New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Triassic Petrified Forest Formation (Chinle Group) at Petrified Forest National Park (pEFO), Arizona, preserves an important sequence of vertebrate faunas from the late CarnianNorian, preserved in superposition. The most common tetrapods are the semi-aquatic phytosaurs. Three grades of phytosaurs are known from PEFO.

The Blue Mesa Member of the Petrified Forest Formation at PEFO contains the type fauna of the Adamanian land-vertebrate faunachron (lvf). A diverse phytosaur fauna was collected by Charles Camp for the University of California Museum of Paleontology (UCMP) in the 1920's and 1930's, and subsequently a few other specimens have been found. The Adamanian phytosaurs are of Rutiodon grade and are characterized by several features, notably external nares at level of skull roof, pendulous squamosal processes in lateral view and supratemporal fenestrae partially obscured in dorsal view. There are dolichorostral and altirostral variants of this phytosaur grade present at PEFO.

The Sonsela Member of the Petrified Forest Formation has yielded one partial phytosaur skull of Nicrosaurus-grade, which is characterized by external nares at level of skull roof and dorso-ventrally flattened squamosal processes in lateral view. The one specimen is altirostral.The Painted Desert Member of the Petrified Forest Formation contains a Revueltian lvf fauna and has yielded a few skulls of Pseudopalatus -grade that are characterized by external nares elevated above level of skull roof, dorso-ventrally flattened squamosal processes in lateral view and supratemporal fenestrae almost obscured in dorsal view. Both dolichorostral and altirostral forms are known.

PEFO phytosaurs represent a sequence of grades of phytosaurs known elsewhere from the American Southwest and provide strong support for phytosaur-based biochronologies of the Late Triassic. The presence of a Nicrosaurus-grade phytosaur in North America is unusual, and the only other published specimen is from the Petrified Forest Formation of north-central New Mexico. In Germany, Nicrosaurus is common, and its rarity in North America may be related to facies differences or biogeographic provincialism.