluate the impact of river management in areas where no pre-impact studies were made. © 2003 Elsevier Ltd. All rights reserved.
• Schöne, B. R., Goodwin, D. H., Flessa, K. W., Dettman, D. L., & Roopnarine, P. D. (2002). Sclerochronology and growth of the bivalve mollusks Chione (Chionista) fluctifraga and C. (Chionista) cortezi in the northern Gulf of California, Mexico. Veliger, 45(1), 45-54.
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  Abstract: Sclerochronology and analysis of oxygen isotopes reveal the age, growth rate, and growth patterns of Chione (Chionista) cortezi and Chione (Chionista) fluctifraga. Chione (C.) cortezi grows more quickly than Chione (C.) fluctifraga, but has a shorter life span (8 years versus 16 years). Microgrowth increments form with tidal periodicity, and their width is mostly influenced by temperature. Microincrement patterns reveal that maximum growth occurs from April to June and again in October. Growth is reduced during the hottest part of the summer and the coldest part of the winter. Growth breaks often occur in December/January and August. Timing of shell growth and environmental conditions were verified by high-resolution oxygen isotope measurements.
• Schöne, B. R., Lega, J., Flessa, K. W., Goodwin, D. H., & Dettman, D. L. (2002). Reconstructing daily temperatures from growth rates of the intertidal bivalve mollusk Chione cortezi (northern Gulf of California, Mexico). Palaeogeography, Palaeoclimatology, Palaeoecology, 184(1-2), 131-146.
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  Abstract: We establish a model for the reconstruction of average daily sea surface temperatures from calcification rates of an intertidal bivalve mollusk. The rate of shell production in Chione cortezi (Carpenter, 1864, ex Sloat MS) is mainly controlled by water temperature, ontogenetic age and the effect of tidal cycles. Statistical methods developed by dendrochronologists can successfully extract the water temperature signal from daily growth increment chronologies. After removal of noise, the growth rates are expressed as scaled daily growth indices. Average daily water temperatures during the first half of the year are highly correlated with the scaled daily growth index values of recent and subrecent specimens, using the multi-valued function presented here. Increment width analysis can reconstruct daily average water temperatures with a mean error of less than 3%. This technique provides an independent method for reconstructing temperatures in fossil specimens of species with living representatives and can supplement high-resolution temperature reconstructions based on geochemical analysis. © 2002 Elsevier Science B.V. All rights reserved.
• Glenn, E. P., Zamora-Arroyo, F., Nagler, P. L., Briggs, M., Shaw, W., & Flessa, K. (2001). Ecology and conservation biology of the Colorado River delta, Mexico. Journal of Arid Environments, 49(1), 5-15.
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  Abstract: The Colorado River delta in Mexico has been partially revegetated following 20 years of water flows from the United States. Lake Powell, the last major impoundment built on the river, filled in 1981. Since then, flood flows in the main channel of the river have occurred in El Niño cycles, and have returned native trees and other vegetation to the riparian corridor. This vegetation provides a migration route for endangered southwestern willow flycatchers (Empidonax traillii) and other migratory birds moving from Mexico to the United States for summer nesting. Agricultural drain water from the Wellton-Mohawk Irrigation District conveyed to the delta since 1977 has created Cienega de Santa Clara, a 4200-ha Typha domengensis marsh containing the largest remaining population of the endangered Yuma clapper rail (Rallus longirostris yumanensis), plus numerous species of migratory and resident waterfowl. Populations in the marine part of the delta have been severely affected by the lack of river flow, but some species have responded positively to renewed flows. Currently, there are 170,000 ha of natural areas in the lower delta in Mexico, containing riparian, wetland and intertidal habitats. Much of this land as well as the adjacent marine zone is protected in the Biosphere Reserve of the Upper Gulf of California and Colorado River Delta. Natural resource managers, scientists and non-governmental environmental groups in Mexico and the United States are exploring conservation measures that can provide water and protection for these areas for the future. © 2001 Academic Press.
• Goodwin, D. H., Flessa, K. W., Schone, B. R., & Dettman, D. L. (2001). Cross-calibration of daily growth increments, stable isotope variation, and temperaturre in the Gulf of California Bivalve Mollusck Chione cortezi: Implications for paleoenvironmental analysis. Palaios, 16(4), 387-398.
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  Abstract: Annual-oxygen isotope profiles from two live-collected specimens of Chione cortezi Carpenter were analyzed in conjunction with daily growth-increment width profiles and high-resolution temperature records from the same site in the northern Gulf of California. The daily growth-increment profiles serve to date the deposition of the δ 18O samples. Then the δ 18O values were compared with high-resolution temperature records from the same site. Shell deposition began in late March or early April and ended in late November or early December. δ 18O-derived estimates of the maximum and minimum temperature thresholds of growth agree well with those obtained from the dated increment width profile. Shell deposition in these two specimens of C. cortezi from the northern Gulf began when temperature warmed above ∼17°C and slowed or halted when temperature rose above ∼31°C. The temporal resolution of stable isotope samples varies throughout the year. Samples with the coarsest resolution (>3 weeks) were taken from parts of the shell deposited near the minimum and maximum temperature thresholds of growth. Higher resolution samples have intermediate δ 18O values and most represent less than five days of growth. Calculated temperatures from the dated oxygen-isotope samples are similar to observed temperatures. Differences reflect the effects of daily temperature variation, tidal emergence, and enrichment in δ 18O of the water in which the clams grew. Stable oxygen-isotope samples used in conjunction with increment-width profiles can provide paleoenvironmental information at sub-weekly to submonthly resolution.
• Rodriguez, C. A., Flessa, K. W., & Dettman, D. L. (2001). Effects of upstream diversion of river water on the estuarine bivalve mollusc Mulinia coloradoensis. Conservation Biology, 15(1), 249-258.
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  Abstract: The Colorado River of the United States and Mexico is extensively dammed and diverted; only a fraction of its previous flow still reaches its estuary. How has the lack of freshwater inflow affected the estuary's biota? Because no prediversion studies are available for comparison, we examined the composition and geochemistry of subfossil shells dating from before the Colorado's diversion. The bivalve mollusc Mulinia coloradoensis was once the most abundant species of clam inhabiting the Colorado Delta. Today, however, only a small population survives near the mouth of the river. The relative abundance of empty shells of this species decreases with increasing distance from the mouth of the Colorado River, indicating that M. coloradoensis was dependent on the flow of the river. The δ18O values in shells of subfossil M. coloradoensis are significantly more negative than δ18O values in live-collected shells of Chione fluctifraga, the most common bivalve living on the delta today. This indicates that M. coloradoensis lived in water lower in salinity than is now typical of the delta. The decline in abundance of M. coloradoensis is most likely due to the post-1930 decrease inflow of Colorado River water to its estuary. Paleontological and geochemical analyses of subfossils can provide environmental baselines for communities that existed prior to human alteration of the habitat.
