Evolution of archosaurian body plans: skeletal adaptations of an air‐sac‐based breathing apparatus in birds and other archosaurs

O’Connor, Patrick M.

doi:10.1002/jez.548

Cited by 70 publications

(170 citation statements)

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“…In the earliest known pterosaurs [33], sauropodomorphs [29], and neotheropods [6], [12], [15] with unambiguous PSP, evidence for pneumaticity is limited to the postaxial cervical and anterior dorsal vertebrae and associated ribs. This matches the “common pattern” of PSP observed in extant birds [5], [8] as well as the earliest ontogenetic stages in extant birds with more extensive PSP [30]: a similar “common pattern” of cervical and anterior dorsal pneumaticity also applies almost universally to non-avian neotheropods [12]. It is striking that the ambiguous evidence of PSP (vertebral fossae and laminae) in non-dinosaurian dinosauromorphs ( Silesaurus ), non-neotheropod theropods ( Tawa ), and ‘prosauropods’ ( Pantydraco , Plateosaurus ) is restricted to, or most strongly expressed in, the cervical and anterior dorsal vertebral column, and this topographical and phylogenetic congruence provides support for these features being pneumatic.…”

Section: Discussionsupporting

confidence: 85%

Reassessment of the Evidence for Postcranial Skeletal Pneumaticity in Triassic Archosaurs, and the Early Evolution of the Avian Respiratory System

2012

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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.

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Section: Discussionsupporting

confidence: 85%

Reassessment of the Evidence for Postcranial Skeletal Pneumaticity in Triassic Archosaurs, and the Early Evolution of the Avian Respiratory System

2012

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“…Although there are no definitive hypotheses for the functions(s) of the air sac system, there is growing consensus that highly efficient lung ventilation in pterosaurs and dinosaurs evolved with enhanced activity levels and endothermy (Claessens et al, 2009;O'Connor, 2009;Benson et al, 2012). Under the hypoxic conditions of the Early Triassic, a unidirectional air flow in the lungs facilitated by a bellows-like action of the air sacs enhanced oxygen extraction and may have been a critical factor in the relative ecological dominance of the sauropsids over the synapsids at this time (Farmer & Sanders, 2010).…”

Section: (B) Cardiovascular Innovationsmentioning

confidence: 96%

“…In the Early Triassic, pneumatized bones appeared in Dinosauromorpha (O'Connor, 2009;Benson et al, 2012;Butler et al, 2012). Bone is a metabolically active tissue so innovations which reduced bone mass without compromising structural integrity would have reduced metabolic demands and increased the options for energy allocation to alternative demands, such as those of locomotion, foraging and reproduction (O'Connor, 2009).…”

Section: Millions Of Years Agomentioning

confidence: 96%

“…Bone is a metabolically active tissue so innovations which reduced bone mass without compromising structural integrity would have reduced metabolic demands and increased the options for energy allocation to alternative demands, such as those of locomotion, foraging and reproduction (O'Connor, 2009). In moderns birds, for example, the ability to pneumatize and thus alter the density of selected bones in the postcranial skeleton allowed birds to diversify into 'pneumaticity phenotypes', such as swimming, diving, gliding and soaring forms (O'Connor, 2009). Pneumaticity of the sacrum and vertebral column are hypothesized to have lowered the centre of gravity and increased stability in bipedal dinosaurs, and controlled pitch, roll, and yaw in birds (Farmer, 2006).…”

Section: Millions Of Years Agomentioning

confidence: 99%

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A phenology of the evolution of endothermy in birds and mammals

2016

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Recent palaeontological data and novel physiological hypotheses now allow a timescaled reconstruction of the evolution of endothermy in birds and mammals. A three-phase iterative model describing how endothermy evolved from Permian ectothermic ancestors is presented. In Phase One I propose that the elevation of endothermy - increased metabolism and body temperature (T ) - complemented large-body-size homeothermy during the Permian and Triassic in response to the fitness benefits of enhanced embryo development (parental care) and the activity demands of conquering dry land. I propose that Phase Two commenced in the Late Triassic and Jurassic and was marked by extreme body-size miniaturization, the evolution of enhanced body insulation (fur and feathers), increased brain size, thermoregulatory control, and increased ecomorphological diversity. I suggest that Phase Three occurred during the Cretaceous and Cenozoic and involved endothermic pulses associated with the evolution of muscle-powered flapping flight in birds, terrestrial cursoriality in mammals, and climate adaptation in response to Late Cenozoic cooling in both birds and mammals. Although the triphasic model argues for an iterative evolution of endothermy in pulses throughout the Mesozoic and Cenozoic, it is also argued that endothermy was potentially abandoned at any time that a bird or mammal did not rely upon its thermal benefits for parental care or breeding success. The abandonment would have taken the form of either hibernation or daily torpor as observed in extant endotherms. Thus torpor and hibernation are argued to be as ancient as the origins of endothermy itself, a plesiomorphic characteristic observed today in many small birds and mammals.

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“…Hard tissue correlates of the other two systems are less common, but widespread enough throughout theropod diversity to suggest that at least the last common ancestor of ceratosaurs and tetanurans possessed abdominal air sacs (O'Connor & Claessens, 2005;, and that most tetanurans potentially had a clavicular air sac (Makovicky et al, 2005;. It should be noted that air sacs do not always invade skeletal elements in living birds and the degree of pneumaticity is observed to correlate with life history parameters such as body size and ecological habits (O'Connor, 2009), so absence of pneumatic traces in bones of extinct theropods cannot be taken as evidence for absence of air sacs, especially if such taxa are bracketed phylogenetically by taxa with positive evidence for air sacs.…”

Section: Metabolism and Respirationmentioning

confidence: 99%