Mandibles of Juvenile Phytosaurs (archosauria: Crurotarsi) from the Upper Triassic Chinle Group of Texas and New Mexico, Usa

Here we describe five specimens of juvenile phytosaurs from several localities in the Upper Triassic Chinle Group of Texas and New Mexico. These include three specimens from localities in the Tecovas Formation (Texas) of Adamanian age and one each from the Revueltian-age Bull Canyon Formation and Apachean-age Travesser Formation of New Mexico. Although all of the specimens are incompletely preserved mandibles, they are unquestionably phytosaurian based on their elongate mandibles with an extensive symphyseal region that is of essentially constant height and has at least some contribution from the splenial. We use the length (in mm) per preserved tooth socket as a proxy for ontogenetic stage, ranking the specimens from very young (~4-6 mm/socket) to juvenile (~9 mm/socket). There are distinct differences in tooth spacing, contribution of the splenial to the symphysis, angle of tooth eruption, and other features that vary in these specimens independent of age. This suggests that taxonomic information is available from the mandible pending further description and characterization of adult specimens of known taxa.