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The evolution of archosaur body plans and air-sac based lungs

J Exp Zool Part A Ecol Genet Physiol. 2009 Jun 2. [Epub ahead of print] 
Evolution of archosaurian body plans: skeletal adaptations of an air-sac-based 
breathing apparatus in birds and other archosaurs.O'Connor PM.
Department of Biomedical Sciences, Ohio University, Athens, Ohio.
Living birds represent the only extant sauropsid group in which pulmonary air 
sacs pneumatize the postcranial skeleton. Notable in this regard is an 
extraordinary degree of variability, ranging from species that are completely 
apneumatic to those characterized by air within the entire postcranial 
skeleton. Although numerous factors (e.g., body size) have been linked with 
"relative" pneumaticity, comparative studies examining this system remain 
sparse. This project sought to (1) characterize whole-body patterns of skeletal 
pneumaticity in distantly related neognath birds and (2) evaluate putative 
relationships among relative pneumaticity, body size and locomotor 
specializations. Pneumaticity profiles were established for 52 species 
representing 10 higher-level groups. Although comparisons reveal relatively 
conserved patterns within most lower-level clades, apparent size- and 
locomotor-thresholds do impart predictable deviations from the clade norm. For 
example, the largest flying birds (vultures, pelicans) exhibit 
hyperpneumaticity (i.e., pneumaticity of distal limb segments) relative to 
smaller members of their respective clades. In contrast, skeletal pneumaticity 
has been independently lost in multiple lineages of diving specialists (e.g., 
penguins, auks). The application of pneumaticity profiling to extinct 
archosaurs reveals similar trends in body size evolution, particularly when 
examining patterns of pneumaticity in a size-diverse assemblage of pterosaurs 
(flying "reptiles"). As a fundamental organizing system, skeletal pneumaticity 
may play a role in relaxing constraints on body size evolution by allowing 
volumetric increases without concomitant increases in body mass. Not only might 
this be critical for taxa (birds, pterosaurs) exploiting the energetically 
costly aerial environment, but could be beneficial for any large-bodied 
terrestrial vertebrates such as the dinosaurs.