The interglenoid tubercle of the atlas is ancestral to lissamphibians

Lissamphibians, represented today by frogs, salamanders, and caecilians, diverged deep in the tetrapod tree of life. Extensive morphological adaptations to disparate lifestyles have made linking extant lissamphibians to one another and to their extinct relatives difficult and controversial. However, the discovery of a feature on the atlas of the frog Xenopus laevis, may add to the small set of osteological traits that unite lissamphibians. In this study, we combine our observations of atlas development in X. laevis with a deep examination of atlantal interglenoid tubercle (TI) occurrence in fossil taxa. The TI is shown herein to occur transiently on the ossifying atlas of roughly one-third of X. laevis tadpoles but is absent in adults of this species. In ancestral character state estimations (ACSE), within the evolutionary context of lissamphibians as dissorophoid temnospondyls, this feature is found to be ancestrally shared among lissamphibians, its presence is uncertain in stem batrachians, and then the TI is lost in extant caecilians and frogs. However, our data suggests apparent TI loss around the origin of frogs may be explained by its ontogenetically transient nature. The only nonamphibian tetrapods with a TI are “microsaurs,” and this similarity is interpreted as one of many convergences that resulted from convergent evolutionary processes that occurred in the evolution of “microsaurs” and lissamphibians. The TI is thus interpreted to be ancestral to lissamphibians as it is found to be present in some form throughout each extant lissamphibian clade's history.