• Rodriguez, C. A., Flessa, K. W., Téllez-Duarte, M. A., Dettman, D. L., & Ávila-Serrano, G. A. (2001). Macrofaunal and isotopic estimates of the former extent of the Colorado River estuary, upper Gulf of California, México. Journal of Arid Environments, 49(1), 183-193.
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  Abstract: Faunal and isotopic evidence can be used to reconstruct the zone of fresh water influence of the Colorado River prior to its diversion for agricultural and domestic uses. The beaches and islands of the Colorado delta are predominantly composed of shells of the bivalve mollusk Mulinia coloradoensis. The shells date from before the construction of upstream dams, and δ18O values from the shells are significantly more negative than δ18O values from species living in the delta today. Both faunal and isotopic evidence indicate that M. coloradoensis is a brackish water species that thrived when the river flowed into the Gulf. The proportion of empty shells of M. coloradoensis ranges from 80-95% near the river's mouth to only 25% 65 km to the south. Shells of the species are rare to absent 80 km south of the mouth of the river. Macrofaunal evidence indicates a mixing zone extending as far as 65 km along the western shore of the upper Gulf of California. Average δ18O values in shells of M. coloradoensis become more positive with increasing distance from the river's mouth, reflecting the greater dilution of river water with normal salinity Gulf water. Average δ18O values in the fossil shells approach values in live bivalve mollusks at a distance 65 km south of the mouth of the river indicating that the mixing zone of the former Colorado River extended at least 65 km from its mouth. The effect of virgin Colorado River flow in the upper Gulf of California was geographically extensive. © 2001 Academic Press.
• Kowalewski, M., & Flessa, K. W. (2000). Seasonal predation by migratory shorebirds recorded in shells of lingulid brachiopods from Baja California, Mexico. Bulletin of Marine Science, 66(2), 405-416.
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  Abstract: Repair scars attributed to shorebirds are very frequent in intertidal populations of Glottidia palmeri (Lingulidae, Brachiopoda) from the Gulf of California. The multimodal distribution of the scars along the anterior-posterior axis of the brachiopod shells suggests the existence of strong temporal variation in the intensity of predation. Bootstrap analyses of the scar distributions, size-frequency population data, and growth ring data all indicate that the scars are seasonal and originated in the late fall and/or winter months. The scars record predation by migratory shorebirds that have wintering grounds in Baja California. The anterior-posterior distributions of scars can offer insights into seasonal variation in predation on marine benthos. Also, because seasonal repair scars represent a periodic signal, they may estimate the longevity and growth rates in prey and offer a "sclerochronological clock" analogous to oxygen isotopes or growth rings. The approach proposed here may be applicable also to the fossil record of some shelly organisms.
• Kowalewski, M., Serrano, G. A., Flessa, K. W., & Goodfriend, G. A. (2000). Dead delta's former productivity two trillon shells at the mouth of the Colorado River. Geology, 28(12), 1059-1062.
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  Abstract: The diversion of the Colorado River by dams and irrigation projects, started in the 1930s, triggered the collapse of the Colorado delta ecosystem. Paleontological, ecological, geochronological, stable isotope, field, and satellite image data provide estimates of the delta's benthic productivity during the 1 k.y. directly preceding the artificial shutdown of the river. At least 2 × 1012 shells of bivalve mollusks make up the current beaches and islands of the delta. The 125 individual valves dated using 14C-calibrated amino acid racemization indicate that these shells range in age from A.D. 950 to 1950. Seasonal intrashell cycles in δ180 values indicate that average-sized bivalves lived at least 3 yr. The most conservative calculation based on these numbers indicates that during the time of natural river flow, an average standing population of ∼6 × 109 bivalve mollusks (population density ∼50/m2) thrived on the delta. In contrast, the present abundance of shelly benthic macroinvertebrates is ∼94% lower (3/m2 in 1999-2000). The dramatic decrease in abundance testifies to the severe loss of benthic productivity resulting from diversion of the river's flow and the inadequacy of its partial resumption (1981 to present). An integration of paleontological records with geomorphological, geochemical, and geochronological data can provide quantitative insights into human impact on coastal ecosystems.
• Flessa, K. W. (1998). Well-traveled cockles: Shell transport during the Holocene transgression of the southern North Sea. Geology, 26(2), 187-190.
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  Abstract: Radiocarbon dates from 34 shells of the intertidal bivalve Cerastoderma edule demonstrate that shells were transported in both landward and seaward directions during the Holocene transgression of the southern North Sea. Old shells on the beaches of the East Frisian Islands of Germany document landward transport and young shells in the German Bight and Dogger Bank document seaward transport. The area's sea-level curve and the shell ages were used to predict the original depth of each specimen. The difference between a specimen's predicted age and its present depth is a measure of depth displacement. Depth displacements ranged from +35 to -37 m. Eight shells remained within 2.5 m of their original depth and roughly equal proportions of the rest moved landward and seaward. Specimens transported into deeper water are a very small fraction of the shells at that depth, whereas specimens transported into shallow water occur alongside abundant indigenous individuals of the same species. Rare fossils should not be used to estimate paleodepths.
• Kowalewski, M., Goodfriend, G. A., & Flessa, K. W. (1998). High-resolution estimates of temporal mixing within shell beds: the evils and virtues of time-averaging. Paleobiology, 24(3), 287-304.
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  Abstract: This study quantifies the fine structure of time-averaging by using large samples of dated shells collected from within individual strata. Time-averaging results in both good and bad news for interpreting bioclasic deposits. Nine samples of shells were collected from four Holocene cheniers on the Colorado Delta (Gulf of California) and 165 shells of the bivalve Chione fluctifraga were dated using 14C-calibrated amino acid racemization (D-alloisoleucine/L-isoleucine). The age range of shells within samples averages 661 years and, in seven out of nine samples, exceeds 500 years. The sample standard deviation ranges from 73 to 294 years and averages 203 years, far exceeding the dating errors (≪100 years) and potential variation in the life span of Chione (
• Cadée, G. C., Walker, S. E., & Flessa, K. W. (1997). Gastropod shell repair in the intertidal of Bahia la Choya (N. Gulf of California). Palaeogeography, Palaeoclimatology, Palaeoecology, 136(1-4), 67-78.
