Abstract
The shape of the brain influences skull morphology in birds, and both traits are driven by phylogenetic and functional constraints. Studies on avian cranial and neuroanatomical evolution are strengthened by data on extinct birds, but complete, 3D-preserved vertebrate brains are not known from the fossil record, so brain endocasts often serve as proxies. Recent work on extant birds shows that the Wulst and optic lobe faithfully represent the size of their underlying brain structures, both of which are involved in avian visual pathways. The endocasts of seven extinct birds were generated from microCT scans of their skulls to add to an existing sample of endocasts of extant birds, and the surface areas of their Wulsts and optic lobes were measured. A phylogenetic prediction method based on Bayesian inference was used to calculate the volumes of the brain structures of these extinct birds based on the surface areas of their overlying endocast structures. This analysis resulted in hyperpallium volumes of five of these extinct birds and optic tectum volumes of all seven extinct birds. Phylogenetic ANCOVA (phyANCOVA) were performed on regressions of the brain-structure volumes and endocast structure surface areas on various brain size metrics to determine if the relative sizes of these structures in any extinct birds were significantly different from those of the extant birds in the sample. Phylogenetic ANCOVA indicated that no extinct birds studied had relative hyperpallial volumes that were significantly different from the extant sample, nor were any of their optic tecta relatively hypertrophied. The optic tectum of Dinornis robustus was significantly smaller relative to brain size than any of the extant birds in our sample. This study provides an analytical framework for testing the hypotheses of potential functional behavioral capabilities of other extinct birds based on their endocasts.
Highlights
Our deep fascination with extinct animals such as dinosaurs is driven not by their dry bones that remain with us today but by thinking of them as living and breathing animals, interacting with each other and their environments
The phyANCOVA tests performed on phylogenetic generalized least squares (PGLS) regressions of different combinations of hyperpallium size metrics and size correction factors indicated that none of the extinct birds in the sample had significantly different intercepts from the rest of the sample when slope was held constant (Table 3; Figure 4)
This study presents a quantitative, replicable workflow for predicting the volumes of the optic tecta and hyperpallia of extinct birds and identifying extinct birds in which these brain structures are significantly different in size from those of a broad sample of birds
Summary
Our deep fascination with extinct animals such as dinosaurs is driven not by their dry bones that remain with us today but by thinking of them as living and breathing animals, interacting with each other and their environments These interactions are determined by the various behaviors in which organisms engage, which are in turn enabled by the functional capabilities of their brains. Correlations are drawn between the size of a brain structure in a species of extant animal and the relative emphasis on the behavior it mediates Efforts to use these correlations to infer functional capabilities in extinct animals have been stymied by the fact that whole soft-tissue organs, including the brain, do not preserve in three dimensions in the fossil. Fossil record isorgans, much better at preserving hard such bone, and fact thatThe whole soft-tissue including the brain, do nottissues preserve in as three dimensions in the the brain is surrounded by bone to varying extents. The fossil record is much better at preserving hard tissues such as bone, and endocranial which can to serve as a mold to generate a brain endocast
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