I. The lower Carboniferous microsaurs

Abstract

Redescriptions of Adelogyrinus simnorhynchus and Dolichopareias disjectus are given. The skulls of these forms are characterized by the very anteriorly placed small orbits and long post-orbital skull table. Further diagnostic characters are the exclusion of the ‘post-orbital’ from the orbit, the long straight suture between the squamosal and parietal and the absence of anotic notch. A parasternal process is present on the interclavicle. A new genus and species Palaeomolgophus scoticus of early Carboniferous age is described. It is an aquatic form with branchial arches, yet the characters it displays are essentially reptiliomorph, quite unlike those of the labyrinthodonts and resembling more those of lower Permian reptiles. It has microsaur-type vertebrae in which the centrum is a pleurocentrum, winged ribs, a differentiated series of cervical ribs and an interclavicle with a posterior parasternal process. The limbs are small, but well developed, with ossified condylar surfaces. Comparisons are made with Microbrachis from the Upper Carboniferous of Nyran, the type microsaur, which is represented mainly by larval forms, but the disappearance of the lateral line canals in larger skulls indicates a metamorphosis. Microbrachis is more primitive than Palaeomolgophis in that small pre-sacral as well as post-sacral intercentra are present. Evidence is given which leaves little doubt that in microsaurs generally the vertebrae are of apsidospondylus type and the centrum is a pleurocentrum. They are not lepospondylous and the validity and usefulness of the term ‘Lepospondyli’ is questioned. The evidence presented here supports the view that there was a deep and early split in tetrapods separating the labyrinthodonts (batrachomorphs) from the reptiliomorph types, (the Lower Carboniferous microsaurs, microbrachids, gymnarthrids, seymouriomorphs, etc.). All display more or less a series of structures never found in any labyrinthodont and which indicate either a very early divergence from the labyrinthodont stock or a separate origin from fishes. The emergence of essentially reptilian characters in Palaeomolgophis , an apparently aquatic form of Lower Carboniferous age contradicts the assumption that these characters arose as adaptations to land life and indicates that the first move toward the reptilian condition was structural and that it was only at a later date that the life-history was modified and that terrestrial tetrapods, reptiles in the full sense, arose.