First Therapsids
Therapsids mark the point where the mammal-side branch became more active, more specialized, and more clearly aimed toward the later mammal body plan.
~203 MYA Mammaliaforms
Mammaliaforms mark the near-mammal threshold. Early forms such as Morganucodon had mammal-like chewing teeth, a more mammal-like jaw joint, and bones near the jaw that were shrinking and shifting toward the middle ear. They were not crown mammals yet, but they crossed into the fuzzy zone where cynodont descendants became recognizably mammal-like.
Last Non-Mammal Mammaliaforms
Not every mammaliaform became a mammal; some were late-surviving side branches near the mammal line.
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First Synapsids
Early synapsids looked reptile-like, but they were not on the reptile line. They were the first recognizable step on the long road toward mammals.
First Mammaliaforms: Morganucodon
Morganucodon was an early mammaliaform from about 200 million years ago. Crown mammals emerged from within this group about 170 million years ago.
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