Therapsids are the advanced synapsids that began tightening the mammal-side body plan. Their most visible shift was toward a more upright posture, with limbs moving farther under the body instead of sprawling far out to the sides. Alongside that came a more powerful skull, more specialized teeth, and signs of a more active lifestyle. They were not mammals yet, but this is where the old synapsid line began to look more focused, more predatory in some branches, more specialized in others, and more clearly aimed toward the later mammal story.
To set the stage, picture the first therapsids living during the Middle Permian, around 272 million years ago. This is when Pangaea dominated the planet. Their world was warmer, more seasonal, and often drier than the earlier Carboniferous coal forests. A more upright posture allowing for running on dry land had real advantages. Picture open floodplains, river channels, muddy basins, patchy wetlands, seed ferns, horsetails, early conifers, and broad stretches of exposed earth. This was a changing world: less swamp-jungle, more seasonal land. In that setting, faster, more active synapsids had room to rise.
Raranimus dashankouensis is a good example of one of the earliest therapsids. It is known from a partial skull from Middle Permian China, and a 2021 reassessment confirmed it as a basal member of Therapsida. A 2024 paper also notes that until a newer early-middle Permian gorgonopsian find, Raranimus was considered the oldest known unequivocal therapsid. It still carried older synapsid features, but its skull and teeth show the early therapsid direction: sharper jaws, predatory adaptations, and the beginning of the line that would eventually include cynodonts, mammaliaforms, and mammals.
