Best practices for digitally constructing endocranial casts: examples from birds and their dinosaurian relatives

Abstract

The rapidly expanding interest in, and availability of, digital tomography data to visualize casts of the vertebrate endocranial cavity housing the brain (endocasts) presents new opportunities and challenges to the field of comparative neuroanatomy. The opportunities are many, ranging from the relatively rapid acquisition of data to the unprecedented ability to integrate critically important fossil taxa. The challenges consist of navigating the logistical barriers that often separate a researcher from high-quality data and minimizing the amount of non-biological variation expressed in endocasts - variation that may confound meaningful and synthetic results. Our purpose here is to outline preferred approaches for acquiring digital tomographic data, converting those data to an endocast, and making those endocasts as meaningful as possible when considered in a comparative context. This review is intended to benefit those just getting started in the field but also serves to initiate further discussion between active endocast researchers regarding the best practices for advancing the discipline. Congruent with the theme of this volume, we draw our examples from birds and the highly encephalized non-avian dinosaurs that comprise closely related outgroups along their phylogenetic stem lineage.

Sensory Evolution and Ecology of Early Turtles Revealed by Digital Endocranial Reconstructions

“…Due to poor grayscale attenuation (in particular for SMNS 16980), segmentation was performed manually using the paintbrush and interpolation tools in the Avizo segmentation editor (both reconstructions performed by the first author for consistency following Balanoff et al, 2016). 3D surface models and volumes were created to visualize the endocranial components.…”

Comparative morphology of snake (Squamata) endocasts: evidence of phylogenetic and ecological signals

“…Endocasts are generated at the interface between the skeleton (typically bone or cartilage) and the soft tissues (or fluid) lying immediately near it (Balanoff et al. ). In the cranial cavity, the soft tissue forming the interface with the surrounding skeleton is not the brain but the superficial surface of the dural meninges, blood vessels and vasculature enveloping the brain (Walsh & Knoll, ).…”

“…Mammals and birds, which are generally considered as highly encephalized taxa (large brains relative to body size; Balanoff et al. ), tend to have brains that nearly fill the cranial cavity, resulting in a strong correlation between the volume and morphology of the brain endocast and those of the brain (Balanoff et al. ).…”

The Endocranial Cavity of Oviraptorosaur Dinosaurs and the Increasingly Complex, Deep History of the Avian Brain

“…One can expect their fidelity to the actual brain regions to erode as the BEC index (brain-endocast index) [Balanoff et al, 2016] is reduced down the stem lineage and towards the ancestral crown archosaur. The rate and complexity of that declination remains unclear but represents an area of active research [Morhardt et al, 2012].…”

“…Endocasts were constructed and measured using the original TIFF or DICOM images in the volume-rendering program VGStudioMax © and following the recommended protocol of Balanoff et al [2016]. For ease of description, we refer to the digitally constructed features of the endocranial space by the names of the softtissue neural structures they are inferred to reflect (e.g., cerebrum rather than cast of cerebrum).…”