Metabolic features of enantiornithines (Avialae: Ornithothoraces): evidence supporting a semi-precocial/precocial developmental trajectory.

The enantiornithes, a group of Mesozoic birds, have morphological features suggesting a precocial mode of development; however, their detailed physiological characteristics, particularly hatchling metabolic rates, have yet to be explored. Here, we retrodicted metabolic rates for four Lower Cretaceous enantiornithines: one hatchling, one adult from Las Hoyas, one early post-hatching juvenile and one post-juvenile individual from the Jehol Biota, using phylogenetic comparative analyses and a comprehensive dataset of extant birds spanning the altricial-precocial spectrum. By examining vascular density (related to resting metabolic rate (RMR)) and nutrient foraminal size (proportional to blood flow rate) as proxies, we inferred the metabolic scope of the hatchling enantiornithine and determined that it fell toward the precocial end of the altricial-precocial spectrum. The RMR values of the two enantiornithine chicks were retrodicted as 0.67 mlO2 h-1 g-0.93. The peak metabolic rate (PMR) retrodicted is twice the resting metabolism for the Las Hoyas hatchling. These results are consistent with a semi-precocial/precocial development characterized by independent locomotion by climbing using the wing claws soon after hatching. However, flapping flight may have been delayed, owing to incomplete ossification of the sternum, as observed in one hatchling specimen.