Haptodus longicaudatus (Credner 1888, H. garnettenis Currie 1977) lived ~305 mya, Kasimovian (Late Carboniferous) , 60 cm long, was similar in size to its predecessor, Archaeothyris and was a sister to the other pelycosaurs, Ophiacodon, Edaphosaurus, Haptodus and Dimetrodon. Haptodus specimens started off small in the Late Carboniferous and continued to grow into the Early Permian. H. garnettensis most closely resembled Ophiacodon. H. saxonicus most closely resembles Sphenacodon and Dimetrodon.
Distinct from Ophiacodon the skull of Haptodus was short, like that of Archaeothyris. The maxilla was deeply convex ventrally but beneath the lateral emporal fenestra the jawline was deeply concave as the quadrate dropped below the jawline. The lateral temporal fenestra was enlarged, approaching the jaw line. The quadratojugal was reduced to a nearly invisible vestige at the tip of the quadrate. The mandible developed a reflected lamina, a thin sheet of bone that projected posteriorly off the angular.
The cervicals were slightly longer. The neural spines were slightly shorter.
The ilium developed an anterior process.
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A sister to Haptodus, Stenocybus nests at the base of the Therapsida.