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Theropod air-filled postcranial bones and bird transition



From: Ben Creisler
bh480@scn.org

New online article:

Roger B. J. Benson, Richard J. Butler, Matthew T. Carrano &  Patrick M.
O?Connor (2011)
Air-filled postcranial bones in theropod dinosaurs: physiological
implications and the 'reptile'?bird transition.
Biological Reviews (advance online publication) 
DOI: 10.1111/j.1469-185X.2011.00190.x
http://onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.2011.00190.x/abstract

Pneumatic (air-filled) postcranial bones are unique to birds among extant
tetrapods. Unambiguous skeletal correlates of postcranial pneumaticity
first appeared in the Late Triassic (approximately 210 million years ago),
when they evolved independently in several groups of bird-line archosaurs
(ornithodirans). These include the theropod dinosaurs (of which birds are
extant representatives), the pterosaurs, and sauropodomorph dinosaurs.
Postulated functions of skeletal pneumatisation include weight reduction in
large-bodied or flying taxa, and density reduction resulting in energetic
savings during foraging and locomotion. However, the influence of these
hypotheses on the early evolution of pneumaticity has not been studied in
detail previously. We review recent work on the significance of
pneumaticity for understanding the biology of extinct ornithodirans, and
present detailed new data on the proportion of the skeleton that was
pneumatised in 131 non-avian theropods and Archaeopteryx. This includes all
taxa known from significant postcranial remains. Pneumaticity of the
cervical and anterior dorsal vertebrae occurred early in theropod
evolution. This 'common pattern' was conserved on the line leading to
birds, and is likely present in Archaeopteryx. Increases in skeletal
pneumaticity occurred independently in as many as 12 lineages, highlighting
a remarkably high number of parallel acquisitions of a bird-like feature
among non-avian theropods. Using a quantitative comparative framework, we
show that evolutionary increases in skeletal pneumaticity are significantly
concentrated in lineages with large body size, suggesting that mass
reduction in response to gravitational constraints at large body sizes
influenced the early evolution of pneumaticity. However, the body size
threshold for extensive pneumatisation is lower in theropod lineages more
closely related to birds (maniraptorans). Thus, relaxation of the
relationship between body size and pneumatisation preceded the origin of
birds and cannot be explained as an adaptation for flight. We hypothesise
that skeletal density modulation in small, non-volant, maniraptorans
resulted in energetic savings as part of a multi-system response to
increased metabolic demands. Acquisition of extensive postcranial
pneumaticity in small-bodied maniraptorans may indicate avian-like
high-performance endothermy.


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