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  Abstract: Models of evolutionary escalation between gastropods and their shell-breaking predators rely on the presence of a strong relation between predation intensity and repair frequency. Some previous work has suggested that both predation intensity and repair frequency have increased through geologic time. Repair frequency (the percentage of shells with at least one repair scar) in four Recent gastropods from the northern Gulf of California shows both high interspecific (7.6% in Cerithium stercusmuscarum to 87.9% in Turritella gonostoma) and interhabitat variation (11.9-30.7% in Theodoxus luteofasciatus and 26.8-64.9% in Cerithidea albonodosa). Habitat-mixing, time-averaging and collecting practices might diminish variation in shell repair in fossil populations. Nevertheless, the high microhabitat variation observed here indicates that trends in shell repair through geologic time should consider the variation in shell repair at any one time. Reliable estimates of repair frequencies in fossil gastropods requires samples of several species and several habitats. Measuring shell repair should be better standardized, published data are now often difficult to compare.
• Goodfriend, G. A., & Flessa, K. W. (1997). Radiocarbon reservoir ages in the Gulf of California: Roles of upwelling and flow from the Colorado River. Radiocarbon, 39(2), 139-148.
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  Abstract: We measured apparent radiocarbon ages of live-collected, pre-bomb mollusk shells from the northern and central Gulf of California to determine the source of the reservoir ages and the reservoir age correction offsets for calibrating 14C dates of fossil samples. Reservoir ages average 860 yr in the northern Gulf and 725 yr in the central Gulf. The corresponding AR values (the deviation from typical worldwide values) are 540 yr and 395 yr, respectively, with variabilities (SD) of 90 and 110 yr. This variability significantly limits the precision of calibrated 14C ages. The apparent 14C age of Colorado River water (as measured in a freshwater mussel, collected in the 1890s, before diversion of river flow) is not sufficiently high (1420 yr) to account for the high reservoir ages in the Gulf. The lack of a relation between the stable isotope composition of Gulf mollusks and their reservoir ages is further evidence that the Colorado River does not make a significant contribution to Gulf reservoir ages. Upwelling of old, deep Pacific-derived water appears to be the cause of the large reservoir ages.
• Goodfriend, G. A., Flessa, K. W., & Hare, P. E. (1997). Variation in amino acid epimerization rates and amino acid composition among shell layers in the bivalve Chione from the Gulf of California. Geochimica et Cosmochimica Acta, 61(7), 1487-1493.
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  Abstract: Variation in the isoleucine epimerization rate and amino acid composition and concentration among and within the three layers comprising the shell of the marine bivalve Chione fluctifraga from the northern Gulf of California was determined in a time series of ten shells, from modern back to 900 yr BP (reservoir-age-corrected radiocarbon age). Differences in epimerization rates of up to 30% were found between various portions of the shells. In samples taken from the middle layer of the shell (two different positions) and the hinge area of the inner layer, D-alloisoleucine/L-isoleucine (A/I) values showed excellent age prediction ability, whereas in samples from the outer layer and the central part of the inner layer, A/I values showed greater variability. Isoleucine epimerization rates were found to differ between sampling positions within both the inner and outer layers of the shell. Average rate differences were also found among layers. The amino acid composition of the three layers is rather similar (with Asp, Glu, and Gly the most abundant amino acids) but variable. Significant differences in amounts of amino acids were found, with the middle layer showing the lowest amounts and the inner layer the highest amounts. Careful choice of sampling position may improve the accuracy of age estimates from amino acid racemization. Copyright © 1997 Elsevier Science Ltd.
• Kowalewski, M., Dyreson, E., Marcot, J. D., Vargas, J. A., Flessa, K. W., & Hallman, D. P. (1997). Phenetic discrimination of biometric simpletons: Paleobiological implications of morphospecies in the lingulide brachiopod Glottidia. Paleobiology, 23(4), 444-469.
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  Abstract: The extreme morphological simplicity of lingulide brachiopod shells makes them particularly useful for investigating the species-level taxonomic resolution of the fossil record as well as the relationships between taxonomy, morphological complexity, and evolutionary rates. Lingulides have undergone little change in shell morphology and have had low taxonomic diversity since the Paleozoic. Is this pattern an evolutionary phenomenon or an artifact of the shell's simplicity? Multivariate methods were used to establish morphogroups among seven populations of four extant species of Glottidia. Six characters (three shell dimensions and three internal septa) were measured for 162 specimens from field and museum collections. All populations follow similar allometric trajectories: internal septa display positive allometry and shell dimensions display negative allometry. The allometric pattern may reflect D'Arcy Thompson's Principle of Similitude. Principal component analysis does not reveal any distinct clusters in Glottidia morphospace but suggests that some differences independent from ontogeny exist among the populations. Size-free canonical variate analysis indicates the presence of five size-invariant groups that are statistically distinct. Bootstrap-corrected error rates indicate that four specimens are enough to classify a sample correctly at α = 0.05 and eight specimens at α = 0.01. The groups are consistent with neontological classification with the exception of two populations of G. pyramidata identified by discriminant analysis as two distinct groups. The size-free morphogroups reflect geographic separation rather than ontogenetic or substrate differences among the populations. Despite the morphological simplicity of the shell, size-free multivariate analysis of Glottidia delineates groups that offer taxonomic resolution comparable with the neontological classification. The method offers a promising tool for identifying natural morphogroups on the basis of few morphological characters. Moreover, the agreement between neontological taxonomy and the morphogroups suggests that the size-free approach can be applicable for evaluating the reality of the low diversity and turnover rates observed in the fossil record of lingulide brachiopods (= Family Lingulidae). Assuming that the neontological species of Glottidia are biologically meaningful, this study shows that morphological simplicity of lingulides does not necessarily result in taxonomic underresolution. Our analysis, as well as several previous case studies, suggests that taxonomic diversity and turnover rates do not have to be dependent on the morphological complexity of preservable parts. In many cases, when rigorous quantitative methods are employed, the differences in the rates of morphological evolution may be a real evolutionary phenomenon and not artifacts of morphological complexity.
• Kowalewski, M., Flessa, K. W., & Marcot, J. D. (1997). Predatory scars in the shells of a recent lingulid brachiopod: Paleontological and ecological implications. Acta Palaeontologica Polonica, 42(4), 497-532.
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  Abstract: The paper presents the detailed quantitative study of predatory scars in the shells of an inarticulate brachiopod: the lingulid Glottidia palmeri Dall, 1870. The scars include four morphological types: u-shaped, pocket, crack, and miscellaneous scars. They concentrate and open up toward the anterior shell edge. They commonly consist of a pair of scars on the opposite valves. The analysis of 820 specimens live-collected from two intertidal localities in the northern Gulf of California indicates that (1) 23.4% specimens bear repair scars; (2) the scars vary in size from 1.5 to 24 mm2 (mean = 2.5 mm2) and all scar types have similar size-frequency distributions; (3) the spatial distribution of scars on the shell is non-random; (4) the anterior-posterior distribution of scars is strongly multimodal and suggests seasonal predation in the late fall and winter months; and (5) the frequency of scarred specimens increases with brachiopod size and differs between the two sampled localities, but does not vary among brachiopod patches from the same locality. The repair scars record unsuccessful attacks by epifaunal intertidal predators with a scissors-type weapon (birds or crabs). The high frequency of attacks, seasonal winter predation, and previous ecological research suggest that scars were made by wintering shorebirds (willets or/and curlews). However, crabs cannot be entirely excluded as a possible predator. Because repair scars represent unsuccessful predation, many of the quantitative interpretations are ambiguous. Nevertheless, the study suggests the existence of strong seasonal interactions between inarticulate brachiopods and their predators. Because shorebirds, crabs, and lingulids may have co-existed in intertidal ecosystems since the late Mesozoic, predatory scars in lingulid shells may have potentially a 100 million year long fossil record.
