Marine vertebrates from the Hartland Shale (Upper Cretaceous: Upper Cenomanian) in southeastern Colorado, USA

The Hartland Shale Member of the Greenhorn Limestone was deposited in the middle of the Late Cretaceous Western Interior Seaway of North America. Rock samples rich in micro-vertebrate fossils were collected from the lower part of the Hartland Shale (ca. 94.6 Ma: early Late Cenomanian) in southeastern Colorado, USA. Through acid treatment of the rock samples, 25 marine vertebrate taxa are identified including chondrichthyans, osteichthyans, and a reptile. Chondrichthyans are represented by seven species: Ptychodus anonymus, Squalicorax curvatus, Carcharias saskatchewanensis, Archaeolamna kopingensis, Cretoxyrhina mantelli, Cretomanta canadensis, and Rhinobatos incertus. Osteichthyan fishes consist of 17 taxa: Micropycnodon kansasensis, cf. Palaeobalistum sp., Caturidae indet., Protosphyraena sp., Plethodidae indet., Elopopsis sp., Pachyrhizodus minimus, cf. Pachyrhizodus sp., Albulidae indet., Cimolichthys nepaholica, Enchodus cf. E. gladiolus, E. cf. E. shumardi, Apateodus sp., and four unidentified teleosts. The only reptilian recognized is the small aquatic lizard Coniasaurus crassidens (Dolichosauridae). The taxonomic composition of the Hartland Shale fauna is similar overall to the extensively sampled, underlying mid-Cenomanian Lincoln Limestone fauna in Colorado and Kansas, although the occurrence of Apateodus and Cimolichthys from the Hartland Shale is notable as they represent geologically the oldest record for the two genera. The vertebrates identified are mostly carnivores that include piscivorous and durophagous forms, providing new insights into the trophic structure of the palaeocommunity.