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, 275 (1631), 157-61

Avian-like Breathing Mechanics in Maniraptoran Dinosaurs

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Avian-like Breathing Mechanics in Maniraptoran Dinosaurs

Jonathan R Codd et al. Proc Biol Sci.

Abstract

In 1868 Thomas Huxley first proposed that dinosaurs were the direct ancestors of birds and subsequent analyses have identified a suite of 'avian' characteristics in theropod dinosaurs. Ossified uncinate processes are found in most species of extant birds and also occur in extinct non-avian maniraptoran dinosaurs. Their presence in these dinosaurs represents another morphological character linking them to Aves, and further supports the presence of an avian-like air-sac respiratory system in theropod dinosaurs, prior to the evolution of flight. Here we report a phylogenetic analysis of the presence of uncinate processes in Aves and non-avian maniraptoran dinosaurs indicating that these were homologous structures. Furthermore, recent work on Canada geese has demonstrated that uncinate processes are integral to the mechanics of avian ventilation, facilitating both inspiration and expiration. In extant birds, uncinate processes function to increase the mechanical advantage for movements of the ribs and sternum during respiration. Our study presents a mechanism whereby uncinate processes, in conjunction with lateral and ventral movements of the sternum and gastral basket, affected avian-like breathing mechanics in extinct non-avian maniraptoran dinosaurs.

Figures

Figure 1
Figure 1
Uncinate processes (arrows) of (a) a running bird, the Cassowary (Casuaris casuaris), (b) a flying bird, the Eagle Owl (Bubo bubo), (c) a diving bird, the Razorbill (Alca torda); canonical variate analyses indicates that the uncinate processes are shorter in running, long in diving and intermediate in all other birds (Tickle et al. 2007), (d) AMNH 6517, (Oviraptor philoceratops), (e) MPD 100/25 (Velociraptor mongoliensis). Anterior is to the right in all figures. Scale bars, 5 cm. Photographs taken by J.R.C.
Figure 2
Figure 2
Summary cladogram of maniraptoran dinosaurs showing the occurrence of ossified uncinate processes. Uncinate processes: plus, ossified uncinate processes present; minus, ossified uncinate processes absent; question mark, presence of ossified uncinates unknown. Material: C, complete postcranial skeleton; I, incomplete postcranial skeleton; F, fragmentary postcranial skeleton. (Adapted from Clark et al. (2002).)
Figure 3
Figure 3
Uncinate processes (arrows) of IVPP V13352 (Microraptor gui). Anterior is to the left. Scale bar, 13 cm. (Adapted with permission from Xu et al. (2003).)
Figure 4
Figure 4
Hypaxial musculature of tuatara (Sphenodon punctatus) demonstrating insertion of external oblique muscle from the gastralia to the uncinate processes. Anterior is to the left. (Adapted with permission from Maurer (1896).)

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