Rapid Ossification Helps Explain Small Body Size in Urspelerpes brucei (Caudata: Plethodontidae)

Urspelerpes brucei (Patch-nosed Salamander) is a miniaturized lungless salamander (family Plethodontidae) native to select headwater tributaries of the Tugaloo River along the border of Georgia and South Carolina. Due to the rarity and relatively recent discovery of this species, many aspects of its biology are unstudied. In particular, there has not yet been any description of its larval skeleton. We provide the first description of larval osteology for U. brucei, and we compared cleared and stained specimens of U. brucei close to the minimum size at metamorphosis to similarly sized specimens of Eurycea cirrigera (Southern Two-lined Salamander). Our results showed that the skull and long bones of U. brucei are more heavily ossified at smaller sizes than in E. cirrigera. In addition, U. brucei possesses more robust vomers and pterygoids, coronoids that are already incorporated into the dentary, and an ossified os thyroideum. These discrepancies in ossification suggest that the skeletons of U. brucei complete development at smaller sizes than E. cirrigera, a result likely tied to heterochronic accelerations of mineralization and sexual maturation in U. brucei. Taken together, these factors appear to help explain how U. brucei has developed and maintained its small body size.