Protoclepsydrops
The skeletal remains of Protoclepsydrops suggest it may have been a synapsid. If so, it is the oldest synapsid known, though its status is unconfirmed because its remains were fragmentary. Protoclepsydrops lived slightly earlier than Archaeothyris, which was definitely a synapsid. Like Archaeothyris, Protoclepsydrops had a body shape which resembled modern lizards.