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

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.