title: 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.
authors.affiliation: Division of Paleontology, American Museum of Natural History, New York, New York, United States of America.|Richard Gilder Graduate School, American Museum of Natural History, New York, New York, United States of America.
authors.forename: Akinobu
authors.initials: A
authors.lastname: Watanabe
authors.affiliation: Division of Paleontology, American Museum of Natural History, New York, New York, United States of America.|Richard Gilder Graduate School, American Museum of Natural History, New York, New York, United States of America.
authors.forename: Maria
authors.initials: M
authors.lastname: Eugenia Leone Gold
authors.affiliation: School of GeoSciences, University of Edinburgh, Scotland, United Kingdom.
authors.forename: Stephen L
authors.initials: SL
authors.lastname: Brusatte
authors.affiliation: Department of Earth Sciences, University of Oxford, Oxford, United Kingdom.|Evolutionary Studies Institute and DST/NRF Centre of Excellence in Palaeosciences, University of the Witwatersrand, Johannesburg, South Africa.
authors.forename: Roger B J
authors.initials: RB
authors.lastname: Benson
authors.affiliation: Evolutionary Studies Institute and DST/NRF Centre of Excellence in Palaeosciences, University of the Witwatersrand, Johannesburg, South Africa.
authors.forename: Jonah
authors.initials: J
authors.lastname: Choiniere
authors.affiliation: Division of Paleontology, American Museum of Natural History, New York, New York, United States of America.
authors.forename: Amy
authors.initials: A
authors.lastname: Davidson
authors.affiliation: Division of Paleontology, American Museum of Natural History, New York, New York, United States of America.|Richard Gilder Graduate School, American Museum of Natural History, New York, New York, United States of America.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Norell
mesh_terms: D000818:Animals; D005075:Biological Evolution; D025061:Dinosaurs / Q000033:anatomy & histology*; D010802:Phylogeny; D012886:Skull / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D013131:Spine / Q000033:anatomy & histology / Q000000981:diagnostic imaging*; D014057:Tomography, X-Ray Computed / Q000379:methods; D014463:Ultrasonography