Tetraceratops, once classified as a pelycosaur, is now considered the most primitive therapsid which is intermediate between the pelycosaurs and therapsids. It possessed three pairs of horns on its premaxillary, prefrontal, and angular bones. There are a number of characteristics of the teeth and jaws which unite Tetraceratops and all therapsids (the loss of precanine and ectopterygoid teeth; reduction of quadrate, epipterygoid, and interpterygoid vacuity; and the modification of the lower temporal fenestra). There are also a number of traits which unite all therapsids other than Tetraceratops as being more advanced (such as the increased size of the septomaxilla, maxilla, loss of the basipterygoid articulation, equalization of incisor size, and the exclusion of the lacrimal bone from the narial region). Pelycosaurs are first known from the Middle Pennsylvanian, reach their greatest diversity in the Early Permian and become extinct by the Late Permian. Tetraceratops, the oldest and most primitive therapsid, is known from the Early Permian (Laurin, 1990).
The next oldest therapsids are known from
In primitive therapsids, there was a combination primitive and derived features.
A number of therapsid groups evolved.
Eotitanosuchians were the earliest and most primitive therapsids. The pterygoid bone in the roof of the mouth still included teeth (unlike mammals which only have teeth in the maxillary and premaxillary) and the 2 vomer bones of the nasal septum were only partly fused (in mammals, there is only one vomer). The stapes had become a smaller bone that was no longer has to support the back of the skull. It contacted the quadrate bone of the upper jaw and both these bones would be middle ear bones in mammals. Phtinosaurus and Pthinosuchus were the most primitive eotitanosuchians. They retained primitive pelycosaur features including a curved top of the skull and more post-canine teeth. Biarmosuchus had slender and more agile limb bones. The pelvis still retained a large pubis and the ilium was only slightly expanded. The femur included modifications seen in later therapsids such as a more medial head (which would allow therapsids to walk more upright) and 2 bumps called trochanters. Biarmosuchus was related to the gorgonopsids given that they both had a preparietal bone between frontal and parietal bones of the skull (Kemp, 1982; Carroll, 1988).
Dinocephalians were large reptiles that
dominated the therapsid fauna of
Dicynodonts evolved from the dinocephalians and later replaced them. Venyukoviamorphs seem to be intermediates between the two groups. They were exclusively herbivores. In terms of numbers, they were the most successful of the early therapsids and by the Lower Permian, their fossils can represent more than 90% of the specimens found. They radiated again in the Triassic, persisting almost to the end of the period (Kemp, 1982; Carroll, 1988).
Gorgonopsids first appeared in the Lower Permian and were the main carnivores of the Permian (probably feeding dinocephalians and dicynodonts). They become extinct at the end of the Permian. Most of their derived characteristics were adaptations for feeding on large prey. Because of their large canines, they are sometimes called “saber-toothed reptiles”. The postcanine teeth were reduced to 5 small teeth, the temporal opening was enlarged, a preparietal bone was present (showing a relationship to Biarmosuchus), the toes were more equal in size to each other, there were no intercentra in the trunk vertebrae (although there were still intercentra in neck to support neck movements), there were 3 sacral ribs, the tail was smaller, the vertebral column no longer laterally undulated during movement (as in lizards), the pineal foramen was still present, and the stapes was smaller resting on the quadrate. It is unknown whether there was an eardrum. The palatine bones (small bones of the roof of the mouth whose role will be discussed in the next section) met in the midline of the oral cavity to form the palate (also found in eotitanosuchians) (Kemp, 1982; Carroll, 1988).
Although gorgonopsid forelimbs were just as sprawling as those of pelycosaurs, the hindlimbs were held more erect. They would have been capable of both a sprawling and a semi-erect gait as seen in modern crocodiles. Their ankle was structured to support both gaits. The brain was still long and tubular and there was no increase in size since the pelycosaur. Lycaenops resembles mammals postcranially. On other words, other than the skull, the skeleton was approaching the mammalian form.
5) Ictidorhinids (hipposaurids)
Ictidorhinids were a South African group of gorgonopsids with primitive features similar to eotitanoscuhians.