The major split in the amniote family tree separated the synapsid lineage, which includes mammals, from the sauropsid lineage, which includes reptiles and birds, roughly 312 million years ago. It was not a Triassic split between mammals and diapsid reptiles.
Synapsids were not “mammal-like reptiles”
Early synapsids and later non-mammalian synapsids belong on the stem leading toward mammals, but calling them reptiles obscures the branching pattern. Features associated with mammals, including jaw and ear changes, hair, endothermy and secondary palates, evolved step by step in different lineages.
From stem mammals to mammals
Therapsids contain many important non-mammalian synapsids, while animals such as Morganucodon are better described as early mammaliaforms or stem mammals. Evolution does not work as a straight ladder in which one modern group turns into another.
Why the timing matters
The Carboniferous origin of synapsids, later diversification of therapsids and early mammaliaforms, and much later Neogene history are separate chapters. Accurate time scales help explain both shared ancestry and the deep differences among living amniotes.
Use current terminology
Modern terms such as synapsid, sauropsid and stem mammal communicate relationships more clearly than outdated labels. They also make fossils more interesting by placing them on a branching family tree.
