Evolutionary stasis in synapsid encephalization during the end-permian mass extinction.

Brain size and encephalization are correlated with extinction risks, making them central to understanding evolutionary outcomes in the face of the climatic and ecological challenges posed by mass extinction events. Yet, the end-Permian mass extinction-the most significant extinction event in the history of Life-has received little attention from a paleoneurological perspective. Here we use Synchrotron Radiation and CT scanning to study the evolution of brain endocasts of mammalian forerunners, the Permo-Triassic Synapsida, across this extinction event. During mass extinction events, it is expected that the brain would adapt either by growing larger to enhance behavioral response to the challenges, or shrinking to save energy. To test these predictions, we conducted a broad phylogenetic survey of synapsids across the end of the Permian and, unexpectedly, found stasis of the encephalization quotient. Furthermore, analysis of evolutionary rates indicates that stabilizing selection does not account for the observed stasis. We hypothesize that a lack of resources may have impeded neuroplasticity and prolonged a period of stasis that began in the late Permian. Brain size resumed varying after the crisis, marking the first steps toward the evolution of the mammalian brain.