Meeting Abstract
Brains do not fossilize, so endocasts provide the most direct evidence of neuroanatomy and potential functional capability in extinct birds. The optic lobe and the Wulst are structures that are visible on the endocasts of a majority of birds. They overlie the optic tectum and hyperpallium, respectively, two brain structures that have roles in the visual pathways of birds and whose relative size seems to correlate with emphasis on visual information. Researchers have inferred potential functional capabilities of extinct birds from relative endocast structure size, but it is unknown if the size of the endocast structure faithfully represents the size of the brain structure. We measured the size of the endocast and brain structures of interest in a diverse evolutionary sample of extant birds. We regressed the volumes of optic tecta on the surface areas of the optic lobes and the volumes of the hyperpallia on the surface areas of the Wulsts. Both regressions yielded a strong, significant, positive relationship between the volume of the brain structure and the surface area of the overlying endocast structure. A phylogenetic prediction method based on Bayesian inference was used to calculate the volumes of the brain structures of a few extinct birds based on the surface areas of their endocast structures. Phylogenetic ANCOVAs indicated that no extinct birds studied had hypertrophied optic tecta relative to brain-rest volume, nor were their hyperpallial volumes significantly different from the extant sample. The optic tectum of Dinornis robustus was significantly smaller than any of the extant birds in our sample. Our results provide an analytical framework within which hypotheses of potential functional capabilities of extinct birds based on their endocasts can be tested.
