A New Locality Of Fossil Marine Vertebrates From The Basal Greenhorn Formation (Upper Cretaceous) In Southeastern Colorado, USA

The Lincoln Limestone Member of the Greenhorn Formation is a mid-Cenomanian sedimentary rock unit deposited under the Late Cretaceous Western Interior Seaway (WIS) in North America. Fossiliferous rock samples from the basal Lincoln Limestone, that formed sometime between 95 and 94.7 Ma, were obtained from a locality (‘Table Mesa locality’) in southeastern Colorado where its vertebrate contents have never been examined before. Vertebrate fossils were collected through acid treatment of the rock samples. Thirty marine vertebrate taxa are identified including chondrichthyan and osteichthyan fishes and reptilian taxa. The chondrichthyans consist of 14 taxa, including Ptychodus occidentalis, P. rhombodus, Cretoxyrhina mantelli, Aechaeolamna cf. A. kopingensis, Megachasma comanchensis, Microcorax crassus(?), Squalicorax curvatus, Rhinobatos cf. R. incertus, Rajidae incerate sedis, and Cretomanta canadensis. Osteichthyan fishes consist of 14 taxa, including Micropycnodon kansasensis, Pycnodontidae indet., Protosphyraena sp., Elopopsis sp., Pachyrhizodus minimus, Enchodus cf. E. gladiolus, Enchodus cf. E. shumardi, and three additional unidentified teleosts.  The reptilian remains are represented by two squamate taxa: an aquatic form Coniasaurus crassidens and a terrestrial form Scincomorpha indet. The fauna reported here is overall similar to other mid-Cenomanian marine vertebrate faunas from the Western Interior, supporting high taxonomic homogeneity in the WIS. The locality is thought to have been located at or close to the deepest part of the WIS during the mid-Cenomanian, and thus the occurrence of a terrestrial squamate is intriguing.  Likewise, the occurrence of the Late Cretaceous magamouth shark, Megachasma comanchensis, is intriguing because its fossil record is so far confined to southeastern Colorado.