A NEW COLOSTEID AMPHIBIAN FROM HANCOCK COUNTY, KENTUCKY

The Hancock County tetrapod site has yielded fossilized remains of a diverse assemblage of Mississippian fish and tetrapods. Three distinct facies have yielded vertebrate remains with a dark gray shale of the Buffalo Wallow Formation (Chesterian) producing the most diverse fauna including: dipnoans, palaeoniscoids, rhizodonts, xenacanths, and a colosteid amphibian. The colosteid specimen represents a previously undescribed taxon. The specimen is a nearly complete individual preserving the anterior two-thirds of the skull and the entire post-crania excluding the distal half of the caudal series. It is identified as a colosteid by the following characters: dentary teeth larger than maxillary teeth, triradiate parasphenoid, rhachitomous vertebrae, uniradiate ilium, and deeply pitted ornamentation on the skull. The Hancock specimen can be distinguished from other colosteids by its possession of thick osteodermal squamation that covers the dorsal surface of the specimen. The squamation begins slightly anterior to the pectoral girdle and continues posteriorly to the pelvic girdle. The rectangular osteoderms are arranged in rows of 5-7 individual elements angling posterolaterally about 55º to the axis of the vertebral column. Individual osteoderms are arranged with their long axis oriented with the axis of each row. Dorsal squamation is seen among some advanced temnospondyls and Permian chroniosuchids. Among these more advanced tetrapods the osteoderms likely served to strengthen the vertebral column as these taxa were highly terrestrial and the osteoderms are closely aligned with, or articulate to, the neural spines. The relatively small size and mail-like arrangement of the squamation in the Hancock colosteid, along with its likely aquatic habits, argue against such a function for this squamation. The presence of numerous large (5+ meter) rhizodonts within the ox-bow deposit suggests that this squamation may have been a defensive adaptation. Mississippian colosteids without thick squamation are not found associated with predators larger than themselves and were likely apex predators in their respective ecosystems.