Morphological and histological description of small metoposaurids from Petrified Forest National Park, AZ, USA and the taxonomy of Apachesaurus

ABSTRACT

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, relatively mature individuals, and as a result, the early stages of ontogeny are still poorly characterised. Small-bodied metoposaurids from North America have traditionally been assigned to Apachesaurus gregorii, interpreted as a diminutive taxon, but this interpretation has not been rigorously tested. Here we provide a morphological description of two new small-bodied metoposaurid specimens from 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 perform a histological analysis on associated intercentra of these specimens, which indicates that these are relatively immature individuals. 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.

KEYWORDS:

• Metoposauridae
• Temnospondyli
• Triassic
• ontogeny
• histology

Supplemental meterial

Supplemental data for this article can be accessed here.

Acknowledgments

We would like to thank Matt Smith (PEFO) for granting collections access and for facilitating the specimen loan and Brad Traver (PEFO) for granting the specimen loan and permission for the destructive sample analysis. Thanks to Matt Brown (UT Austin) for moulding and casting of the sampled intercentra. Adam Marsh (PEFO) assisted with fieldwork. We thank Michelle Stocker and Dorota Konietzko-Meier and the editor, Gareth Dyke, for constructive reviews that improved the manuscript. This is Petrified Forest National Park Paleontological Contribution No. 57. Any opinions, findings, or conclusions of this study represent the views of the authors and not those of the U.S. Federal Government.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by the Petrified Forest Museum Association.