Archaeothyris was a small, lizard-like animal that lived in the late Carboniferous, specifically 300 mya in the Pennsylvanian Epoch, making it the oldest pelycosaur. Pelycosaurs were a family of mammal-like reptiles that all had strong jaws and sharp, varied teeth, and were therefore carnivorous. Fossils have only been found in Nova Scotia, Canada. During the time it lived, this area was a warm, humid jungle with many trees and ferns. It had a long tail, sharp teeth, and strong jaws. The jaws could be opened very wide. The sharp teeth, because they were various sizes and included large canines at the front of the mouth, suggest that it was a carnivore, feeding on small reptiles (it was about 20 inches long).