Wikipedia
Archaeothyris
Archaeothyris is an extinct genus of ophiacodontid synapsid that lived during the Late Carboniferous and is known from Nova Scotia. Dated to 306 million years ago, Archaeothyris, along with a more poorly known synapsid called Echinerpeton, are the oldest undisputed synapsids known. Protoclepsydrops also from Nova Scotia is slightly older but is known by very fragmentary materials.