Trophic ecology outweighed intrinsic constraints in shaping skull evolution of carnivorous Permian synapsids

The first truly terrestrial apex predators were carnivorous synapsids, which emerged in the Permian over 260 million years ago and evolved against a backdrop of harsh ecological change. In many ways, these predators mirrored feeding modes and evolutionary trends seen in their much later descendants, the flesh-eating mammals; could apparent resemblances indicate evolutionary constraints on form, or were they shaped by natural selection? Here we show that the skulls of carnivorous Permian synapsids were shaped primarily by adaptation, their shapes reflecting trophic function, and with similarities between distant relatives arising by convergence through natural selection. Conversely, we find no evidence for constraint in terms of the direction or rate of evolution based on patterns of topological modularity. These findings illustrate methods of identifying evolutionary processes in deep time and emphasise the role of competition and adaptation over intrinsic constraints in macroevolution.