A clash of clades : the evolution and ecology of terrestrial amniotes through the Permo-Triassic

The late Palaeozoic and early Mesozoic saw the birth of modern terrestrial ecosystems and clades as tetrapods embarked on conquest of the land. The onset of full tetrapod terrestrialisation in the Carboniferous sparked diversification that saw two amniote clades become the principal actors in the terrestrial realm, the Synapsida and Archosauromorpha, which came to dominate terrestrial faunas in the late Palaeozoic and early Mesozoic, respectively. Their patterns of diversification may help shed light on the way intrinsic and extrinsic pressures direct the course of evolution, as their relative success ebbed and flowed against a backdrop of the profound environmental changes that punctuated their evolution. Extrinsic changes are undoubted agents of macroevolutionary change, as they are associated with multiple mass extinction events and faunal turnovers. Extinction events help reset eco-evolutionary dynamics by giving new or minor clades the chance to diversify across previously occupied niches, but what drives differential survival and success through these events and in their aftermath? Here, I investigate how susceptibility to extinction and radiation are linked to ecology, as an animal's ecology determines its capacity to respond to environmental changes and their potential intrinsic interactions, which are key to survival and success through and after extinction events. Using the functional anatomy of synapsids and archosauromorphs through the late Palaeozoic and early Mesozoic macroevolution, I assess how their ecology varied in response to intrinsic and extrinsic changes to identify the key driving forces behind their macroevolution. I find that morpho-functional diversity is highly segregated by clade, particularly through times of environmental stability, reflecting strong niche partitioning and intrinsic constraint. However, I find the largest changes in ecomorphology are associated with large-scale extrinsic events, with the loss of competitors through these events often preceding eco-morphological diversification in the survivors. Consequently, intrinsic factors such as innate anatomy and competition appear to be key controls on ecological diversity, but environmental events are the primary drivers of large-scale changes in a clade's macroevolution.