Origins of Avian Hyperactive Mitochondria, Genome Compaction, and Air-Sac Physiology in Early Theropods During the Carnian Pluvial Episode.

Extant birds and the earliest dinosaurs may share fundamental metabolic features essential for aerobic exercise, suggesting that the extraordinary physical performance typical of avian species originated when dinosaurs first appeared during the Carnian Pluvial Episode (CPE). This physiological adaptation is complemented by hyperactive mitochondria that exhibit high oxygen consumption and low reactive oxygen species production. Molecular genomics of fossils, the so-called "Jurassic Genome," indicates that these early dinosaurs possessed compact genomes, 50-60% the size of the human genome, and small cells, implying a highly stringent metabolic regime. We suggest that hyperactive mitochondria, closely associated with compact genomes and small cells, drive theropod adaptation to the hot, dry, and hypoxic environments of the Late Triassic period, ultimately enabling their ecological dominance. Early dinosaurs such as Herrerasaurus are hypothesized to have possessed advanced physiological traits shared with modern birds, including hyperactive mitochondria, compact genomes, small cells, and a developing air-sac system. Collectively, these features most likely may have contributed to exceptional metabolic capacity, locomotor performance, and adaptation to the harsh environment of the CPE.