Specimen-level phylogenetic analysis and taxonomic revision of North American metoposaurids: a case for monospecific genera in vertebrate paleontology.

The Metoposauridae were a widespread family of aquatic Late Triassic temnospondyl amphibians known from numerous localities in non-marine strata of the Northern Hemisphere, India, and Madagascar. Recent descriptions of intraspecific variation in skulls from hyper accumulations suggest some characters used in species diagnoses may be variable within hypothesized populations. This realization necessitates the reevaluation of taxonomic diagnoses for previously established species. Multiple monospecific hyper accumulations of different metoposaurid taxa are known in North America providing a relatively large sample to assess the validity of the North American taxa: Anaschisma browni, Buettnererpeton bakeri, and Apachesaurus gregorii. The purported 'small-bodied' metoposaurid A. gregorii has recently been suggested to be predominantly represented by juvenile material; whether this material represents a juvenile of a previously described large-bodied metoposaurid or is indeed a distinct taxon has not been tested. A specimen-level character matrix incorporating scores from all name-bearing specimens diagnostic to the species level as well as numerous referred specimens was constructed, and the interpretations are validated by the inclusion of photographs and interpretive drawings (where necessary for clarity) of specimens included in the matrix. The synonymy of specimens from Texas, New Mexico, and Arizona referred to An. browni was not supported, and to avoid exacerbating taxonomic instability of the Metoposauridae, two novel genera are proposed to encompass two previously described species from Texas (Paranaschisma gen. nov. howardensis and Tecovaserpeton gen. nov. perfectum). A putative adult specimen of A. gregorii was identified, and ontogenetic variation in B. bakeri and A. gregorii was compared to propose evolutionary scenarios related to heterochronic shifts in metoposaurids that could be tested in future studies. Reconciling outstanding challenges with the assessment of ontogenetic variation in metoposaurids will rely on the recovery, description, and phylogenetic analysis of specimens that occupy the size gap between the large-bodied and small-bodied Late Triassic metoposaurids in the western USA in this specimen-level framework.