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.
~170 MYA Crown Mammals
Crown mammals mark the ancestry of all living mammals: monotremes, marsupials, and placentals. By this point, the mammal line had moved beyond near-mammal forms into the branch that still survives today. The defining story is not just mammal-like anatomy, but shared descent from the last common ancestor of all living mammal groups.
First Cynodonts
Cynodonts were a more advanced branch of therapsids where the mammal-like jaw, teeth, palate, and breathing package began to come together.
First Crown Mammals
Crown mammals are the branch that includes the common ancestor of all mammals alive today and every descendant of that ancestor.
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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.