Archaeothyris

Archaeothyris was the earliest definite synapsid. It lived in the late Carboniferous period, 306 million years ago. 
It was found in Nova Scotia, at the same locality as "Hylonomus", and "Petrolacosaurus", which are similar to "Archaeothyris", but probably sauropsids. 
"Protoclepsydrops" is slightly older, but its status as a synapsid is unclear as the remains are more fragmentary.
Systematics.
"Archaeothyris" belonged to a group of early pelycosaurs that evolved early in the Pennsylvanian. It is thus seen as the precursor of all synapsids (which include mammals).
Appearance and lifestyle.
Unlike the early sauropsids, "Hylonomus" and its kin, "Archaeothyris" was relatively large, measuring head to tail. It was also more advanced than the early sauropsids (~reptiles), with strong jaws that could open wider than those of the sauropsids. Its sharp teeth were all of the same shape, and it had a pair of enlarged canines, suggesting that it was a carnivore.
"Archaeothyris" lived in what is now Nova Scotia, about 306 million years ago in the Carboniferous Period (Pennsylvanian). Nova Scotia at this time was a swamp, similar to today's Everglades in Florida. The 'trees' (actually giant clubmosses) were very tall. Some, such as "Lepidodendron", were up to tall. "Archaeothyris" and the other early amniotes lived on the forest floor.