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Thrinaxodon (Therapsida) cranial morphology ontogeny

Ben Creisler

A recent online paper not yet mentioned:

Sandra C. Jasinoski, Fernando Abdala and Vincent Fernandez (2015)
Ontogeny of the Early Triassic Cynodont Thrinaxodon liorhinus
(Therapsida): Cranial Morphology.
The Anatomical Record (advance online publication)
DOI: 10.1002/ar.23116

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