EVIDENCE FOR HIGH BONE GROWTH RATE IN EUPARKERIA OBTAINED USING A NEW PALEOHISTOLOGICAL INFERENCE MODEL FOR THE HUMERUS

The study of bone growth rate and metabolic rate evolution in archosaurs (crocodiles, dinosaurs including birds, and pterosaurs) and close outgroups has become 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 in understanding the ancestral condition for archosaurs because this taxon is considered the closest relative to the archosaur crown group. We obtained an instantaneous growth rate of 6.12m/day, suggesting that Euparkeria shared with other non-archosaurian archosauromorphs (Prolacerta, Proterosuchus, and Erythrosuchus) a condition of high growth rate compatible with endothermy. This derived state may have been inherited by some Triassic crurotarsans, as suggested by the high instantaneous bone growth rate (14.52m/day) estimated in this study for Postosuchus. Jurassic crurotarsans may have lost endothermy during the transition from terrestrial habitats and active predation to aquatic habitats and sit-and-wait predation behaviors, so that Cretaceous crocodiles may be secondarily ectothermic, as suggested by O-18 values. In conclusion, we provide new evidence for the hypothesis of an ancestral endothermic state for the last common ancestor of archosaurs, and show that non-archosaurian archosauromorphs and Triassic crurotarsans may have been characterized by a thermometabolism more similar to that of dinosaurs than to that of lepidosaurs and turtles.