Intervertebral joint polarity reversions in extant and extinct crocodylians

Abstract

AbstractProcoely in presacral and caudal vertebrae except the biconvex first caudal vertebra is regarded as a key diagnostic feature of eusuchian crocodylomorphs, although its individual variation, which may have an impact on the evolutionary and functional interpretations, remains unexplored. Here, we examined 19 extant species as well as a few fossil species of crocodylians to identify individual (intraspecific) variation in the intervertebral joint polarities. We found joint alteration in eight individuals from seven extant species and three individuals from two extinct species. Most variant specimens show sacrocaudal joint alteration, while single Osteolaemus tetraspis peculiarly displays multiple joint polarity reversions (cotyle to condyle and vice versa) in anterior caudal vertebrae. Because the sacrocaudal joint polarity is frequently reversed and the first caudal vertebra becomes procoelous, caution should be taken when identifying an isolated first caudal vertebra of basal and outgroups of eusuchians. Almost exclusive occurrences of joint alteration at the less mobile sacrocaudal joint in crocodylians are consistent with the hypothesis that the joint alteration takes place at joints with a lesser range of motion.