Cranial Ontogeny of the Early Triassic Basal Cynodont Galesaurus planiceps

JASINOSKI, S. C.; ABDALA, F.

ANATOMICAL RECORD

WILEY-LISS, DIV JOHN WILEY & SONS INC

Lugar: New York; Año: 2017

0003-276X

Ontogenetic changes in the skull and mandible of thirty-one specimensof Galesaurus planiceps, a basal non-mammaliaform cynodont from theEarly Triassic of South Africa, are documented. The qualitative survey indicatedeight changes in the craniomandibular apparatus occurred duringgrowth, dividing the sample into three ontogenetic stages: juvenile, subadult,and adult. Changes in the temporal region, zygomatic arch, occiput,and mandible occurred during the transition from the subadult to adultstage at a basal skull length of 90 mm. At least four morphological and allometricdifferences divided the adult specimens into two morphs, indicatingthe presence of sexual dimorphism in Galesaurus. Differences includeextensive lateral flaring of the zygomatic arches in the ?male? morph resultingin a more anterior orientation of the orbits, and a narrower snout in the?female?. This is the first record of sexual dimorphism in a basal cynodont,and the first time it is quantitatively documented in a non-mammaliaformcynodont. An ontogenetic comparison between Galesaurus and the morederived basal cynodont Thrinaxodon revealed differences in the timing andextent of sagittal crest development. In Galesaurus, the posterior sagittalcrest, located behind the parietal foramen, developed relatively later inontogeny, and the anterior sagittal crest rarely formed suggesting the anteriorfibres of the temporalis were less developed than in Thrinaxodon. Incontrast, craniomandibular features related to the masseters became moredeveloped during the ontogeny of Galesaurus. The development of theadductor musculature appears to be one of the main factors influencingskull growth in these basal non-mammaliaform cynodonts.