diversity Article Beyond Endocasts: Using Predicted Brain-Structure Volumes of Extinct Birds to Assess Neuroanatomical and Behavioral Inferences 1, , 2 2 Catherine M. Early * y , Ryan C. Ridgely and Lawrence M. Witmer 1 Department of Biological Sciences, Ohio University, Athens, OH 45701, USA 2 Department of Biomedical Sciences, Heritage College of Osteopathic Medicine, Ohio University, Athens, OH 45701, USA; [email protected] (R.C.R.); [email protected] (L.M.W.) * Correspondence: [email protected] Current Address: Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA. y Received: 1 November 2019; Accepted: 30 December 2019; Published: 17 January 2020 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. Keywords: endocast; neuroanatomy; brain; bird; evolution 1. Introduction 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. The brain-behavior connection in extant and extinct organisms has led to rapid growth of the field of comparative neuroanatomy [1–3]. In this field, 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 Diversity 2020, 12, 34; doi:10.3390/d12010034 www.mdpi.com/journal/diversity Diversity 2020, 12, 34 2 of 23 Diversity 2020, 12, x FOR PEER REVIEW 2 of 23 record.fact that The whole fossil soft record-tissue is organs, much betterincluding at preserving the brain, do hard not tissues preserve such in asthree bone, dimensions and fortunately, in the thefossil brain record. is surrounded The fossil record by bone is much to varying better at extents. preserving The hard space tissues within such these as bone, bones and constitutes fortunately, the endocranialthe brain is cavity, surrounded which canby bone then to serve varying as a mold extents. to generate The space a brain within endocast these bones (or simply constitutes “endocast” the forendocranial the remainder cavity, of which this article). can then These serve can as a be mold physical to generate endocasts a brain (e.g., endocast latex; [ 4(or]) or,simply more “endocast” commonly infor recent the remainder years, virtual of this endocasts article). These reconstructed can be physical from computed endocasts tomography (e.g., latex; [4 (CT),]) or, amore non-destructive commonly samplingin recent techniqueyears, virtual [5]. endocasts The endocast reconstructed can be a very from accurate computed representation tomography of the(CT), size a non and-destructive shape of the externalsampling surface technique of the [5] brain. 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