Turtle Diversity and Abundance through the Lower Eocene Willwood Formation of the Southern Bighorn Basin

The early Eocene is a period of significant change in the composition of North American turtle faunas. Over the first 2.2 million years of the Eocene, the fossil record documents several immigrations into depositional basins of the Western Interior and an expansion in the ecological niches of turtles toward herbivory. Focusing on the Willwood Formation, we document change in generic and species richness in greater detail than previously available and place this in a better-constrained temporal framework, confirming that the two major episodes of change in the turtle faunas coincide with the appearance of Wasatchian mammals and with an interval of climate change at biohorizon B-C within the Wasatchian land-mammal age. We report changes in relative abundances of turtles and discuss possible explanations for these changes. Climate change (at least as estimated by proxies for mean annual rainfall or temperature) does not provide an adequate explanation for the observed changes. Rather, we think that changes in habitat heterogeneity may be a better explanation and that changes in the turtle fauna can best be understood by examining how change in climate, in combination with effects of local tectonic controls on floodplain development, affected the availability and preservation of diverse habitats. INTRODUCTION the only record of faunal change that can be correlated with The mammalian portion of the terrestrial early Eocene vertebrate fauna has attracted much attention, but an understanding of the herpetofauna (amphibians and reptiles) is essential for evaluating and testing scenarios of coevolution of early Eocene climates and biotas. Testudines are the best represented order of the ancient herpetofauna. Turtles are preserved in great numbers in a variety of lithologies and depositional settings, and even isolated elements of the shell are readily recognizable and identifiable. The Bighorn Basin record of turtles studied here (Fig. 1), is significant in being one of the longest (approximately 2.2 million years) and most complete records of a local fauna in the Western Interior and because it is currently similarly detailed studies of local changes in other aspects of the fauna (e. g., Gunnell, 1998; Hartman and Roth, 1998), climate (Wing et al., 2000) and hydrology (Bown and Kraus, 1993). The Bighorn Basin testudine record was originally and preliminarily summarized by Hutchison (1980), who first recognized this major reorganization of the ecologic and taxonomic composition of turtle fauna. Subsequently, Hutchison (1982, 1992) explored the relationship between reptile diversity and climate change through the Paleogene, identifying the linked roles that climate and hydrology have in affecting changes in diversity and herpetofaunal composition. Systematic work on Bighorn Basin turtles in the last twenty years is somewhat more limited. Gingerich (1989) reported the earliest records of the In: Paleocene-Eocene Stratigraphy and Biotic Change in the Bighorn immigrant Echmatemys from Wa-0 localities in the Clarks Fork and Clarks Fork Basins, Wyoming (P. D. Gingerich, ed.), University of Basin. Hutchison described the (mud Michigan Papers on Paleontology, 33: 97-107 (2001). turtles) of the Bighorn Basin. Hutchison (1998) summarized