THE PERMIAN-TRIASSIC CLIMATE CRISES AS KEY DRIVERS FOR THE RADIATION OF REPTILE BODY PLANS (Invited Presentation)

The fossil record provides a unique opportunity to understand long-term organismal responses to major environmental disruptions throughout Earth’s history. Although biotic response to environmental change has been extensively studied in marine invertebrates, comparably little data exists for many vertebrate groups due to various challenges: (1) limited sample sizes; (2) absence of appropriate analytical tools to analyze the available data, and finally; and (3) long-standing conflicts regarding fundamental aspects of vertebrate phylogeny.

Particularly contentious have been the relationships between the major groups of reptiles (notably turtles). Though reptiles have survived several major environmental crises, the influence of these on their evolutionary history has been difficult to test in the absence of a robust phylogeny. Here we provide a new morphological dataset inclusive of all major lineages of early reptiles, early synapsids, and their closest relatives (early amniotes), representing the largest taxon sample ever assembled for part of the tree of life. We explored this dataset with emerging techniques from Bayesian evolutionary analyses and new approaches developed by us to measure macroevolutionary dynamics in fossil organisms to understand Earth-life system interactions during this critical time in Earth’s history.

Our results provide a new, highly precise evolutionary time-tree for early amniotes. We overhaul traditional hypothesis of amniote evolution; reconstructing several “reptile” groups as actually belonging to the amniote stem. Additionally, we provide the first agreement between morphological and molecular hypotheses on the position of turtles as the sister group to archosaurs. By combining phenotypic rate, selection mode, body size, and global temperature data, we reveal an intimate association between reptile evolutionary dynamics and climate change in the deep past. We show that the origin and phenotypic radiation of reptiles was not solely driven by ecological opportunity following the end-Permian extinction as previously thought, but also the result of multiple adaptive responses to climatic shifts spanning 57 million years bracketing the Permian-Triassic boundary.