New Mexico Geological Society Annual Spring Meeting — Abstracts

Redondasaurus is the geologically youngest known North American phytosaur and an index fossil of the Apachean land-vertebrate faunachron. In the Rock Point Formation (Chinle Group) at the Whitaker quarry, Ghost Ranch, New Mexico, a nearly complete juvenile skull of Redondasaurus was preserved among the many skeletons of the dinosaur Coelophysis. Approximately 220 m total length, it is the best preserved and most complete juvenile phytosaur skull of its ontogenetic stage known. The skull and lower jaws are nearly complete, only missing the anterior snout tip and anterior end (symphyseal tip) of the lower jaw. This skull shows that many of the diagnostic cranial features of Redondasaurus are present in small specimens and thus not subject to ontogenetic change, including septomaxillae that wrap around the outer margin of the external narial opening, thickened orbital margins, and inflated posterior nasal behind the external narial opening. However, the juvenile skull does not possess some features that diagnose adult Redondasaurus, including supratemporal fenestrae concealed in dorsal view and reduced antorbital fenestrae. These differences are attributable to differential growth of selected parts of the skull, particularly the great expansion of the squamosals to conceal the supratemporal fenestrae in the adult. Thus, the juvenile skull of Redondasaurus demonstrates that juvenile phytosaurs can be diagnosed and assigned to taxa defined on adult characters. We assembled a growth series of five Redondasaurus skulls from which we assessed skull metrics and allometry. Relative growth data for this dataset of Redondasaurus skulls, which range in length from 220 to 1205 mm, as log-transformed metrics better fit a linear regression than a polynomial, which indicates simple allometry where shape changes during ontogeny occurred along a single constant trajectory. The allometry of the Redondasaurus skull is similar to that of numerous other phytosaur species, except in the postorbital region, which apparently grows in negative allometry as opposed to the positive allometry seen in others. The data thus imply that this is an important ontogeny-based diagnostic character of Redondasaurus, but a larger sample size will be required to confirm the hypothesis.