The evolution of respiratory systems in Theropoda and Paracrocodylomorpha, the end-Triassic extinction, and the role of Late Triassic atmospheric O2 and CO2

During the Late Triassic Period (235–201.3 Ma), Paracrocodylomorpha and Theropoda switched predatory roles, with the former filling the subsidiary predator niche and the latter filling the top predator niche at the end-Triassic extinction. Reasons for the transition in predator guilds remain unknown, but atmospheric conditions during this time, which involved high CO2 and low O2 concentration levels, may be associated with the event. Evidence of bird-like pneumatic post-crania, present in both groups as foramina and fossae, correlates with an avian-like respiratory system. This may have allowed organisms to cope with declining atmospheric O2 environments of the Late Triassic. This study estimated pO2 throughout the late Triassic using organic carbon isotope measurements from Newark-Hartford basin paleosols and examined the morphological change in skeletal remains of Theropoda, focusing on pneumatic bones, compared with Paracrocodylomorpha during the Late Triassic and across the Triassic-Jurassic boundary. With data compiled from the Paleobiology Database and published anatomical literature, the change of pneumaticity in taxa is quantified by a Pneumatic Index (PI), in which the number of pneumatized units is divided by the total number of bones examined. Patterns of pneumaticity are individually scored for the presence or absence of pneumatic bones. Morphological data are compared to corresponding femur length, estimated atmospheric O2 and CO2 level reconstructions, and an ancestral state reconstruction depicting how PI values change throughout various clades. Our results suggest that PI values in Theropoda and Paracrocodylomorpha correspond with the vacillating pO2 and pCO2 throughout the Late Triassic and into the Jurassic. High PI values are prevalent in advanced theropod taxa, while Paracrocodylomorpha PI values vary by clade with a generally negative trend throughout the Triassic. Ancestral state reconstruction analysis highlights the increasing PI values within Theropoda and clade dependent trends in Paracrocodylomorpha. This study is instrumental in reconstructing how Theropoda became evolutionarily successful and perhaps how and why the avian-like respiratory system originated. •Pneumaticity in theropods and paracrocs during the Late Triassic•Theropod and paracroc pneumaticity evolution•Pneumaticity evolution compared with the paleoclimate of the Late Triassic•Increased pneumaticity in theropods but decreased in paracrocs in the Triassic•O2 levels