Cranial ontogenetic patterns in Permo-Triassic basal cynodonts from Southern Africa.

JASINOSKI, S. C.; ABDALA, F.; HOPSON, J.

Calgary

Conferencia; 77th Annual Meeting of the Society of Vertebrate Paleontology; 2017

Society of Vertebrate Paleontology

The cranial ontogeny of Thrinaxodon liorhinus and Galesaurus planiceps, basalepicynodonts from the Early Triassic of South Africa, was recently documented. These comprehensive studies using qualitative and quantitative analyses revealed the ontogenetic trends within each taxon. Nine cranial features divided the large sample of Thrinaxodon into four ontogenetic stages; whereas eight craniomandibular features in Galesaurus separated the comparatively smaller sample into three ontogenetic stages. Several of the ontogenetic changes were related to the development of the adductor musculature. A new study of Procynosuchus delaharpeae, a basal cynodont from the Late Permian, allows comparison to the cranial ontogenetic trends previously described in the two basal epicynodonts. More than forty specimens of Procynosuchus from southern Africa were included in the qualitative analysis; however, poor preservation and deformation of several skulls limited the sample size for the quantitative analyses. A few unequivocal changes were documented in the skull and mandible, which separated the sample of Procynosuchus into immature and adult stages. The adult stage of Procynosuchus is characterized by the development of the posterior sagittal crest, masseteric fossa, and nasofrontal protuberance. The development of the posterior sagittal crest was also documented in thebasal epicynodonts Galesaurus and Thrinaxodon, although it developed relatively earlier in the ontogeny of Thrinaxodon. There are fewer ontogenetic changes recognized in Procynosuchus in comparison to the basal epicynodonts, possibly due to the poor preservation of small and intermediate size specimens. The most significant finding is the discovery of sexual dimorphism in adult Procynosuchus, with differences in the snout, temporal region, and canine. The presence of sexual dimorphism in Procynosuchus and the basal epicynodont Galesaurus suggests that this condition might be more widespread in basal non-mammaliaform cynodonts.