• Meldahl, K. H., Flessa, K. W., & Cutler, A. H. (1997). Time-averaging and postmortem skeletal survival in benthic fossil assemblages: Quantitative comparisons among Holocene environments. Paleobiology, 23(2), 207-229.
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  Abstract: We used radiocarbon ages on dead Holocene shells of the venerid bivalve Chione spp. to investigate how time-averaging and taphonomy in shallow marine benthic assemblages vary with sedimentary and tectonic setting. We compared shells collected from the sediment surface in five depositional environments from two regions of the Gulf of California, Mexico: Bahía Concepción, a young faulted rift basin with high rates of terrigenous and carbonate sedimentation; and Bahia la Choya, an intertidal system along a sediment-starved shelf. Frequency distributions of shell ages in all environments form a hollow curve, with a mode at young ages and a long tail toward older ages. This pattern suggests that shells are added to the taphonomically active zone (TAZ) at roughly constant rates (via continuous shell deaths), and removed from the TAZ at random, either through destruction or by achieving final burial. Shell half-lives (the amount of time to remove half the shells from the TAZ) provide a comparative measure of time-averaging. Time-averaging varies with sedimentary and tectonic setting. The lowest amounts of time-averaging (shell half-lives of 90 to 165 years) occur in Bahia Concepcion, where rapid rates of terrigenous sedimentation (on fan-deltas) and carbonate sedimentation (in pocket bays) bury shells rapidly. Time-averaging is higher in the sediment-starved environments of Bahía la Choya (shell half-lives of 285 to 550 years). The highest amounts of time-averaging occur the inner tidal flats of Bahía la Choya (shell half-life of 550 years). Here the conjunction of low sedimentation rates with low rates of shell destruction (due to periodic tidal emergence) permits shells to persist in the TAZ for very long time spans. There is no systematic relationship between a shell's age and its taphonomic condition (taphonomic grade) in any environment, probably because of the complex and random nature of burialexhumation in the TAZ. Age variance tends to increase with increasing taphonomic alteration: highly altered shells range in age from young to several thousand years old, while less altered shells are mostly young. The correspondence between time-averaging and the taphonomic condition of entire shell assemblages is also weak, but might be resolved with further study. These results provide quantitative data on time-averaging in benthic assemblages as a function of sedimentary and tectonic setting, and suggest some guidelines for facies appropriate for particular studies. Shallow marine rift basins like Bahía Concepción can potentially contain within-horizon fossil assemblages representing time spans of only a few hundred years - time resolution often beyond reach in paleontology. In contrast, sediment-starved shelf habitats like Bahía la Choya are unlikely to yield assemblages with time resolution finer than several thousands of years.
• Hallman, D. P., Flessa, K. W., Kowalewski, M., Hertweck, G., Aggen, J., & Carlton, J. (1996). Ternary taphograms and the comparative taphonomy of recent mollusks from the north sea and the Gulf of California. Senckenbergiana Maritima, 27(1-2), 67-75.
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  Abstract: We examined the taphonomic condition of bivalve shells from the East Frisian Islands of the German North Sea coast (Cerastoderma edule), and two localities in the northern Gulf of California: Bahia la Choya (Chione spp.) and Colorado Delta (Mulinia coloradoensis). We classified each shell according to its taphonomic condition with respect to seven features: abrasion, edge preservation, bioerosion, encrustation, internal and external color, and fragmentation. The northern Gulf of California has a macrotidal regime and the German North Sea coast has a mesotidal regime. The Gulf of California has a hotter and drier climate than the German North sea coast. The Bahia la Choya locality has coarser sediments and a lower rate of sedimentation than the other two localities. We used ternary diagrams (taphograms) to characterize the frequency of preservational grade within a sample and to compare the taphonomic signatures of the three regions. Comparison of North Sea, Bahia la Choya, and Colorado Delta samples shows: (a) high wear among North Sea shells; (b) high fragmentation and encrustation among Bahia la Choya shells; and (c) high variation in surface condition and limited bioerosion and encrustation among Colorado Delta shells. The variation in shell condition among these three localities reflects (1) the relatively high energy regime of the German North Sea localities, as compared to the northern Gulf of California; (2) the low sedimentation rates of Bahia la Choya, as compared with the North Sea and Colorado Delta localities; and (3) the soft substrates and limited subaqueous exposure of Colorado Delta shells, as compared with the Bahia la Choya and North Sea shells. This study demonstrates that regional variation in the preservational quality of fossil shells can reflect many of the differences in the original environments of deposition.
• Kidwell, S. M., & Flessa, K. W. (1996). The quality of the fossil record: populations, species, and communities. Annual Review of Earth and Planetary Sciences, 24, 433-464.
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  Abstract: Paleontologists have always been concerned about the documentary quality of the fossil record, and this has also become an important issue for biologists, who increasingly look to accumulations of cones, shells, and plant material as possible ways to extend the time-frame of observation on species and community behaviors. Quantitative data on the postmortem behavior of organic remains in modern environments are providing new insights into death and fossil assemblages as sources of biological information. Important findings are discussed. Although a complex array of processes and circumstances influences preservation, death assemblages of resistant skeletal elements are for many major groups good to excellent records of community composition, morphological variation, and environmental and geographic distribution of species, and such assemblages can record temporal dynamics at ecologically and evolutionarily meaningful scales.
• Kowalewski, M., & Flessa, K. W. (1996). Improving with age: The fossil record of lingulide brachiopods and the nature of taphonomic megabiases. Geology, 24(11), 977-980.
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  Abstract: Variation in the actual preservation of fossils can be used to detect taphonomic megabiases - large-scale distortions caused by changes in the quality of the fossil record. For example, temporal changes in the taphonomy of lingulide brachiopods suggest that Paleozoic lingulides had a higher fossilization potential than the post-Paleozoic ones. The frequency of stratigraphic occurrences of lingulides covaries with their fossilization potential. Thus, the apparent decline in the importance of lingulides in Phanerozoic communities may be due to changes in their fossilization potential. Taphonomic megabiases may be very common. We distinguish four major types of megabiases: within taxon, static among taxon, dynamic among taxon, and global.
