Vertebral Development in the Oldest Microsaur and the Problem of “Lepospondyl” Relationships

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Abstract
A newly discovered specimen of the oldest known microsaur, Utaherpeton, shows evidence that its caudal vertebrae may have developed via paired pleurocentral elements. Comparison of other aspects of the vertebrae of other lepospondyls demonstrates that few derived features are shared by all the major groups. Vertebral structure does not provide strong support for the monophyly of lepospondyls. Both structural and functional relationships of the articulation between the skull and the atlas distinguish aïstopods sharply from other lepospondyls. There is little evidence to support sister-group relationships between any of the lepospondyl orders and any of the known labyrinthodont groups. All of the lepospondyl orders may have diverged prior to the appearance of any of the Carboniferous labyrinthodont lineages. Many of the similar derived features of lepospondyls may be attributed to small size, rather than common ancestry.