Evidence for Endothermic Ancestors of Crocodiles at the Stem of Archosaur Evolution

Physiological, anatomical, and developmental features of the crocodilian heart support the paleontological evidence that the ancestors of living crocodilians were active and endothermic, but the lineage reverted to ectothermy when it invaded the aquatic, ambush predator niche. In endotherms, there i...

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Bibliographic Details
Published in:Physiological and biochemical zoology 2004-11, Vol.77 (6), p.1051-1067
Main Authors: Seymour, Roger S., Bennett‐Stamper, Christina L., Johnston, Sonya D., Carrier, David R., Grigg, Gordon C.
Format: Article
Language:English
Subjects:
Alligators and Crocodiles - genetics
Alligators and Crocodiles - physiology
Animals
Biological Evolution
Birds
Blood
Blood pressure
Bone and Bones - physiology
Dinosaurs
Heart
Heart - anatomy & histology
Heart valves
Lung - anatomy & histology
Lung - physiology
Lungs
Mammals
Nexus
Paleontology
Phylogeny
Reptiles
Septum
Thermogenesis - genetics
Thermogenesis - physiology
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Summary:Physiological, anatomical, and developmental features of the crocodilian heart support the paleontological evidence that the ancestors of living crocodilians were active and endothermic, but the lineage reverted to ectothermy when it invaded the aquatic, ambush predator niche. In endotherms, there is a functional nexus between high metabolic rates, high blood flow rates, and complete separation of high systemic blood pressure from low pulmonary blood pressure in a four‐chambered heart. Ectotherms generally lack all of these characteristics, but crocodilians retain a four‐chambered heart. However, crocodilians have a neurally controlled, pulmonary bypass shunt that is functional in diving. Shunting occurs outside of the heart and involves the left aortic arch that originates from the right ventricle, the foramen of Panizza between the left and right aortic arches, and the cog‐tooth valve at the base of the pulmonary artery. Developmental studies show that all of these uniquely crocodilian features are secondarily derived, indicating a shift from the complete separation of blood flow of endotherms to the controlled shunting of ectotherms. We present other evidence for endothermy in stem archosaurs and suggest that some dinosaurs may have inherited the trait.
ISSN:1522-2152
1537-5293
DOI:10.1086/422766