One temporal opening behind each eye. Started when early amniotes evolved a skull with one temporal opening behind each eye. That opening marks the mammal-side branch apart from the reptile/bird side.
For the end of the older synapsid stage, I’d use late caseids as your marker. Caseids are basal/non-therapsid synapsids that persisted into the Middle Permian, with the last known forms around the Roadian–Wordian range, before disappearing as therapsids took over.
Last Non-Therapsid Synapsids Read More »
Echinerpeton and Archaeothyris are among the oldest known synapsids. They mark the mammal-side branch of amniotes, recognized by the single temporal opening behind the eye. UCMP frames Synapsida as a 300-million-year-old group with mammals as one surviving branch, while Archaeothyris is commonly treated as one of the oldest undisputed synapsids.
