The evolution of thermometabolism in pseudosuchians (Late Triassic to the present) remains a partly unsolved issue: extant taxa (crocodilians) are ectothermic, but the clade was inferred ancestrally endothermic. Here we inferred the thermometabolic regime of two neosuchian groups, Goniopholididae (Early Jurassic to Late Cretaceous) and Dyrosauridae (Middle Cretaceous to Late Eocene), close relatives of extant crocodilians, in order to elucidate the evolutionary pattern across Metasuchia (Early Jurassic to the present), a clade comprising Neosuchia (Early Jurassic to the present) and Notosuchia (Middle Jurassic until Late Miocene). We propose a new integrative approach combining geochemical analyses to infer body temperature from the stable oxygen isotope composition of tooth phosphate and palaeohistology and phylogenetic comparative methods to infer resting metabolic rates (RMR) and red blood cells dimensions. †Dyrosaurus and †Goniopholis share with extant crocodilians similar lifestyles, body forms, bone tissue organization, body temperatures (Tb), metabolic rates and red blood cells dimensions. Consistently, we infer ectothermy for †Dyrosaurus and †Goniopholis with the parsimonious implication of neosuchians and metasuchians as being primitively ectothermic.

Data used in PEM inferences, which are the results of the quantification of microstructural bone structures (osteons, osteocytes lacunae) on pictures of femoral cross-section using ImageJ. Values were obtained using the equations given in the paper. V1 for Table Sup 2 and V1+V2 for Table Sup 3 were obtained from the literature given in the paper.