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.
First Synapsids
~308 Million years ago
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