A NEW SHORT-FACED ARCHOSAURIFORM FROM THE UPPER TRIASSIC (ADAMANIAN:NORIAN) PLACERIAS/DOWNS QUARRY COMPLEX, APACHE COUNTY, ARIZONA, EXPANDS THE MORPHOSPACE OCCUPATION OF TRIASSIC ARCHOSAURIFORMES

The Placerias/Downs quarry (PDQ) complex in east-central Arizona is the most diverse Late Triassic nonmarine vertebrate locality in the world, yielding dozens of taxa since the first excavations in the 1930s. The PDQ is low in the Chinle and yields an Adamanian (early Norian) assemblage with a published maximum depositional age of ~219 Ma. Excavations led by us since 2010 resulted in the discovery of a new, short-faced archosauriform (SFA) that increases the known morphospace occupancy of Triassic archosauromorphs. The SFA is represented by 4 lower jaws (2 left, 2 right) that represent 2-4 individuals. Each jaw is relatively short and deep (only 2.5x longer than tall), robust, and contains a unique dentition that is restricted to the anteriormost portion and consists of 3-4 anterior, strongly procumbent teeth, a larger, caniniform tooth, and 1-3 more posterior, dorsally directed teeth, although the latter appear to diminish with age. The shorter, more gracile jaws have more teeth (4-1-3) than does the most robust jaw (3-1-2). This short-faced morphology is unique among Triassic taxa but convergent with the notosuchid crocodyliform Simosuchus from the Upper Cretaceous of Madagascar. Other local autapomorphies of the SFA include a strongly developed surangular ridge that completes a 90˚ arc posteriorly, uniquely coarsely sculptured lateral surfaces of the angular, surangular, and dentary, and a finely sculptured medial surface of the splenial. CT data reveal a replacement tooth and extensive vascular network associated with tooth positions 1-3. An associated posterior skull with articulated braincase preserves a sagittal crest, fused parietals, a laterosphenoid-parabasisphenoid contact, and a prootic-quadrate contact, all characteristics of crocodylomorphs, although we cannot be certain that this element pertains to the SFA. The SFA is particularly important because, in addition to expanding the known diversity of Triassic archosauriforms, it is another example of the Triassic archosauriform radiation occupying morphospace that would later be occupied by younger, later diverging taxa, in this case the superficially convergent Simosuchus. The SFA also demonstrates the importance of continuing to sample known localities, as it was renewed excavations at a well-known site that led to its discovery.