Fossil vertebrates from a unique marine bonebed of the Upper Cretaceous Smoky Hill Chalk, western Kansas, U.S.A.: new insights into the paleoecology of the Niobrara Formation

ABSTRACT

The Smoky Hill Chalk Member of the Upper Cretaceous Niobrara Formation is a sedimentary deposit formed in the Western Interior Seaway of North America. Here, we examined a fossiliferous bonebed from an uncertain horizon within the Smoky Hill Chalk in Gove County, Kansas, U.S.A. Over 40,450 taxonomically identifiable specimens were collected, consisting of at least four chondrichthyans, 24 actinopterygians, and two tetrapods, including taxa such as Nanocorax, Squalicorax, Rhinobatos, pycnodontids, Lepisosteidae, Protosphyraena, Pachyrhizodus, ichthyodectiforms, plethodids, Albulidae, Apateodus, Cimolichthys, Enchodus, Acanthomorpha, Platecarpus, and Hesperornithiformes. The diversity of these vertebrates suggests the presence of a very complex ecosystem when the bonebed formed, which is determined to be mid- or late Santonian in age based on biostratigraphic, geographic, and geologic evidence. Based on published literature and this new study, at least 117 vertebrate taxa are now known from the Smoky Hill Chalk of Kansas, comprising 18 chondrichthyans, 63 actinopterygians, and 36 tetrapods. Comparisons of faunal data from the bonebed with databases on vertebrate fossils from the Niobrara Formation of Kansas in seven major museum collections indicate that the Niobrara fossil record based on museum collections is biased towards large reptilian remains compared with fish fossils. In particular, this study indicates that Enchodus spp. were especially common when the bonebed formed. A major corollary from this study is that a more accurate reconstruction of the paleoecology of the Western Interior Seaway requires the need for more analyses on smaller vertebrate taxa.

ACKNOWLEDGMENTS

The completion of this project could not have happened without many people and organizations’ help. The study was made possible by R. Zakrzewski (FHSM) who initially loaned FHSM VP-644 in the early 2000s to the second author (KS), as well as by L. Wilson and A. Baumgartner’s (FHSM) continued support and curation of specimens derived from FHSM VP-644. We would also like to thank A. Gishlick (AMNH), S. McLeod (LACM), A. Millhouse (USNM), V. Rhue (YPM), W. Simpson (FMNH), and M. Sims (KUVP) for providing a comprehensive list of Niobrara vertebrate specimens housed in their respective museum. Under KS’s supervision, the initial fossil picking stage was conducted by E. Beeson followed by the following former DePaul undergraduate students and local high school student volunteers: B. Armand, L. Elward, M. Gearon, L. Guerrero, A. Meglei, F. Ruiz, and S. Zavala. We also thank C. Fielitz (Emory & Henry College) and T. Konishi (University of Cincinnati) for help identifying some of the materials described in this study, M. Everhart for useful stratigraphic discussion, and DePaul University’s Department of Biological Sciences and the College of Science and Health for various financial support. Furthermore, we sincerely thank C. Schröder-Adams (Carleton University) and J. Craig (Alberta, Canada) for analyzing the nannofossil components of the bonebed. Comments and suggestions made by S. C. Bennett and J. Diaz-Cruz as well as the JVP editorial team greatly improved the quality of this paper. This present study represents the M.S. thesis project of the first author (JGA) under KS’s supervision, and JGA also thanks the other two members of his thesis committee, J. Bystriansky and T. Sparkes, as well as DePaul University’s Department of Biological Sciences and the College of Science and Health for various support.