A TEMNOSPONDYL AMPHIBIAN FROM THE MISSISSIPPIAN OF SCOTLAND ROBERT HOLMES and ROBERT CARROLL 1 Abstract. The edopoid labyrinthodont, Caeror-hachis bairdi (new genus and species), a specimen from Thomas Stock's collection and therefore probably from the locality of Loanhead, Midlothian, Scotland (Namurian A), has a skull structure that suggests affinities with the family Dendrerpetontidae. It has 31 presacral vertebrae, and limb proportions suggesting semiterrestrial hab-its, in contrast to the necessarily aquatic nature of most other known Mississippian labyrinthodonts. The vertebral structure is not of the rhachitomous type, but resembles the "protoreptilian" structure normally associated with primitive anthracosaurs. The vertebral anatomy of this animal is not con-sistent with the pattern expected according to Romer's theory of the evolution of the labyrintho-dont centrum. Evidence from Caerorhachis and rhipidistian fish suggests that the specific pattern of die vertebral centra among Mississippian am-phibians may be quite variable, and may not in itself be an adequate criterion for establishing the phylogenetic relationships of primitive tetrapods. INTRODUCTION During the late Paleozoic, amphibians were the dominant terrestrial vertebrates. The most common of the Paleozoic amphib-ians were the labyrinthodonts, so called be-cause the dentine of their teeth was infolded in the form of a labyrinth. A more impor-tant characteristic of the group is that the centra of their vertebrae were formed from more than a single ossification per segment. The pleurocentrum, comparable to the de-finitive centrum of amniotes, forms the pos-terior part of the vertebral centrum and usually supports the neural arch. The inter-centrum, a relatively insignificant ventral element in the vertebrae of lower amniotes and absent as a discrete element in the pre-sacral region of mammals, was an important structural element in the vertebrae of nearly all labyrinthodonts. The intercentrum forms the anterior part of the vertebra and is ex-pressed as the haemal arch in the caudal re-gion. 1 Redpath Museum, McGill University, Montreal, Quebec, Canada. The earliest labyrinthodonts, the ichthy-ostegids, are known from the latest Devo-nian. Remains of other labyrinthodonts are common in Pennsylvanian and Permian de-posits, but the group is represented by less than a dozen genera in the fossil record of the Mississippian. Consequently, our con-cept of the early evolution of the group is based largely on evidence provided by the study of the Pennsylvanian and Permian forms. Among the few labyrinthodonts from the Mississippian is a nearly complete skeleton of a previously undescribed form in the col-lection of the Museum of Comparative Zool-ogy at Harvard University. The description of any Mississippian amphibian is of con-siderable importance, but the structure of the vertebrae of this specimen makes it of particular significance in understanding the phytogeny of Paleozoic labyrinthodonts. ACKNOWLEDGEMENTS We wish to thank the late Dr. Alfred S. Romer for allowing us to examine and make casts of the specimen of Caerorlwchis. An interpretation of the structure of this animal Bull. Mus. Comp. Zool, 147(12): 489-511, April, 1977 489
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