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Triassic Archosaur Skeletal Pneumaticity

From: Ben Creisler

New in PLoS ONE:

Butler, R,J,, Barrett, P.M. & Gower, D.J. (2012)
Reassessment of the Evidence for Postcranial Skeletal Pneumaticity in
Triassic Archosaurs, and the Early Evolution of the Avian Respiratory
PLoS ONE 7(3): e34094.

Uniquely among extant vertebrates, birds possess complex respiratory
systems characterised by the combination of small, rigid lungs,
extensive pulmonary air sacs that possess diverticula that invade
(pneumatise) the postcranial skeleton, unidirectional ventilation of
the lungs, and efficient crosscurrent gas exchange. Crocodilians, the
only other living archosaurs, also possess unidirectional lung
ventilation, but lack true air sacs and postcranial skeletal
pneumaticity (PSP). PSP can be used to infer the presence of
avian-like pulmonary air sacs in several extinct archosaur clades
(non-avian theropod dinosaurs, sauropod dinosaurs and pterosaurs).
However, the evolution of respiratory systems in other archosaurs,
especially in the lineage leading to crocodilians, is poorly
documented. Here, we use µCT-scanning to investigate the vertebral
anatomy of Triassic archosaur taxa, from both the avian and
crocodilian lineages as well as non-archosaurian diapsid outgroups.
Our results confirm previous suggestions that unambiguous evidence of
PSP (presence of internal pneumatic cavities linked to the exterior by
foramina) is found only in bird-line (ornithodiran) archosaurs. We
propose that pulmonary air sacs were present in the common ancestor of
Ornithodira and may have been subsequently lost or reduced in some
members of the clade (notably in ornithischian dinosaurs). The
development of these avian-like respiratory features might have been
linked to inferred increases in activity levels among ornithodirans.
By contrast, no crocodile-line archosaur (pseudosuchian) exhibits
evidence for unambiguous PSP, but many of these taxa possess the
complex array of vertebral laminae and fossae that always accompany
the presence of air sacs in ornithodirans. These laminae and fossae
are likely homologous with those in ornithodirans, which suggests the
need for further investigation of the hypothesis that a reduced, or
non-invasive, system of pulmonary air sacs may be have been present in
these taxa (and secondarily lost in extant crocodilians) and was
potentially primitive for Archosauria as a whole.