Archaeothyris

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Archaeothyris was an extinct, early synapsid that lived about 306 million years ago in what is now Nova Scotia, Canada. It is one of the oldest undisputed synapsids, along with Echinerpeton. The name means "ancient window," referring to a hole in its skull called the temporal fenestra.

Archaeothyris belonged to the ophiacodontid family of early pelycosaurs, animals that predate mammals. It had strong jaws and teeth that were all similar in size, but it also had a pair of larger canine teeth, suggesting it ate meat. Its legs faced out to the sides, giving it a sprawling, reptile-like gait.

Fossils were first described in 1972 from the Joggins Fossil Cliffs, a site also known for early reptiles Hylonomus and Petrolacosaurus. At that time, Nova Scotia was part of a coal forest with tall club mosses like Lepidodendron, where Archaeothyris lived alongside other early amniotes and amphibians.