Beyond Endocasts: Using Predicted Brain-Structure Volumes of Extinct Birds to Assess Neuroanatomical and Behavioral Inferences

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.

Endocast structures are reliable proxies for the sizes of corresponding regions of the brain in extant birds

“…Abbreviations: H, hyperpallium; OL, optic lobe; OT, optic tectum; W, Wulst. Scale bar = 5 mm Early et al, 2014Early et al, , 2016Early et al, , 2020Torres and Clarke, 2018), based on the assumption that the surface area of the optic lobes reliably estimates the volume of the underlying optic tectum. However, beneath the optic tract and optic tectum are the inferior colliculus and tegmental nuclei, and changes in the sizes of these regions could affect the surface area of the optic lobes.…”

“…As shown in Figure 7c, there is considerable scatter around the regression line, the confidence and prediction intervals are quite large, and the slope and correlation coefficients are low (Table 2). (Ashwell and Scofield, 2008;Milner and Walsh, 2009;Scofield and Ashwell, 2009;Walsh and Milner, 2011b;Ksepka et al, 2012;Smith and Clarke, 2012;Walsh et al, 2013;Early et al, 2014Early et al, , 2016Early et al, , 2020Kawabe et al, 2014;Tambussi et al, 2015;Carril et al, 2016;Gold et al, 2016;Proffitt et al, 2016;Torres and Clarke, 2018).…”

“…The clearly described boundaries we used to define the endocast structures of interest will make it easy for similar measurements to be taken from these endocasts. And the sample of virtual endocasts of extant birds published in this study provide the context of a comparative framework within which previously published endocasts of extinct birds could be analyzed (Early et al, 2020).…”

“…The close relationship between endocranial size and brain size, as well as the increasingly widespread use of CT scanning to generate virtual endocasts, has led to a large number of paleoneurological studies of extinct birds (Domínguez Alonso et al, 2004; Ashwell and Scofield, 2008; Milner and Walsh, 2009; Scofield and Ashwell, 2009; Walsh and Milner, 2011b; Zelenitsky et al, 2011; Ksepka et al, 2012; Smith and Clarke, 2012; Balanoff et al, 2013; Walsh et al, 2013, 2016; Kawabe et al, 2014; Tambussi et al, 2015; Gold et al, 2016; Proffitt et al, 2016; Early et al, 2020). The most popular approach to studying these virtual endocasts is inspired by the Principle of Proper Mass, which states that the size of a brain region is proportional to its ability to process information (Jerison, 1973).…”

Cranial evolution in the extinct Rodrigues Island owl Otus murivorus (Strigidae), associated with unexpected ecological adaptations

“…1 ), from Rodrigues Island 5 , 38 , and ten crania of extant owls, were imaged through x-ray microtomography at ENS de Lyon (GE Phoenix Nanotom 180 device (platform US8/UMS3444 SFR BioSciences), in order to obtain 3D reconstructions of cranium, endocast and inner ear, as well as virtual sections of cranium. Brain endocast is considered to be an excellent proxy of the underlying brain structure in birds 39 . Avizo Lite 9.0.1 and Meshlab 2016 were used for image treatment and measurements (provided in electronic supplementary material) (Supplementary Figs.…”