Does the Maximum Body Size of Theropods Increase across the Triassic–Jurassic Boundary? Integrating Ontogeny, Phylogeny, and Body Size

Mass extinctions change global ecosystems, and the end-Triassic mass extinction was hypothesized to have precipitated the rise of dinosaur dominance, with dinosaurs filling resource zones of eliminated large-bodied reptilian lineages. This replacement has been explicitly hypothesized for theropod dinosaurs, and the eastern North American theropod footprint record suggests an increase in maximum body size across the Triassic-Jurassic boundary. Without taking ontogenetic stage in account, the maximum size of the rare large Triassic theropods worldwide supports this hypothesis, with the size of the largest individuals corresponding to the largest Triassic theropod tracks. However, both morphological data and histological examination suggest that known large-bodied Triassic theropods are represented by immature individuals still growing rapidly at the time of death, indicating that the maximum body size of Triassic theropods was much larger than that a strict reading of the body fossil record would suggest. The size increase recorded in the sediments of eastern North America is not part of a global trend. Instead of a simple ecological replacement of non-dinosaurian archosaurs by dinosaurs, the rise in theropod dinosaurian ecological dominance was an extended process across the end of the Late Triassic into the Jurassic. Anat Rec, 303:1158–1169, 2020.