Archaeothyris is an extinct genus of ophiacodontid synapsid that lived during the Late Carboniferous and is known from Nova Scotia.

[3] The name means ancient window (Greek), and refers to the opening in the skull, the temporal fenestra, which indicates this is an early synapsid.

Protorothyris archeri Caseasauria Ianthodon schultzei Edaphosauridae Sphenacodontia Varanopidae Archaeothyris florensis Varanosaurus acutirostris Ophiacodon spp.

Archaeothyris lived in what is now Nova Scotia, about 306 million years ago in the Carboniferous Period (Pennsylvanian).

Archaeothyris and the other early amniotes lived in the moist vegetation on the forest ground, together with the more terrestrially adapted labyrinthodont amphibians.