DESCRIPTION OF MATERIAL FROM A POTENTIAL NEW SPECIES OF METOPOSAUR FROM THE PETRIFIED FOREST NATIONAL PARK, AZ, USA

Metoposaurids are late Triassic temnospondyl amphibians characterized by broad, flat skulls, laterally placed orbits in the rostral half of the skull, cylindrical intercentra, and heavy ossification of the skeleton. Two species of metoposaur are commonly recovered from the Chinle Formation of the Petrified Forest National Park (PEFO): the larger Koskinonodon perfectus and the smaller Apachesaurus gregorii. Material of large metoposaurs is more abundant in the lower units of the Chinle (late Carnian), while material of smaller ones is more abundant in the upper units (early Norian). This is proposed to be associated with the hypothesized Adamanian-Revueltian faunal turnover, which is thought to be the result of regional desiccation and reduction of aquatic habitats.

In October 2013, a team from the Natural History Museum of Los Angeles County (LACM) recovered a partial metoposaurid skull from the Lot’s Wife Beds (lower Sonsela Member) of the Chinle Formation exposure within the park. This stratum underlies the Jim Camp Wash Beds, in which the Adamanian-Revueltian boundary occurs. The specimen is the only metoposaurid skull known from the Lot’s Wife Beds, which are relatively sparse in vertebrate remains compared to other strata, and the majority of large metoposaurid specimens in the park come from lower strata, primarily the Blue Mesa Member. The specimen contains well-preserved dentition, as well as the honeycomb ornamentation on the dorsal portion of the skull that characterizes temnospondyls. Most of the original sutures are unidentifiable because of post-mortem weathering and slight displacement of some elements along their sutures. The specimen features several traits characteristic of K. perfectus: a large skull, a well-developed tabular horn, a deep otic notch, and a lacrimal that enters the rostral margin of the orbit. However, its dentition is uncharacteristic of that of the species in that it lacks the anterior vomerine tusk and the arrangement of vomerine teeth around the choana. Based on comparison with other large metoposaurs, the lack of these features does not appear to be ontogenetic or due to sexual dimorphism. These characteristics, along with the specimen’s stratigraphic position, support a taxonomic reevaluation and classification of metoposaurs in the Petrified Forest National Park.