STATE OF THE CLEITHRUM IN EARLY PERMIAN NORTH AMERICAN TETRAPODS

The cleithrum is a major pectoral bone in fish. As tetrapods evolved to live on land, the cleithrum was reduced and lost in most lineages. Each tetrapod lineage lost their cleithra independently. Today, anurans are the only tetrapods to have a cleithrum. During the Cisuralian Epoch (Early Permian), the cleithrum was present as an independent bone in most lineages. There has not been a concerted effort to describe the cleithrum in pelycosaurs since Romer and Price (1940). Little has been done to delineate the differences in the shape of the cleithrum among Early Permian synapsids. This lack of study is primarily due to a paucity of material. The cleithrum in pelycosaurs is not articulated with the rest of the pectoral girdle, and it strongly resembles a cervical rib. This presents the challenge of both finding and correctly identifying this bone. In comparison, amphibian cleithra from the same time period, such as those found in the genus Eryops, are much larger and more distinct from other skeletal elements. Due to the evolutionarily transitional nature of this bone, this study attempts to understand the state of the cleithrum in the lower Permian. The Craddock Bonebed in Baylor County, Texas, is a well-known Early Permian locality where multiple articulated vertebrate skeletons have been recorded. The bonebed is part of the Clear Fork Group’s Arroyo Formation, and the site is representative of an oxbow lake environment. Samples from various taxa (such as the genera Dimetrodon, Edaphosaurus, and Eryops) have been photographed and illustrated to showcase diagnostic features of the bones. Formal descriptions have been generated for the bones, and comparisons have been made across taxa.