ONTOGENETIC CHANGE IN THE MANDIBLE OF LATE JURASSIC NORTH AMERICAN GONIOPHOLIS (NEOSUCHIA, CROCODYLIFORMES)

The lower jaw of a young goniopholidid crocodilian (MWC 5625) from the Late Jurassic Morrison Formation of northeastern Wyoming demonstrates near starting points for allometric growth of the mandible as North American Goniopholis attained adult size. During growth from the young juvenile stage, the number of teeth increased slightly, the bones of the front half of the jaw became proportionately longer, the symphysis lengthened, and the shape and angle of the retroarticular process changed. The new mandible is ~13cm long and represents an animal approximately 1m in total length. There are 18 alveoli restricted to the anterior 39% of the jaw length (measured from the anterior end of the symphysis to the posterior process of the glenoid fossa); in adult specimens of Goniopholis from the Morrison Formation at Bone Cabin Quarry (lower jaw lengths ~41cm), there are 22–24 teeth and the first 18 of these occupy the anterior 46% of the jaw. Thus, the dentary (and the splenial) in the juvenile appear to be relatively short compared to the back half of the mandible, suggesting greater growth in these elements during ontogeny. The depth to length ratio of the symphysis in MWC 5625 is 0.42; in adults it is about 0.32, indicating a relative anteroposterior lengthening of this surface during growth. The ratio of the greatest dorsoventral depth of the mandible to its total length is 0.21 in the juvenile specimen and 0.15–0.17 in adult Goniopholis and Eutretauranosuchus. The dorsal border of the retroarticular process in North American Goniopholis is angled posteroventrally relative to the line of the dorsal border of the mandible; this angle is 47 degrees in MWC 5625 vs. ~25 degrees in adults. Most proportional changes in the mandible of Goniopholis suggest significantly greater growth of the dentary and splenial compared to that of the angular, surangular, and articular as the animals aged.