THE VERTEBRATE FAUNA FROM THE LOWER CRETACEOUS HOLLY CREEK FORMATION. THE FIRST MULTI-TAXA VERTEBRATE ASSEMBLAGE FROM THE MESOZOIC OF ARKANSAS, USA

In the late 1980’s an assemblage of vertebrate material was discovered at the Briar Site Gypsum Mine, the site of extensive sauropod and theropod tracks from the overlying De Queen Limestone outside of Dierks, Arkansas. The assemblage was discovered in an organic-rich, pyrite-bearing dark grey mudstone. Many of the large vertebrate material contain abundant “pyrite-disease,” and, in combination with the lithology, the assemblage is interpreted as having been deposited in a lagoonal environment tied to marine facies. Within the assemblage, a diverse fauna of micro and macrovertebrates were discovered. We provide a preliminary description of the assemblage. Of the macrovertebrate remains, elements from a very large theropod (likely Acrocanthosaurus atokensis), a large nodosaur-type ankylosaur, a large ceolognathosuchian crocodile, a very large cryptodiran turtle, smaller solemydid turtles, and a large titanisauriform sauropod are identified. The microvertebrate assemblage is dominated by aquatic and semi-aquatic taxa, with an abundance of osteichthyans (Pycnodontiformes, Semionotidae, Amiidae, and Teleostei) and chondrichthyans (Lamiformes and Hybodontidae). Semi-aquatic taxa contain neosuchian crocodylomorphs (Bernissartiidae and Coelognathosuchia, which include Goniopholididae and Pholidosauridae), a solemydid turtle (Naomichelys sp.), and Lissamphibia (Albanerpeton sp.). Terrestrial taxa include indeterminate scincomorphan squamates, dinosaurs (Deinonychus antirrhopus and Richardoestesia sp.), and an avian ungual. The assemblage is consistent with most of the sparse material documented from Arkansas, namely tracks, but lacks any identifiable material attributable to an ornithomimid, “Arkansaurus,” found in the same region. Combined, the macrovertebrate and microvertebrate assemblage represents faunas identified from the Trinity Group of Texas and the Antlers Formation of Oklahoma and Texas, but also contains taxa identified from Laramidia (Cloverly and Cedar Mountain formations) and Appalachia (Arundel Formation). The assemblage supports the presence of a southern passageway for faunas between the West and East that lasted for only a few more million years before being isolated from each other as the Western Interior Seaway inundated North America.