NEWLY DESCRIBED PHYTOSAUR SACRUM REVEALS OVERLOOKED POSTCRANIAL DIVERSITY IN PHYTOSAURIA

Phytosaurs are an extinct clade of crocodile-like reptiles found as fossils in Upper Triassic rocks globally. Their skulls are well-studied and form the basis of our current understanding of their ingroup relationships, while their postcrania have mostly been overlooked. Recent phylogenies place Phytosauria within Archosauriformes as the sister group to Archosauria or as a basal clade of pseudosuchian (‘crocodile-line’) archosaurs. They are generally described as possessing two sacral vertebrae, the plesiomorphic condition for archosaurs. Recently, it has been shown that some phytosaurs possessed a third sacral vertebra, namely a sacralized last dorsal vertebra (dorsosacral) in addition to the two primordial sacral vertebrae. We describe a phytosaur specimen recovered from the Petrified Forest Member of the Chinle Formation within Petrified Forest National Park, Arizona (PEFO 31219, Machaeroprosopus mccauleyi) that possesses at least three sacral vertebrae, namely two primordial sacral vertebrae plus a sacralized first caudal (caudosacral) vertebra. Using other phytosaur specimens for reference, we identify the two primordial sacral vertebrae and ribs based on their morphological characteristics, including: the posterodorsal taper of the second sacral ribs, the dip angles of the sacral ribs in lateral view, and the locations of the sacral rib scars on the medial ilia. Given our identification of the primordial first and second sacral vertebrae, we identify the first caudal vertebra as a caudosacral based on the clear articulation of its ribs with the medial postacetabular process of the ilium laterally and with the second sacral ribs anterodistally. The last presacral vertebra is preserved in PEFO 31219, however, it lacks the distal ends of the ribs, making it impossible to firmly identify this element as a dorsosacral. Thus, PEFO 31219 possessed at least three sacral vertebrae—a primordial first sacral, primordial second sacral, and a caudosacral—a derived condition relative to that of the putative ancestral archosauriform and a unique condition among phytosaurs. Incorporation of such characters into phylogenetic analyses may be useful in resolving relationships within Phytosauria.