Abstract

The study of the evolution of bone growth rates and metabolic rates in archosaurs (crocodiles, dinosaurs including birds, and pterosaurs) and close outgroups became a subject of major interest among paleontologists in recent years. In this paper, we estimate the bone growth rate of Euparkeria using a new statistical inference model for the humerus. We modified the taxonomic range of extant species used in previous studies, on which we performed quantitative measurements of histological features and bone growth rates. Bone growth rate values estimated for Euparkeria are crucial to understand the ancestral condition for archosaurs because this taxon is considered the closest relative to the archosaur crown-group. We obtained an instantaneous growth rate value of 6.12 µm/day, suggesting that Euparkeria shared with other non-archosaurian archosauriforms (Prolacerta, Proterosuchus and Erythrosuchus) a condition of high growth rate compatible with endothermy. This derived state may have been inherited by some Triassic crurotarsi, as suggested by the high instantaneous bone growth rate (14.52 µm/day) estimated in this study for Postosuchus. Jurassic crurotarsi may have lost endothermy during the transition from terrestrial habitats and active predation behaviors to aquatic habitats and sit-and-wait predation behaviors, and Cretaceous crocodiles may be secondarily ectotherms, as suggested by δ18O values. In conclusion, we provide new evidence for the hypothesis of an ancestral endothermic state for the last common ancestor of archosaurs, and predict that non-archosaurian archosauromorphs and Triassic crurotarsi may have been characterized by a thermometabolism more similar to that of dinosaurs than to that of lepidosaurs and turtles.