Ontogeny of the Early Triassic Cynodont Thrinaxodon liorhinus (Therapsida): Cranial Morphology.

JASINOSKI, S. C.; ABDALA, F.; FERNANDEZ, V.

ANATOMICAL RECORD

WILEY-LISS, DIV JOHN WILEY & SONS INC

Lugar: New York; Año: 2015 vol. 298 p. 1440 - 1464

0003-276X

The cranial morphology of 68 Thrinaxodon liorhinus specimens, rangingin size from 30 to 96 mm in basal skull length, is investigated using bothqualitative and quantitative analyses. From this comprehensive survey, wedetermined that nine cranial features, including five in the temporal region,separated the sample into four ontogenetic stages. A bivariate analysis of 60specimens indicated that the skull generally increased in size isometrically,with the exception of four regions. The orbit had negative allometry, a resultconsistent with other ontogenetic studies of tetrapods, whereas the length ofthe snout, palate, and temporal region showed positive allometry. The lasttrend had strong positive allometry indicating that during ontogeny thelength of the sagittal crest increased at a much faster rate than the rest ofthe skull. The large number of changes in the temporal region of the skull ofThrinaxodon may indicate a greater development of the posterior fibres ofthe temporalis musculature from an early ontogenetic stage. For example,the posterior sagittal crest developed much earlier in ontogeny than theanterior crest that formed in adults, and bone was deposited dorsally creatinga unified posterior sagittal crest rather than having a suture thatspanned the entire depth of the skull roof. In combination with the isometricheight of the zygomatic arch and the almost complete absence of the zygomaticarch angulation, these ontogenetic changes suggest that there wasgreater development of the temporalis relative to the masseter muscles, indicatinga strong posterodorsal movement of the mandible in Thrinaxodon.