Newly Recognized Specimens from the White River Formation (Eocene/Oligocene) Represent Wyoming's Last Crocodyliforms
FARKE, Andrew A.; Raymond M. Alf Museum of Paleontology at The Webb Schools, Claremont, CA. CHAI, Benjamin; The Webb Schools, Claremont, CA. GERARD, Isabel; The Webb Schools, Claremont, CA. GRAF, Peter; The Webb Schools, Claremont, CA. HUEN, Alex Chun Ting; The Webb Schools, Claremont, CA. ZHANG, Shuwen; The Webb Schools, Claremont, CA.
Despite their current absence, crocodyliforms have a deep geological history in the Great Plains region of North America. The ebb and flow of their distribution is an important indicator of long-term environmental change and biogeographic corridors, and even fragmentary material provides crucial data points. The White River sequence of Nebraska, Wyoming, North Dakota, South Dakota, and Colorado preserves an exquisite faunal assemblage spanning the Eocene/Oligocene boundary. Among these fossils, the oldest definitive Alligator (A. prenasalis) is represented by numerous individuals from the Chadron and Brule formations in South Dakota and probably also Nebraska. In contrast, crocodyliform material from the White River Formation of Wyoming is extremely poorly known. Isolated elements have been mentioned in the literature, but not described, from the Yoder Member of the White River Formation in Goshen County (housed at SDSM, Museum of Geology, South Dakota School of Mines & Technology, and MCZ, Museum of Comparative Zoology, Harvard University). Spurred by the recognition of additional material in the collections of the Raymond M. Alf Museum of Paleontology (RAM), we surveyed all known crocodyliform fossils from the White River Formation of Wyoming.
Most material originates in the Yoder Member, representing isolated dorsal osteoderms (SDSM 6346, a single osteoderm; SDSM 53298, three unassociated osteoderms). Vertebrae reported by Schlaikjer cannot be located at the MCZ. Based on biostratigraphy, the Yoder fossils are probably early or mid-Chadronian in age (~35 Ma). A scrappy but associated specimen (RAM 24130) was collected within the Brule Member of the White River Formation in Niobrara County, Wyoming. It includes teeth with roots, cranial bone fragments, a vertebral fragment, and osteoderms. Teeth range in shape from conical to globidont, and the size of all of the elements is consistent with a small individual (<1.5 m body length). Unfortunately, none of the material includes synapomorphies that constrain the fossils beyond Crocodyliformes, although they are morphologically consistent with Alligator prenasalis material from South Dakota. The locality hosting RAM 24130 is likely early Orellan (~33.9 Ma), and thus represents the youngest known crocodyliform from the state of Wyoming. Compared with time-equivalent beds in South Dakota and Nebraska, crocodyliform material is vanishingly rare in Wyoming, which may reflect regional environmental differences.