150 years of synapsid paleoneurology: the origins of the mammalian brain, behavior, sense organs and physiology

Abstract A century and a half of paleoneurological study of synapsids has provided invaluable insight into the evolution of their brain, sense organs, behavior, and physiology. Here, we review and discuss the evidence for parental care, brooding, intraspecific combat, display, and gregariousness, and conclude that evidence for higher levels of social interactions and communication is piling up and may soon push the origin of sociality in the mammalian lineage to the middle Permian. We also review the paleoneurological cues (the trigeminal canals, parietal foramen, and inner ear) that support a new evolutionary hypothesis in which the homeogene MSX2 mutated early in the probainognathian cynodonts and changed their biology towards a more mammalian condition. This includes the loss of the parietal foramen, inflation of the cerebellar vermis, maintenance of a fur pelt, and appearance of mammary glands, some 247 million years ago. This was followed by the origin of the ability to whisk 241 million years ago, and that of endothermy 233 million years ago, as indicated by the evolution of the trigeminal and semicircular canals, respectively. Finally, we review the immense progress made in the study of encephalization and support that probainognathians went through a neurosensory revolution during the Triassic. Their newly acquired small body size, fur, and nocturnal lifestyle generated sensory input that affected the evolution of all their sensory organs, leading up to the development of the modern mammalian brain.