101.3 Saturday, Jan. 7 Early evolution of elevated growth and metabolic rates in archosaurs WERNING, S*; IRMIS, RB; NESBITT, SJ; SMITH, ND; TURNER, AH; PADIAN, K; Univ of California, Berkeley; Univ of Utah; Univ of Washington; The Field Museum; Stony Brook Univ; Univ of California, Berkeley firstname.lastname@example.org
Birds exhibit much higher growth and metabolic rates compared to other extant reptiles. Bone histology establishes that dinosaurs and pterosaurs also grew at elevated rates, but it remains uncertain when these features evolved, temporally or phylogenetically. We expand the histological database of archosaurs and their ancestors to include early archosauromorphs, pseudosuchians, and dinosauromorphs, tracking changes in growth rate and its underlying metabolism through deep time and in taxa whose character states are not represented among living animals. Our study differs from previous works in its approach, phylogenetic breadth, and level of taxonomic sampling, but also in that we used apomorphy-based identifications for all specimens and sampled at homologous locations from individuals of comparable ontogenetic stage. We mapped characters relevant to growth and metabolism (e.g., osteocyte density, collagen organization, osteonal development, vascularity) on a recent phylogeny of archosauromorph reptiles to assess where particular adaptations of growth dynamics first evolved, focusing on the lineages leading to Archosauria, Crocodylomorpha, Dinosauria, and Theropoda. Many histological features associated with high growth and metabolic rates evolved much earlier than the common ancestor of birds and pterosaurs, and several aspects of the accelerated growth syndrome did not evolve simultaneously. Most of these character changes accumulated in a short segment of the archosauriform tree before the end of the Early Triassic. Many physiological features related to the high growth and metabolic rates of living birds evolved before the most recent common ancestor of crocodiles and dinosaurs.