Akinobu Watanabe, Paul M. Gignac, Amy M. Balanoff, Todd L. Green, Nathan J. Kley & Mark A. Norell (2018)
Are endocasts good proxies for brain size and shape in archosaurs throughout ontogeny?
Journal of Anatomy (advance online publication)
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Cranial endocasts, or the internal molds of the braincase, are a crucial correlate for investigating the neuroanatomy of extinct vertebrates and tracking brain evolution through deep time. Nevertheless, the validity of such studies pivots on the reliability of endocasts as a proxy for brain morphology. Here, we employ microâcomputed tomography imaging, including diffusible iodineâbased contrastâenhanced CT, and a threeâdimensional geometric morphometric framework to examine both size and shape differences between brains and endocasts of two exemplar archosaur taxa â the American alligator (Alligator mississippiensis) and the domestic chicken (Gallus gallus). With ontogenetic sampling, we quantitatively evaluate how endocasts differ from brains and whether this deviation changes during development. We find strong size and shape correlations between brains and endocasts, divergent ontogenetic trends in the brainâtoâendocast correspondence between alligators and chickens, and a comparable magnitude between brainâendocast shape differences and intraspecific neuroanatomical variation. The results have important implications for paleoneurological studies in archosaurs. Notably, we demonstrate that the pattern of endocranial shape variation closely reflects brain shape variation. Therefore, analyses of endocranial morphology are unlikely to generate spurious conclusions about largeâscale trends in brain size and shape. To mitigate any artifacts, however, paleoneurological studies should consider the lower brainâendocast correspondence in the hindbrain relative to the forebrain; higher size and shape correspondences in chickens than alligators throughout postnatal ontogeny; artificially âpedomorphicâ shape of endocasts relative to their corresponding brains; and potential biases in both size and shape data due to the lack of control for ontogenetic stages in endocranial sampling.
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