Lower Permian to Middle Trias. Families : Diadectidae, Pariasauridae, Procolop/ionidae.
Super-order Theromorpha (or Anomodontia). Mammal-like reptiles. Reptiles in which the temporal region of the skull is per forated by a single vacuity, bounded primitively by the postorbital and squamosal, but enlarging so that the parietal and jugal also enter its borders. Cranium short and high behind the incisura prooticum, fenestra ovalis below the level of the base of the brain. Stapes articulating with the quadrate. The lower jaw, and espe cially its hinder end, laterally compressed, the angular in all ex cept, perhaps, the most primitive forms, with a notch in its lower border.
A pro-atlas present, the neural arch of the atlas usually a pair of bones, which, with the intercentrum, rest on the anterior end of a trefoil-shaped odontoid. Vertebrae with slender neural arches with oblique articular faces, centra notochordal or deeply amphi coelous. Intercentra usually present, at any rate in the cervical region, ribs always two-headed anteriorly, usually single-headed posteriorly. An ossified sternum sometimes present, abdominal ribs present as slender rods in primitive forms. Pectoral girdle with scapula, precoracoid and coracoid in all forms (except one, Varanops), clavicle with expanded lower end, and interclavicle a wide flat sheet. Cleithra usually present, but small. Pelvis very variable, plate-like in primitive forms, with an obturator foramen in later types. Ilium, directed forwards, vertically, or backward. Two to seven sacral vertebrae. Limbs exhibiting all stages of ad vance from primitive cotylosaur-like organs to a pro-mammalian condition. Digital formula primitively 2, 3, 4, 5, 3 or 4 reduced to 2, 3, 3, 3, 3 in later forms.
Order 1. Pelycosauria. Primitive Theromorpha in which the pterygoids articulate with the basipterygoid processes of the basisphenoid by a movable joint. The quadrate is relatively large and the quadrato-jugal forms part of the lateral surface of the skull. The shoulder girdle has a screw-shaped glenoid cavity shared by the scapula, coracoid and precoracoid, and the limbs are primitive. The pelvis is plate-like.
Upper Carboniferous and Lower Permian. Families : Polio sauridae, Ophiacodontidae, Sphenacodontidae, Edaphostauridae, Caseidae, Bolosauridae, Palaeohatteriidae.
Order 2. Deinocephalia. Theromorpha in which the pterygoids are attached to the basisphenoid by an immovable joint. The basioccipital and basisphenoid are produced downward below the occipital condyle as a thick sheet of bone. The quadrate is unre duced and the quadrato-jugal is on the lateral surface. The shoulder girdle in early forms has the glenoid cavity borne only to a very slight extent the precoracoid, but it is screw-shaped; in later forms this structure disappears and the glenoid cavity is re stricted to the scapula and coracoid. The limbs are of modern ized type. The pelvis is plate-like, the ilium being attached to four sacral ribs.
Middle Permian. Families : Tapinoceplialidae, Titanosuchidae.
Order 3. Dromosauria. Small Theromorpha in which the facial region of the skull is very short, the temporal fossa is bounded above by the postorbital and squamosal and the zygomatic arch is reduced to a narrow rod so that the quadrate and quadrato jugal project below it. Shoulder girdle with the glenoid cav ity on the scapula and coracoid, precoracoid large. No cleithra. Pelvis plate-like. Limbs very long and slender, digital formula 2, 3, 3, 3, 3.
Upper Permian. One family only.
Order 4. Dicynodontia (or Anomodontia). Theromorpha in which the preorbital part of the skull is very short, whilst the temporal vacuity is greatly enlarged. The latter is bounded above by the postorbital and squamosal. The quadrate and quadrato jugal are reduced, and rest in a recess in the front face of the lower end of the T-shaped squamosal, which is widely expanded laterally so as to form a sheet in the plane of the occipital surface. The pterygoids are rigidly fixed to the basisphenoid, and are not pro duced into transverse flanges. The premaxillae are fused and toothless, the maxillae may have a large canine or a series of small cheek teeth, or both, or be toothless. A horny beak like that of a tortoise was always present. The articular of the lower jaw always has the unique feature of a convex articular surface. Intercentra are absent except in the atlas and axis. The tail is short. The glenoid cavity is entirely, or almost entirely, restricted to the scapula and to the coracoid. There is an acromium on the scapula which also shows the beginnings of a mammal-like scapu lar spine.