A detailed redescription of a skeletally immature ‘Redondasaurus’ suggests ontogenetic transformations in the taxon mirror phytosaurian morphological evolution

Abstract

The study of morphological evolution is fundamentally tied to ontogeny, yet studies of these heterochronic processes in the fossil record are rare. Fossils belonging to an ontogenetic series are difficult to assign to an ontogenetic stage due to inconsistent proxies for skeletal ages, challenging to taxonomically assign due to morphological changes that may occur through ontogeny, and scarce due to taphonomic processes that preferentially destroy and disarticulate smaller bones. To contribute a new ontogenetic study to the evolutionary research of extinct taxa, we redescribe the smallest, mostly complete, skeletally immature phytosaur skull (NMMNH P-44920; ‘Redondasaurus’). We introduce another small, nearly complete, and skeletally immature ‘Redondasaurus’ skull (AMNH FR 32182) as well as a small partial rostrum (CMNH 51002), and qualitatively compare all morphological variation to the holotype skull ‘Redondasaurus’ (CM 69727). All four of the aforementioned skulls are from the “siltstone member” of the Chinle Formation at the Coelophysis Quarry at Ghost Ranch, Abiquiu, New Mexico, USA, and likely represent a relatively temporally constrained population. Qualitative observations reveal that smaller, skeletally immature specimens of ‘Redondasaurus’ possess a mosaic of cranial traits that are observed in both early and late-diverging phytosaur species, suggesting a parallelism between ontogenetic transformations and morphological evolution in Phytosauria.