• Kowalewski, M., & Flessa, K. W. (1996). Improving with age: the fossil record of lingulide brachiopods and the nature of taphonomic megabiases. Geology, 24(11), 977-980.
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  Abstract: Variation in the actual preservation of fossils can be used to detect taphonomic megabiases - large-scale distortions caused by changes in the quality of the fossil record. For example, temporal changes in the taphonomy of lingulide brachiopods suggest that Paleozoic lingulides had a higher fossilization potential than the post-Paleozoic ones. The frequency of stratigraphic occurrences of lingulides covaries with their fossilization potential. Thus, the apparent decline in the importance of lingulides in Phanerozoic communities may be due to changes in their fossilization potential. Taphonomic megabiases may be very common. Four major types of megabiases are distinguished within taxon, static among taxon, dynamic among taxon, and global.
• Myrick, J. L., & Flessa, K. W. (1996). Bioturbation rates in Bahía la Choya, Sonora, Mexico. Ciencias Marinas, 22(1), 23-46.
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  Abstract: Experimental determination of sediment reworking rates in a subtropical intertidal flat environment yielded information about the amount, nature and implications of sediment reworking in nearshore deposits. Callianassid shrimp in Bahía La Choya, Sonora, Mexico, overturn the sediment in the inner flats at an average rate of 0.56 m3/m2/year. Elasmobranch rays overturn the sediment in the midflats at an average rate of 1.01 m3/m2/year. Resin castings indicate that the shrimp are capable of burrowing to a depth of at least 1.15 m and, where present, can completely rework this interval in Bahía La Choya in two years. The rays reach a maximum observed burrowing depth of 20 cm and, where present, can completely rework this interval in Bahía La Choya in 72 days. Reworking rates are high enough to preclude the preservation of most physical sedimentary structures under normal conditions. Only large-scale sedimentary structures or those buried deeply and rapidly are likely to escape reworking. Rates of biogenic sedimentation by callianassid shrimp are high enough to generate subsurface shell beds. Short-term biogenic sedimentation rates are higher than long-term rates, indicating that such intertidal sediments are not only thoroughly reworked, but are incomplete at time scales of weeks to months.
• Springer, D. A., & Flessa, K. W. (1996). Faunal gradients in surface and subsurface shelly accumulations from a recent clastic tidal flat, Bahia la Choya, northern Gulf of California, Mexico. Palaeogeography, Palaeoclimatology, Palaeoecology, 126(3-4), 261-279.
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  Abstract: Do the shelly remains already accumulated at depth reflect the faunal composition and environmental gradients evident in surface deposits, or do many years of selective destruction and post-mortem transportation limit the spatial resolution possible in studies of nearshore marine faunas? We sampled shelly molluscan remains at the surface and at depth along two 2 km transects across the intertidal zone of Bahia 1a Choya, Sonora, Mexico. At each site we collected surface samples from the upper 5 cm and depth samples from a ubiquitous shelly layer approximately 25 cm below the surface. Radiocarbon dates indicate that the samples differ by approximately 800 years in age. We used the 25 most common species in subsequent analyses. Faunal composition and distribution patterns vary systematically among the four sets of samples. Cluster analysis and multidimensional scaling (MDS) show that subsurface transects reflect species distributions of their surface counterparts more closely than species distributions in surface (or depth) transects resemble each other. MDS of samples also highlights a distinctive gradient in species distributions: Dimension 1 consistently expresses the depth gradient across the tidal flats. Dimension 2 appears to represent subtle changes in substrate mobility. These results suggest that, despite the potential for shell transport and extensive time-averaging, surface heterogeneity is accurately recorded in the subsurface shelly bed. Environmental gradients may have remained constant during the ~500-1300 years since deposition of the subsurface bed, reworking may have completely mixed the sedimentary interval, or both. Spatial resolution of approximately 100 m may be possible in intertidal flat habitats in the stratigraphic record.
• Flessa, K. W., & Jablonski, D. (1995). Biogeography of recent marine bivalve molluscs and its implications for paleobiogeography and the geography of extinction: a progress report. Historical Biology, 10(1), 25-47.
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  Abstract: The latitudinal diversity gradient in Recent marine bivalve molluscs and the effects of bivalve distributional patterns on the severity of model extinctions was examined using a global biogeographic database for genera of Recent shallow (
• Kidwell, S. M., & Flessa, K. W. (1995). The quality of the fossil record: Populations, species, and communities. Annual Review of Ecology and Systematics, 26(1), 269-299.
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  Abstract: Paleontologists have always been concerned about the documentary quality of the fossil record, and this has also become an important issue for biologists, who increasingly look to accumulations of bones, shells, and plant material as possible ways to extend the time-frame of observation on species and community behaviors. Quantitative data on the postmortem behavior of organic remains in modern environments are providing new insights into death and fossil assemblages as sources of biological information. Important findings include: 1. With the exception of a few circumstances, usually recognizable by independent criteria, transport out of the original life habitat affects few individuals. 2. Most species with preservable hardparts are in fact represented in the local death assemblage, commonly in correct rank importance. Molluscs are the most durable of modern aquatic groups studied so far, and they show highest fidelity to the original community. 3. Time-averaging of remains from successive generations and communities often prevents the detection of short-term (seasons, years) variability but provides an excellent record of the natural range of community composition and structure over longer periods. Thus, although a complex array of processes and circumstances influences preservation, death assemblages of resistant skeletal elements are for many major groups good to excellent records of community composition, morphological variation, and environmental and geographic distribution of species, and such assemblages can record temporal dynamics at ecologically and evolutionarily meaningful scales.
• Kowalewski, M., & Flessa, K. W. (1995). Comparative taphonomy and faunal composition of shelly cheniers from northeastern Baja California, Mexico. Ciencias Marinas, 21(2), 155-177.
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  Abstract: Taphofacies analysis of 4334 shells of Mulinia coloradoensis described by nine variables indicated that the shells accumulated in these cheniers are rarely affected by biological processes and moderately affected by physical processes. The shells collected from the chenier surface have poorer preservation than the subsurface ones, indicating that taphonomic degradation is mostly a surface phenomenon. Multivariate analyses discriminate the chenier generations even when the poorly preservable luster variables are excluded from the analysis. Chenier shelly assemblages are taphonomically distinct from assemblages formed in other marine environments. They are characterized by a very low intensity of bioerosion and encrustation. -from Authors
• Kowalewski, M., Flessa, K. W., & Hallman, D. P. (1995). Ternary taphograms: triangular diagrams applied to taphonomic analysis. Palaios, 10(5), 478-483.
