Vertebral Pneumaticity in the Ornithomimosaur Archaeornithomimus (Dinosauria: Theropoda) Revealed by Computed Tomography Imaging and Reappraisal of Axial Pneumaticity in Ornithomimosauria

Abstract

Among extant vertebrates, pneumatization of postcranial bones is unique to birds, with few known exceptions in other groups. Through reduction in bone mass, this feature is thought to benefit flight capacity in modern birds, but its prevalence in non-avian dinosaurs of variable sizes has generated competing hypotheses on the initial adaptive significance of postcranial pneumaticity. To better understand the evolutionary history of postcranial pneumaticity, studies have surveyed its distribution among non-avian dinosaurs. Nevertheless, the degree of pneumaticity in the basal coelurosaurian group Ornithomimosauria remains poorly known, despite their potential to greatly enhance our understanding of the early evolution of pneumatic bones along the lineage leading to birds. Historically, the identification of postcranial pneumaticity in non-avian dinosaurs has been based on examination of external morphology, and few studies thus far have focused on the internal architecture of pneumatic structures inside the bones. Here, we describe the vertebral pneumaticity of the ornithomimosaur Archaeornithomimus with the aid of X-ray computed tomography (CT) imaging. Complementary examination of external and internal osteology reveals (1) highly pneumatized cervical vertebrae with an elaborate configuration of interconnected chambers within the neural arch and the centrum; (2) anterior dorsal vertebrae with pneumatic chambers inside the neural arch; (3) apneumatic sacral vertebrae; and (4) a subset of proximal caudal vertebrae with limited pneumatic invasion into the neural arch. Comparisons with other theropod dinosaurs suggest that ornithomimosaurs primitively exhibited a plesiomorphic theropod condition for axial pneumaticity that was extended among later taxa, such as Archaeornithomimus and large bodied Deinocheirus. This finding corroborates the notion that evolutionary increases in vertebral pneumaticity occurred in parallel among independent lineages of bird-line archosaurs. Beyond providing a comprehensive view of vertebral pneumaticity in a non-avian coelurosaur, this study demonstrates the utility and need of CT imaging for further clarifying the early evolutionary history of postcranial pneumaticity.

Histology and pneumaticity of Aoniraptor libertatem (Dinosauria, Theropoda), an enigmatic mid‐sized megaraptoran from Patagonia

“…However, postcranial pneumaticity is not unique to birds, being observed in non‐avian dinosaurs (Britt, 1993; Wedel, 2003; O´Connor, 2006, 2007; O’Connor and Claessens, 2006; Sereno et al ., 2008; Makovicky and Zanno, 2011; Benson et al ., 2012; Xu et al ., 2014; Watanabe et al ., 2015; Lambertz et al ., 2018) as well as pterosaurs (Britt, 1993; Butler et al ., 2009; Claessens et al ., 2009; Martin and Palmer, 2014). The presence of this condition in non‐volant taxa indicates that the interaction between postcranial pneumaticity and flight capability was secondarily acquired and suggests that the main function of postcranial pneumatization is not flight.…”

“…Several authors have analyzed the evolution of postcranial pneumaticity towards the avian line (Wedel, 2003; O´Connor, 2006; O’Connor and Claessens, 2006; Benson et al ., 2010; Watanabe et al ., 2015). Most of those studies rest on the presence of external pneumatic openings and, with the exception of Watanabe et al .…”

“…Most of those studies rest on the presence of external pneumatic openings and, with the exception of Watanabe et al . (2015), do not provide an extensive description of the internal structure of the postcranial pneumaticity. Benson et al .…”

Does postcranial palaeoneurology provide insight into pterosaur behaviour and lifestyle? New data from the azhdarchoid Vectidraco and the ornithocheirids Coloborhynchus and Anhanguera

“…; Wedel ; Martin & Palmer ; Watanabe et al . ). As such, we strongly recommend the use of CT as an aid in determining the pneumatic nature of fossil bones, especially when they are embedded in matrix.…”

Are endocasts good proxies for brain size and shape in archosaurs throughout ontogeny?

“…For example, the rarity of soft-tissue preservation requires paleontologists to frequently utilize anatomical correlates of preserved hard tissues to infer unpreserved soft-tissue characteristics of extinct taxa (e.g. Witmer, 1995;Wedel & Sanders, 2002;Watanabe et al 2015). In the field of paleoneurology (Edinger, 1929;Kochetkova, 1978) internal molds of the cranial cavity, called cranial endocasts (hereafter 'endocasts'), have provided crucial information on the brain morphology of extinct vertebrates (e.g.…”