Variation, Variability, and the Origin of the Avian Endocranium: Insights from the Anatomy of Alioramus altai (Theropoda: Tyrannosauroidea)

Gabe S Bever,Stephen L Brusatte,Mark A Norell,Amy M Balanoff,Daphne Soares

doi:10.1371/journal.pone.0023393

Gabe S Bever, Stephen L Brusatte + Show 3 more

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https://doi.org/10.1371/journal.pone.0023393

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Abstract

The internal braincase anatomy of the holotype of Alioramus altai, a relatively small-bodied tyrannosauroid from the Late Cretaceous of Mongolia, was studied using high-resolution computed tomography. A number of derived characters strengthen the diagnosis of this taxon as both a tyrannosauroid and a unique, new species (e.g., endocranial position of the gasserian ganglion, internal ramification of the facial nerve). Also present are features intermediate between the basal theropod and avialan conditions that optimize as the ancestral condition for Coelurosauria—a diverse group of derived theropods that includes modern birds. The expression of several primitive theropod features as derived character states within Tyrannosauroidea establishes previously unrecognized evolutionary complexity and morphological plasticity at the base of Coelurosauria. It also demonstrates the critical role heterochrony may have played in driving patterns of endocranial variability within the group and potentially reveals stages in the evolution of neuroanatomical development that could not be inferred based solely on developmental observations of the major archosaurian crown clades. We discuss the integration of paleontology with variability studies, especially as applied to the nature of morphological transformations along the phylogenetically long branches that tend to separate the crown clades of major vertebrate groups.

Highlights

The potential of developmental systems to produce morphological variation is not static but changes through ontogeny and phylogeny [1,2]
The partial skeleton has a nearly complete braincase preserving all expected neurocranial and dermal elements except the ethmoid complex (Fig. 1). This midline element was present in life—but perhaps only as a cartilaginous structure—based on a clear frontal scar that marks the contact of the ethmoid ring with the dermal skull roof and delimits the rostrocaudal and mediolateral extent of the olfactory bulbs [25]
A high-resolution computed tomography (CT) scan resulted in remarkable contrast between matrix and bone, allowing a detailed study of the internal braincase anatomy

Summary

Introduction

The potential of developmental systems to produce morphological variation (i.e., variability) is not static but changes through ontogeny and phylogeny [1,2]. Considering the importance of this region (both as the skeletal seat of the brain and primary sense organs and as a source of phylogenetically informative characters), the critical role paleontology plays in breaking up the long periods of evolutionary time often separating major crown clades, and the fact that any study of the fossil record likely requires comparing specimens of different ontogenetic ages, the potential implications of dynamic variability are obvious and significant. Grasping these implications requires a better understanding of developmental dynamics in extant lineages. Tyrannosauroids are beginning to be one such clade [6]

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