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  Abstract: Ternary diagrams can be useful in taphonomic analysis. When the quality of preservation of a hard part can be expressed in a three-fold scheme, a single point on a triangular diagram characterizes the frequency distribution of taphonomic alteration within a sample. Comparison of samples with such a "ternary taphogram' is an efficient way to explore variation in the preservation of hard parts. To illustrate the method, samples from the northern Gulf of California and the North Sea are compared. Taphonomic variation in the samples reflects variation in the extent of subaerial exposure, wave and current regime, and shell architecture. -Authors
• Flessa, K. W., & Kowalewski, M. (1994). Shell survival and time-averaging in nearshore and shelf environments: estimates from the radiocarbon literature. Lethaia, 27(2), 153-165.
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  Abstract: Radiocarbon dates provide a means for estimating the time a shell may persist in active sedimentary environments and the actual temporal extent of time-averaging in marine deposits. Information compiled from the published literature on the radiocarbon age of marine shells gave information on a total of 734 radiocarbon dates from 276 localities from nearshore (10 m depth) habitats. The median age of 128 nearshore shells is 2465 years; that of 158 shell from the shelf is 8870 years. The maximum age of a shell in an active sedimentary environment is a measure of time-averaging, because it estimates the amount of time represented in the deposit. The median duration of time-averaging in 63 nearshore deposits is 1250 years; the median duration of time-averaging in 129 shelf deposits is 9190 years. Greater shell survival and longer durations of time-averaging in shelf settings may result from lower rates of sedimentation, lower rates of taphonomic destruction, greater rates of bioturbation, the history of post-glacial sea level, sample bias, or some combination of these factors. -Authors
• Kowalewski, M., Flessa, K. W., & Aggen, J. A. (1994). Taphofacies analysis of recent shelly cheniers (beach ridges), northeastern baja california, Mexico. Facies, 31(1), 209-241.
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  Abstract: This report presents the results of taphofacies analyses of shelly cheniers (mollusk-dominated lag-concentrations) from the tidal flats of northeastern Baja California, Mexico. The three generations of moderm (formed during last 70 years), submodem (younger than 1,500 BP), and subfossil (5,000-2,000 BP) cheniers can be distinguished by their position relative to the shoreline, their topography, and the radiocarbon-age of their shells. The generations differ in the duration and complexity of their taphonomic history. Sixty-one samples from nine localities were collected to test the utility of the taphofacies approach for studying chenier-type shell deposits. The three chenier generations, although all dominated by the bivalve mollusk Mulinia coloradoensis, differ significantly in their taxonomic composition due to taphonomic and/or biologic factors. The taphofacies analysis included 4,334 specimens of M. coloradoensis described by nine taphonomic variables. Univariate analysis of those variables indicated that the shells that accumulated in the cheniers are little-affected by biological processes (bioerosion, encrustation), and moderately affected by physical processes (fragmetation, cracking, peeling, edge preservation). Only the luster features of shells (external luster, internal luster, and internal features) vary substantially and consistently with chenier age -a result of subaerial weathering. Multivariate taphofacies analysis discriminates the three generations of cheniers even when the poorly preservable luster variables are excluded from the analysis. This suggests that taphofacies discrimination is possible for fossil cheniers. The shells collected from the chenier surface have substantially poorer preservation than shells from the subsurface, indicating that taphonomic degradation in the chenier plain environment is a surface phenomenon. Chenier plain shelly assemblages are taphonomically distinct from assemblages formed in other marine environments: they have a very low frequency of macroscopically recognizable bioerosion and encrustation. The existence of preservable taphonomic differences between the cheniers that differ in their age (i.e., duration of preburial history), suggests that fossil lag concentrations may be useful in detecting incompleteness gradients along stratigraphic boundaries. A 'taphonomic clock'-a correlation between a 'time-sincedeath' and shell preservation-was found only for luster features, taphonomic attributes that are unlikely to be preserved in the fossil record. © 1994 Institut für Paläontologie, Universität Erlangen.
• Kowaloewski, M., & Flessa, K. W. (1994). A predatory drillhole in Glottidia palmeri Dall (Brachiopoda; Lingulidae) from Recent tidal flats of northeastern Baja California, Mexico. Journal of Paleontology, 68(6), 1403-1405.
• Flessa, K. W., Cutler, A. H., & Meldahl, K. H. (1993). Time and taphonomy: quantitative estimates of time-averaging and stratigraphic disorder in a shallow marine habitat. Paleobiology, 19(2), 266-286.
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  Abstract: Examines the radiocarbon age, taphonomic condition and stratigraphic position of shells of the venerid bivalve Chione spp. from the tidal flats of Bahia la Choya, Sonora, Mexico. Shells in Bahia la Choya are time-averaged. Long shell survival is interpreted to be the result of frequent shallow burial. The taphonomic condition of shells varied with environment and taphonomic condition proves to be an unreliable indicator of a shell's time-since-death. Stratigraphic disorder is shown to be a consequence of both time-averaging and physical and biogenic mixing. In order to prevent temporal overlap between successive samples in deposits like Bahia la Choya, sample spacing should not be less than approximately 0.5 m. -from Authors
• Flessa, K. W., Kowalewski, M., & Walker, S. E. (1992). Post-collection taphonomy: Shell destruction and the chevrolet. Palaios, 7(5), 553-554.
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  Abstract: The degree of shell fragmentation can be affected by post-collection ta-phonomic processes. Robust shells of the venerid bivalve Chione (Chionis-ta) fluctifraga suffered no damage during vehicle transport, while from 3 to 15% of the fragile shells of the mytilid bivalve Mytella guayanensis were broken. More breakage occurred in the Mytella sample stored in the bottom of the shipping box than occurred in the sample stored on the top. The use of fragmentation as a taphonomic indicator requires careful post-collection treatment of samples. This paper elucidates the obvious. Copyright © 1992, SEPM (Society for Sedimentary Geology).
• Cutler, A. H., & Flessa, K. W. (1990). Fossils out of sequence: computer simulations and strategies for dealing with stratigraphic disorder. Palaios, 5(3), 227-235.
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  Abstract: Microstratigraphic resolution is limited by vertical mixing and reworking of fossils. Stratigraphic disorder is the degree to which fossils within a stratigraphic sequence are not in proper chronological order. Stratigraphic disorder arises through in situ vertical mixing of fossils and reworking of older fossils into younger deposits. We simulated the effects of mixing and reworking by simple computer models, and measured stratigraphic disorder using rank correlation between age and stratigraphic position (Spearman and Kendall coefficients). The effects of mixing-produced disorder can be minimized by increasing sample size at each horizon. Increased spacing between samples is of limited utility in dealing with disordered sequences: while widely separated samples are more likely to be stratigraphically ordered, the smaller number of samples makes the detection of trends problematic. -from Authors
• Flessa, K. W. (1990). All generalizations about NSF are false. Including this one. Palaios, 5(5), 489-495.
• Flessa, K. W. (1990). NSF Funding for paleontology and sedimentary geology. Palaios, 5(5), 393-.
