Downslope Fossil Contamination: Mammal-Bearing Fluvial Conglomerates and the Paleocene–Eocene Faunal Transition (Willwood Formation, Bighorn Basin, Wyoming)

Abstract Downslope fossil contamination is the result of erosion and subsequent redeposition of fossil material onto lower stratigraphic horizons. This produces time-averaged and potentially anomalous faunal records. Here, we describe vertebrate concentrations in Bighorn Basin (Wyoming) conglomerates that are early Wasatchian (earliest Eocene) in age (Wa-1) and rest erosionally upon Wa-0 strata deposited during the Paleocene–Eocene thermal maximum (PETM). The Wa-1 conglomerates were deposited during river channel migration and sheetflooding onto abandoned parts of an avulsion belt immediately after the PETM. Dark-colored Wa-1 fossil teeth eroding from the conglomerates are now mixed in places with the lighter-colored teeth of Wa-0 mammals. The spatial distribution of the conglomerate fossils and a vertical model of downslope contamination, based on species proportions from overlying stratigraphic intervals, are used to calculate an expected contribution of fossil contaminants to an assemblage. Results of this model are applied to address the principal weakness of the hypothesis that transient decreases in mammalian body size during the PETM were an evolutionary dwarfing response to climate change. Rare occurrences of large taxa with congeners of small size would refute the argument for evolutionary dwarfing, but our results indicate that such rare occurrences can be explained by downslope contamination alone. We conclude more generally that alluvial architecture is important for understanding the potential for downslope fossil contamination and that complications imposed by this type of contamination can be assessed quantitatively.

[1]  M. Kraus,et al.  Recognizing Avulsion Deposits in the Ancient Stratigraphical Record , 2009 .

[2]  S. Hasiotis,et al.  Significance of Different Modes of Rhizolith Preservation to Interpreting Paleoenvironmental and Paleohydrologic Settings: Examples from Paleogene Paleosols, Bighorn Basin, Wyoming, U.S.A. , 2006 .

[3]  J. Barry,et al.  BIOSTRATIGRAPHIC SURVEYS IN THE SIWALIKS OF PAKISTAN: A METHOD FOR STANDARDIZED SURFACE SAMPLING OF THE VERTEBRATE FOSSIL RECORD , 2005 .

[4]  M. Kraus,et al.  Mudrock‐dominated fills formed in avulsion splay channels: examples from the Willwood Formation, Wyoming , 2004 .

[5]  James G. Ogg,et al.  A Geologic Time Scale 2004: CONCEPTS AND METHODS , 2005 .

[6]  J. Zachos,et al.  Carbon and oxygen isotope records from Paleosols spanning the Paleocene-Eocene boundary, Bighorn Basin, Wyoming , 2003 .

[7]  P. Gingerich.  Mammalian responses to climate change at the Paleocene-Eocene boundary: Polecat Bench record in the northern Bighorn Basin, Wyoming , 2003 .

[8]  Philip D. Gingerich,et al.  Causes and consequences of globally warm climates in the early Paleogene , 2003 .

[9]  D. Kelly,et al.  Response of Antarctic (ODP Site 690) planktonic foraminifera to the Paleocene–Eocene thermal maximum: Faunal evidence for ocean/climate change , 2002 .

[10]  Catherine Badgley,et al.  Faunal and environmental change in the late Miocene Siwaliks of northern Pakistan , 2002, Paleobiology.

[11]  S. Kemp,et al.  US Gulf Coast vegetation dynamics during the latest Palaeocene , 2001 .

[12]  P. Gingerich.  Paleocene-Eocene Stratigraphy and Biotic Change in the Bighorn and Clarks Fork Basins, Wyoming , 2001 .

[13]  N. Smith,et al.  Fluvial sedimentology VI , 1999 .

[14]  P. Gingerich,et al.  MAMMALIAN COMMUNITY RESPONSE TO THE LATEST PALEOCENE THERMAL MAXIMUM : AN ISOTAPHONOMIC STUDY IN THE NORTHERN BIGHORN BASIN, WYOMING , 1998 .

[15]  J. Ozouf,et al.  TRANSIT, An Experimental Archaeological Program in Periglacial Environment: Problem, Methodology, First Results , 1998 .

[16]  L. Jacobs,et al.  Advances in vertebrate paleontology and geochronology , 1998 .

[17]  A. Fiorillo.  MEASURING FOSSIL REWORKING WITHIN A FLUVIAL SYSTEM : AN EXAMPLE FROM THE HELL CREEK FORMATION (UPPER CRETACEOUS) OF EASTERN MONTANA , 1998 .

[18]  N. Shackleton,et al.  Benthic foraminifera and stable isotope composition in Paleocene-Eocene sediments , 1996 .

[19]  M. Kraus.  Avulsion Deposits in Lower Eocene Alluvial Rocks, Bighorn Basin, Wyoming , 1996 .

[20]  B. Edited,et al.  Correlation of the early Paleogene in northwest Europe: an overview , 1996, Geological Society, London, Special Publications.

[21]  N. Shackleton,et al.  The Paleocene-Eocene benthic foraminiferal extinction and stable isotope anomalies , 1996, Geological Society, London, Special Publications.

[22]  M. Kraus,et al.  Eocene Hydromorphic Paleosols: Significance for Interpreting Ancient Floodplain Processes , 1993 .

[23]  J. Zachos,et al.  Abrupt Climate Change and Transient Climates during the Paleogene: A Marine Perspective , 1993, The Journal of Geology.

[24]  A. Runkel,et al.  Reworking of Cretaceous dinosaurs into Paleocene channel, deposits, upper Hell Creek Formation, Montana , 1990 .

[25]  P. Gingerich.  New Earliest Wasatchian Mammalian Fauna from the Eocene of Northwestern Wyoming: Composition and Diversity in a Rarely Sampled High-Floodplain Assemblage , 1989 .

[26]  C. Badgley.  Taphonomy of mammalian fossil remains from Siwalik rocks of Pakistan , 1986, Paleobiology.

[27]  A. Behrensmeyer.  Time resolution in fluvial vertebrate assemblages , 1982, Paleobiology.

[28]  M. Kraus,et al.  Vertebrate fossil-bearing paleosol units (Willwood Formation, Lower Eocene, Northwest Wyoming, U.S.A.): Implications for taphonomy, biostratigraphy, and assemblage analysis , 1981 .

[29]  K. Rose.  The Clarkforkian Land-Mammal Age and Mammalian Faunal Composition Across the Paleocene-Eocene Boundary , 1981 .

[30]  P. Gingerich.  Early Cenozoic Paleontology and Stratigraphy of the Bighorn Basin, Wyoming , 1980 .

[31]  John W. Rick,et al.  Downslope Movement and Archaeological Intrasite Spatial Analysis , 1976, American Antiquity.