Pseudosuchian thermometabolism: A review of the past two decades.

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Pseudosuchia, one of the two main clades of Archosauria, is today only represented by some 20 extant species, the crocodilians, representing only a fraction of its extinct diversity. Extant crocodilians are ectotherms but present morphological and anatomical features usually associated with endothermy. In 2004, it was proposed that pseudosuchians were ancestrally endothermic and the features observed in extant crocodilians are the remains of this lost legacy. This contribution has two parts: the first part covers 20 years of studies on this subject, first exploring the evidence for a loss of endothermy in extant crocodilians, before covering the variety of proxies used to infer the thermophymetabolic regime of extinct pseudosuchians. In the second part, the quantitative results of these previous studies are integrated into a comprehensive ancestral state reconstruction to discuss a potential scenario for the evolution of thermometabolism. Pseudosuchian endothermy would then have been lost close to the node Crocodylomorpha. The end-Triassic mass extinction is proposed to have played the role of a filter, leading to the extinction of endothermic pseudosuchians and the survival of ectothermic ones. This difference in survival in Pseudosuchia is compared to those of dinosaurs, and difference in their metabolism is also considered. Pseudosuchian endothermy might have been of a different level than the dinosaurian one and more studies are expected to clarify this question.