• Flessa, K. W. (1990). The "facts" of mass extinctions. Special Paper of the Geological Society of America, 247, 1-7.
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  Abstract: The years since Snowbird I have seen an explosive growth of research on the patterns, rates, causes, and consequences of extinction. The fossil record of evolution is better known, new stratigraphic sections have been scrutinized in great detail, and additional markers of environmental change have been discovered in the rock record. Three contrasting interpretations of the fossil record of extinction are possible: catastrophic extinction, gradual extinction, and stepwise extinction. Each interpretation is supported by some of the evidence. However flawed, the fossil record is the only record we have of natural extinction. Compilations from the primary literature provide insight on the magnitude, breadth, and periodicity of mass extinctions. Outcrop-based studies have the potential to resolve controversies over the precise rate and pattern of mass extinctions. Unfortunately, a literal reading of the fossil record is made difficult by the manner in which fossils occur in strata and by the way in which fossils are collected and studied.
• Meldahl, K. H., & Flessa, K. W. (1990). Taphonomic pathways and comparative biofacies and taphofacies in a Recent intertidal/shallow shelf environment. Lethaia, 23(1), 43-60.
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  Abstract: The development of taphonomic approaches to facies analysis requires a foundation in facies-based actualistic studies. Modern intertidal and shallow shelf environments at Provincetown Harbor, northern Cape Cod, Massachusetts provide an opportunity to compare patterns and controlling factors in molluscan biofacies and taphofacies distributions. Variation in faunal composition, ecologic variables and taphonomic attributes of molluscan death assemblages produce distinct patterns of environmental zonation: (1) Faunal composition primarily tracks variation in substrate type among environments (sand, rock, peat, and Zostera marina beds). (2) Ecologic variables (equitability, infauna:epifauna ratio, gastropod:bivalve ratio, and predation on M. mercenaria) appear to reflect tidal exposure time. (3) Taphononic attributes (fragmentation, abrasion, corrosion, bioerosion, and encrustation) of the common bivalve M. mercenaria track environmental energy, in particular its effects on the stability and reworking of hardparts at the sediment surface. Shells in different environments proceed along different taphonomic pathways - the order of acquisition of taphonomic features by hardparts. -from Authors
• Fursich, F. T., & Flessa, K. W. (1987). Taphonomy of tidal flat moluscs in the Northern Gulf of California: Paleoenvironmental analysis despite the perils of preservation.. Palaios, 2(6), 543-559.
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  Abstract: Taphocoenoses can be powerful indicators of ancient environments, even in depostional settings in which rates of sedimentation are low and tidal currents are strong. The rich subtropical molluscan fauna of the N Gulf of California exhibits a distinct zonation across an extensive tidal flat at Bahia la Choya (Sonora, Mexico). The distribution of live faunas is largely controlled by substrate, energy level, availability of food, and period of submergence. Despite strong tidal currents and the effects of time-averaging, the distribution of taphocoenoses closely reflects the distribution of the live communities. Gastropods dominate on hard and firm substrates while bivalves prevail on sandy substrates. Herbivores are most common on rocky and shelly substrates; detritus feeders occur on the firm substrates of the inner flat and marsh; suspension-feeders predominate in sandy and rocky bottoms. Shell abrasion, encrustation, and boring decrease landward. We tested an intergrated approach to paleoenvironmental analysis by examining a Pleistocene shell bed from Bahia la Choya. Taxonomic, ecologic, and taphonomic criteria suggest deposition by a storm event in a very shallow subtidal to lowest intertidal environment.-from Authors
• Flessa, K. W., & Jablonski, D. (1985). Declining Phanerozoic background extinction rates: Effect of taxonomic structure?. Nature, 313(5999), 216-218.
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  Abstract: A decline in the total and per-family extinction rate of marine families1,2 during the Phanerozoic has been attributed to progressive improvements in the ecological properties of species. We believe this is not necessarily the case and suggest that the lower family extinction rates may reflect increases in the number of species per family, on the geological time scale, with higher species/family ratios being an inevitable consequence of the increase in species richness3 and the geometry of the branching evolutionary tree4. Because species-rich families are more likely to survive the stochastically constant background probabilities of species extinction, the biosphere has gradually accumulated species-rich clades and the total and per-family rates of extinction have consequently declined. © 1985 Nature Publishing Group.
• Jablonski, D., Flessa, K. W., & Valentine, J. W. (1985). Biogeography and paleobiology.. Paleobiology, 11(1), 75-90.
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  Abstract: The traditional focus of paleobiogeographic study has been the province, a statistical entity defined by clusters of range endpoints of invididual taxa. The study of such provinces has been useful in inferring past continental positions (although ambiguities remain that must be resolved using independent geological criteria) and in understanding the role of past global geographies in regulating biotic diversity through changes in the numbers and extent of provinces. This approach can be complemented by the treatment of geographic ranges of taxa as irreducible or emergent traits with far-reaching evolutionary effects upward and downward within a genealogical hierarchy. Temperature tolerances in benthic marine organisms appear to be by-products of selection for enzyme structures imparting favorable activity levels within the normal temperature range rather than direct products of selection for resistance to temperature extremes. Thus, geographic range endpoints, which are also influenced by dispersal capability and the resulting scale of gene flow among disjunct populations, are not direct products of selection. However, the magnitudes of geographic range of species and clades behave as emergent properties and significantly influence taxonomic survivorship during background and mass extinctions in ways that are not extrapolations of effects at lower hierarchical levels. Biogeography shapes macroevolutionary patterns of origination and extinction during times of normal, background extinction and mass extinction. Preferential extinction among regions or among endemic rather than widespread clades can result in strong biases in the nature of the survivors of mass extinctions, with taxa being lost not because of selection against attributes of individual organisms but because of higher-order patterns of geographic selectivity.-from Authors
• Heller, P. L., & Flessa, K. W. (1981). Comment on 'Late Oligocene transgression of middle Atlantic Coastal Plain'. Geology, 9(7), 290-.
• Balsam, N. G., Flessa, K. W., & Dubois, L. G. (1980). Planktonic foraminiferal diversity in the interglacial and glacial North Atlantic: a test of diversity gradients as a paleoceanographic technique.. Geology, 8(12), 582-585.
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  Abstract: Modern diversity patterns, especially those based on the Shannon index, reflect present surface circulation. Mapped 18 000 BP diversity values reveal a southward migration of the polar front, a shrinking of transitional areas, a southward deflection of the Gulf Stream, and a slight expansion of Sargasso Sea waters. These changes agree with the CLIMAP reconstruction. -from Authors
• Flessa, K. W., & Murer, A. S. (1979). Comment on 'Geologic implications of the relationship between mammalian faunal similarity and geographic distance'. Geology, 7(7), 327-328.
