Ophiacodontidae is a family of pelycosaur synapsids that first appeared during the Late Carboniferous period. Archaeothyris, and Clepsydrops were among the earliest Ophiacodontids. Archaeothyris and its relatives were the members of this family. Some ophiacodonts were semi-aquatic, and few were fully aquatic, but some were fully terrestrial like Archaeothyris. They resembled lizards in shape. They have strange, elongated skulls, they also have massive shoulder girdles, probably to provide muscle attachment to support the weight of the huge head. This family is the most primitive among synapsids and may have been ancestral to all pelycosaurs - which includes therapsids and their descendants, the mammals. By the Middle Permian, the ophiacodontids were becoming rare, and they were eventually