Archaeothyris

Archaeothyris was a very early mammal-like reptile, which lived in the late Carboniferous period. Dated to 306 million years ago, it is the oldest undisputed synapsid known. It was found in Nova Scotia, at the same locality as Hylonomus, and Petrolacosaurus, all of which resemble Archaeothyris.

Systematics
Archaeothyris belonged to the family Ophiacodontidae, a group of early pelycosaurs that evolved early in the Late Carboniferous. It is thus seen as the precursor of all synapsids (which include mammals).

Appearance and lifestyle
Unlike Hylonomus and its kin, Archaeothyris was relatively large, measuring 50 centimeters (20 in) head to tail. It was also more advanced than the early sauropsids, having strong jaws that could open wider than those of the early reptiles. While its sharp teeth were all of the same shape, it did possess 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 club mosses) were very tall, some, such as Lepidodendron, up to 50 meters (164 ft) tall. Archaeothyris and the other early amniotes dwelled on the forest ground.