HISTOLOGY AND MORPHOLOGY OF SMALL-BODIED METOPOSAURIDS AND THE TAXONOMY OF APACHESAURUS GREGORII

Metoposaurids are Late Triassic temnospondyls that are abundant components of freshwater depositional settings. Although metoposaurids are represented by hundreds of specimens in collections around the world, the vast majority pertain to large-bodied, presumably relatively mature individuals, and as a result, the early stages of ontogeny are still poorly characterized. Small-bodied metoposaurids from North America have traditionally been assigned to Apachesaurus gregorii, interpreted as a dwarf taxon, but this interpretation has not been rigorously tested. Here we provide a morphological description of two new small metoposaurids from the late Norian of Petrified Forest National Park, AZ, USA. Both provide various anatomical details that improve our understanding of small-bodied metoposaurids and their taxonomic placement within Metoposauridae. Furthermore, we performed a histological analysis on associated intercentra of these specimens, which indicates that these are relatively immature individuals, including one individual that is indistinguishable from the holotype of Apachesaurus. These findings support the growing consensus that Apachesaurus is a juvenile metoposaurid, thereby providing additional data regarding the early stages of metoposaurid ontogeny and evidence of the persistence of large-bodied forms into the late Norian. Accordingly, these findings merit a reevaluation of the taxonomic validity and diagnosis of the taxon and of the previous interpretations of its paleobiology.