Protoclepsydrops was an early amniote, and its skeletal remains indicate that it was more closely related to synapsids than to sauropsids, making it a synapsid member. It is the oldest synapsid, though still unconfirmed because its remains were fragmentary. Protoclepsydrops lived slightly earlier than Archaeothyris. Like Archaeothyris, Protoclepsydrops resembled modern lizards. However, Protoclepsydrops still had primitive tetrapod-like vertebrae with tiny neural processes.