• Flessa, K. W., Barnett, S. G., Cornue, D. B., Lomaga, M. A., Lombardi, N., Miyazaki, J. M., & Murer, A. S. (1979). Geologic implications of the relationship between mammalian faunal similarity and geographic distance. Geology, 7(1), 15-18.
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  Abstract: The degree of similarity, at the generic and familial levels, among Holocene terrestrial mammal faunas is strongly and negatively correlated with the geographic distance between them. Oceanic barriers and climatic differences also contribute significantly to the variation in faunal similarity, but distance alone explains as much as 56/ of the variation. This analysis justifies the use of the degree of similarity among fossil mammal faunas as an estimator of paleogeographic distance. The distance between the centers of regions can be predicted by the biogeographic transfer function CENTER D = 18570-156 (SIMFAM), where distance is measured in kilometres and SIMFAM is the faunal similarity at the familial level, as measured by the Simpson Index. Maps of the Holocene world on which this equation is used to estimate distance display relatively minor deviations from actual geography and suggest that this method is a potentially powerful paleogeographic technique. © 1979 Geological Society of America.
• Balsam, W. L., & Flessa, K. W. (1978). Patterns of planktonic foraminiferal abundance and diversity in surface sediments of the western North Atlantic. Marine Micropaleontology, 3(3), 279-294.
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  Abstract: Seventy-two core tops and grab samples from the western North Atlantic were analyzed to determine what aspects of planktonic foraminiferal abundance and diversity are most closely related to ocean circulation. Some species appear to be reliable indicators of the Gulf Stream, a warm surface current. Both Globorotalia menardii and Globigerinoides sacculifer have their highest abundances under the main trend of the Gulf Stream. Globorotalia inflata reaches high abundances in the cold slope water north of the Gulf Stream but its distribution is not as continuous as the Gulf Stream indicators. Contoured values of species diversity, the Shannon diversity index, and species equitability also reflect surface circulation. A plot of species diversity (number of species) shows a poorly defined region of high diversity beneath the major trend of the Gulf Stream. Use of the Shannon diversity index enhances and clarifies this region of high diversity. A map of species equitability shows a broad belt of low species dominance (high equitability) beneath the Gulf Stream. North of the Gulf Stream, a tongue of high dominance (low equitability) corresponds to the increased relative abundance of Globorotalia inflata. High diversity of planktonic foraminifera in bottom sediments characterizes the warm shifting surface currents of the Gulf Stream; low diversity is typical of slope and Sargasso Sea waters. Low equitability (high species dominance) indicates either cold currents or gyre center waters. Maps of foraminiferal diversity and equitability for other intervals of geologic time may be useful in tracing the evolution of ocean circulation. © 1978.
• Flessa, K. W., & Miyazaki, J. M. (1978). Trends in trans-North Atlantic commonality among Phanerozoic invertebrates, and plate tectonic events: Discussion. Bulletin of the Geological Society of America, 89(3), 476-477.
• Flessa, K. W., Constantine, K. J., & Cushman, M. K. (1977). Sedimentation rates in a coastal marsh determined from historical records. Chesapeake Science, 18(2), 172-176.
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  Abstract: Historical records indicate that Flax Pond, a small Spartina alterniflora marsh located on the north shore of Long Island, New York, was opened to marine waters in 1803. This opening is recorded in the sediments by a sharp transition from sedge (brackish or fresh water) peat to Spartina (salt marsh) peat. This dated horizon was used to calculate an average net rate of vertical accretion of 2.5 mm/yr and a maximum net rate of 4.7 mm/yr. These estimates may serve to predict the rate at which Spartina marshes are able to recover from physical disturbance. *** DIRECT SUPPORT *** A00BY066 00002 © 1977 Estuarine Research Federation.
• Flessa, K. W. (1976). Large scale extinctions. Nature, 264(5584), 383-.

Proceedings Publications

• Flessa, K. (2014, Fall). Water for nature in the Colorado River delta. In Global Water Forum.
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  In: Reflections on water reform in the Colorado and Murray-Darling Basins

Presentations

• Flessa, K. W., & Team, T. D. (2014, Oct). Restoration flows to the Colorado River Delta: Monitoring the effects of a large landscape experiment. National Workshop on Large Landscape Conservation. Wshington, DC: Lincoln Institute of Land Policy and many otjhers.
• Flessa, K. W., Kendy, E., Schlatter, K., & Team, T. D. (2014, DEC). The Science and Policy of the First Environmental Flows to the Colorado Delta. American Geophysical Union. San Francisco, CA: American Geophysical Union.

Poster Presentations

• Lester, R., Barton, J., & Flessa, K. (2014, Fall). Opening the flood gates: the compromises necessary to achieve environmental flow releases. Australian Society for Fish Biology & Australian Society for Limnology Congress.

Reviews

• Dietl, G. P., Kidwell, S. M., Brenner, M., Burney, D. A., Flessa, K. W., Jackson, S. T., Koch, P. L., Jeanloz, R., & Freeman, K. (2015. Conservation Paleobiology: Leveraging Knowledge of the Past to Inform Conservation and Restoration(pp 79-103).

Others

• Adler, R., Colby, B. G., Flessa, K. W., Kenney, D., Lettenmeier, D., Overpeck, J. T., Schmidt, J., Udall, B., & Waskom, R. (2017, June). Innovations in Agricultural Water Conservation and Use: Fertile Ground for Lasting Solutions. Colorado River Research Group.
• Flessa, K. W., Kendy, E., & Schlatter, K. (2016, December). Minute 319 Colorado River Limitrophe and Delta Environmental Flows Monitoring Interim Report.. International Boundary and water Commission. https://www.ibwc.gov/Files/Minutes%20319/2016_EFM_InterimReport_Min319.pdf
• Flessa, K. W., Kendy, E., Schlatter, K., & Team, T. D. (2014, DEC). Minute 319 Colorado River Delta Environmental Flows Monitoring Initial Progress report. International Boundary and water Commission. http://www.ibwc.gov/EMD/Min319Monitoring.pdf
• Flessa, K. W., Glenn, E. P., & Lopez Hoffman, L. -. (2012, June). Monitoring program for the Cienega de Santa Clara. A report to: Central Arizona Water Conservation District, Metropolitan Water District of Southern California, Southern Nevada Water Authority, Comisión Nacional de Áreas Naturales Protegidas & Instituto Nacional de Ecología.. Department of Geosciences, University of Arizona. http://www.geo.arizona.edu/cienega/?q=webfm_send/347
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  Complete author list:Flessa, K, J Campoy Favela, F Zamora, J García, E Glenn, O Hinojosa, J Ramírez, L Lopez-Hoffman, L Mexicano, Y Carrillo, L Cadena, R Brambila de Zamora, E Santiago, S Drake, MAS El Vilaly, K Hartfield & M Lomelí.