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Archaeornithomimus Vertebral Pneumaticity

Ben Creisler

New in PLoS ONE:

Akinobu Watanabe, Maria Eugenia Leone Gold, Stephen L. Brusatte, Roger
B. J. Benson, Jonah Choiniere, Amy Davidson & Mark A. Norell (2015)
Vertebral Pneumaticity in the Ornithomimosaur Archaeornithomimus
(Dinosauria: Theropoda) Revealed by Computed Tomography Imaging and
Reappraisal of Axial Pneumaticity in Ornithomimosauria.
PLoS ONE 10(12): e0145168

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.