Reassessment of the Early Triassic trematosaurid temnospondyl Tertrema acuta from the Arctic island of Spitsbergen

ABSTRACT

Trematosaurids were globally distributed Early Triassic temnospondyl amphibians characterized by elongate ‘crocodile-like’ skulls. Some of the most famous trematosaurid fossils were discovered on the island of Spitsbergen in the Arctic Svalbard archipelago. Among these, the short-snouted trematosaurine, Tertrema acuta, is one of the few taxa represented by virtually complete cranial remains. Unusually, however, the type specimens comprise only natural molds that were historically used to reconstruct three-dimensional casts. Here, we re-assess these restorations using the original impressions to phylogenetically analyze and re-diagnose the taxon. Unexpectedly, our first-hand scores differ markedly from previous literature-sourced interpretations and yield conflicting tree topologies that nest T. acuta with long-snouted lonchorhynchines, thus destabilizing the long-favored sub-division of trematosaurids based on their skull shape. We attribute this result to character state conflict and suggest that the traditional classification of trematosaurids may mask more complex evolutionary relationships, as well as possible trophic partitioning, and eco-morphological plasticity.

ACKNOWLEDGMENTS

This paper derived from an unpublished Master thesis by MS, supervised by BPK and TM. We thank P. Eriksson (Museum of Evolution) for fossil conservation and casting. C. Mays and T. Denk (Swedish Museum of Natural History) contributed technical discussions. The Editors, T. Sulej (Polish Academy of Sciences), and a second anonymous reviewer provided constructive comments on our manuscript. We thank ERASMUS+ (European Community Action Scheme for the Mobility of University Students) and the Stockholm University for financial support to MS. BPK acknowledges funding from a Swedish Research Council Project Grant (2020-3423).