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

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. ABSTRACT 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. The University of Chicago Press

A pterosaur manus track from Denali National Park, Alaska Range, Alaska, United States

“…Such an interpretation then makes endothermy a character for the Archosauria. The presence of pterosaurs from high-paleolatitude settings (Alaska and New Zealand) is consistent with the implication of Seymour et al (2004) that pterosaurs, as members of the Archosauria, were endothermic. Recently the skeletons of large pterosaurs have been shown to have increased appendicular pneumaticity and this may have aided in thermoregulation (Claessens et al, 2009).…”

“…The Archosauria includes the Crocodylia, Pterosauria, and Dinosauria, suggesting at least two physiological strategies within this major taxonomic group, with some suggestion that pterosaurs were endothermic (Lu et al, 2005). In a recent study, Seymour et al (2004) suggested that modern crocodiles are secondarily ectothermic and that ancestral forms were endothermic. Such an interpretation then makes endothermy a character for the Archosauria.…”

Preliminary observations on the bone histology of the Middle Triassic pseudosuchian archosaurBatrachotomus kupferzellensisreveal fast growth with laminar fibrolamellar bone tissue

“…The reason why some pseudosuchians grow faster than others, however, remains unclear. Possible triggers for fast growth in Batrachotomus might be in response to environmental parameters (Botha-Brink and Smith, 2011;Botha-Brink et al, 2016), respiration and metabolic rates (Cubo et al, 2012;Seymour et al, 2004), large body size , a possible earlier onset of sexual maturity (Karkach, 2006), or an erect stance combined with an active predatory life style (Benton, 1984;Seymour et al, 2004). For instance, in an unstable environment, fast growth and earlier maturity may have been favorable , which is in agreement with the supposed habitat of Batrachotomus.…”

New insight into the evolution of the vertebrate respiratory system and the discovery of unidirectional airflow in iguana lungs

“…Although monitor lizards are ectotherms, their lifestyles are largely convergent with small predatory mammals (15) and they have high aerobic capacities compared with other lizards (16). In contrast, extant alligators have limited aerobic stamina (17) but their common ancestor with birds may have had a great aerobic capacity (18) or may have been endothermic (19,20). Crocodilians and monitor lizards also share a suite of features of their pulmonary and cardiac anatomy that have been purported to give rise to, or coevolve with, birdlike patterns of flow.…”