There are two groups of advanced therapsids: therocephalians and cynodonts.  Advanced therapsids had a common ancestor (given similarities in their jaws and braincase) and lived from the Late Permian through the Lower Triassic.  Both have a secondary palate (a bony roof of the mouth separating the oral cavity from the nasal cavity) but in therocephalians it included the vomer while in cynodonts (and mammals) it is composed only of palatine and maxillary bones.  The secondary palate ensures a separation between the air in the nasal cavity and the food in the oral cavity.  In reptiles and early synapsids, the nasal cavity emptied directly into the oral cavity (Kemp, 1982; Carroll, 1988). 

palate 2

     The advanced therapsids were more diverse than the gorgonopsids and included insectivorous, herbivorous, and large carnivorous animals.  The jaw muscles expanded over the dorsal region (top part) of the skull.  Mammals evolved from the cynodonts.

Therocephalians included the following genera:

scaloposaurus bauria
theriognauths pristerognathus



   In cynodonts (and mammals), the palatine forms part of the secondary palate while it remains a minor bone in other groups.  The lumbar ribs (ribs of the lower back) were reduced and fused to form part of the vertebrae.  The two heads of each rib join (to form a head and capitulum).  In the Triassic, the cynodonts replaced the therocephalians and gorgonopsids.  The presence of a secondary palate is evidence that the cynodonts needed more oxygen for increased metabolism, given that the secondary palate ensures an air supply, even when the mouth is full.  Other traits which suggest a higher metabolism include advances in tooth structure which allow chewing and thus the better utilization of food (Kemp, 1982; Carroll, 1988). 



     Dvinia is the most primitive known cynodont from Lower Permian.  It possessed 6 upper and lower incisors, it still retained one precanine tooth (as do primitive therocephalians), the secondary palate is only partly fused and still exposed part of vomers, the vomers were still paired (but will be fused to a single bone in all other cynodonts),  and the lower teeth bite against roof of mouth rather than occluding with (grinding against) the upper teeth.

     Procynosuchus was close to the ancestral condition of later cynodonts.  It had all the primitive features of Dvinia except that the vomers are fused.  There was still no fusion of secondary palate and the palate didn’t continue behind the region of the teeth.  There were 4 lower incisors.  The Upper Permian procynosuchids developed a secondary palate, complex cheek teeth, and reorganized the jaw muscles (such as the masseter) (Kemp, 1982; Carroll, 1988). 



    Cynodonts like Thrinaxodon were ancestral to all later cynodonts.. Thrinaxodon is close to the ancestry of mammals and resembled weasels in body form: it was 50 cm long with short limbs.  Thrinaxodon had a solid secondary palate (although in a few species it was still incomplete) and a mammalian dental formula with 4 upper and 3 lower incisors, canines, and 7-9 cheek teeth which are compressed and have cusps.  Unlike mammals, the teeth were still replaced in adulthood (mammals only have 2 sets of teeth in their lifetimes) and there was no occlusion pattern (the pattern in which upper and lower teeth meet to grind).  In the hip, the ilium was larger and the pubis was smaller.  The dentary bone dominated lower jaw but the articular bone still formed the joint with the upper jaw.  The pineal opening was still present.  The quadrate (a future middle ear bone) and quadrajugal bones were reduced in size and laid in sockets in squamosal bone (the part of the temporal bone that contains the middle ear).  Where the occipital region of the skull met the first vertebrae, there were 2 occipital condyles (as in mammals) as opposed to a single condyle in reptiles.

thrin skull thrin skull 2

     The first 2 vertebrae, the atlas and axis, were beginning to assume the mammalian form.  Thrinaxodon still retained atlas and axis intercentra and still retained the proatlas.

The atlas pleurocentrum was attached to the axis but the suture between the two was still visible (in mammals, this forms the dens of the axis and no suture is visible).   The atlas intercentrum was not ossified but cartilage and ligaments attached it to the neural arch (Kemp, 1982; Carroll, 1988). 

atlas axis atlas axis
atlas axis
thrin skull
     Thrinaxodon still had ribs on its cervical (neck) vertebrae but had 7 cervical vertebrae as in most modern mammals.  It still retained cervical intercentra which were lost in trunk region.  For the first time, there were distinct thoracic and lumbar regions of the vertebral column and the lumbar ribs lacked a shaft.  The tail was shorter (10-15 vertebrae) and it stood more erect due to changes in girdles for muscle attachment (Kemp, 1982; Carroll, 1988). 



      Thrinaxodon had jaw musculature that was intermediate between primitive and advanced cynodonts.  In advanced cynodonts, the large holes (foramina) of the snout and lower jaw and the absence of grooves on the bones in this area suggests that there was thick fleshy tissue in this area, raising the possibility that there whiskers here and that cynodonts had thus developed hair. Advanced cynodonts had most mammalian features of the skull.  They lost the postorbital and prefrontal bones (as in mammals) and as a result, there was no postorbital bar (the temporal opening opens into the orbit to form one large opening; many mammals still retain this setup).  The extension of the secondary palate was the same as in mammals.

     In mammals, the braincase is a continuous set of bones, completely enclosing the brain.   In mammalian ancestors, there are more small bones that compose the braincase  and they did not yet form a complete wall around the brain.  In cynodonts, a bone previously called the ectopterygoid was now broader and is given the same name used in mammals, the alisphenoid .  In later cynodonts and mammals, the angular bone of the lower jaw is small and supports the eardrum.  Later cynodonts and early mammals actually have 2 joints between the upper and lower jaws: the reptilian joint between the quadrate and articular (which in mammals become middle ear bones) and the mammalian jaw joint between the dentary and squamosal.

     In later cynodonts, the centrum of the atlas is attached to the axis, forming the dens. Although the atlas neural arch and the atlas intercentrum were joined, it was not yet one fused solid structure.  The coracoid bones of shoulder even further reduced.  The pelvis was mammalian in form but the three bones (ilium, ischium, and pubis) were not yet fused (Kemp, 1982; Carroll, 1988). 

 a) Cynognathidae

     This family evolved from galesaurids like Thrinaxodon and were extinct by the Mid-Triassic.  The skulls could be 40 cm long and they were one of most dangerous predators of Early Triassic.  The occipital region of the skull was wider and the dentary bone of the lower jaw was bigger (with a deep depression for masseter muscle and high coronoid process).  The inner surface of the lower jaw had a bar that retained a number of primitive ancestral bones: the articular, prearticular, angular, and surangular bones.  The coronoid bone was still part of the lower jaw (Kemp, 1982; Carroll, 1988). 

b) Gomphodonts

     The gomphodonts were 2 families of herbivores from Early Triassic.


c) Chiniquodontidae:

   This family was composed of small, conservative cynodonts of the Mid- to Upper Triassic.  Probainognathus may have been the first animal in which the dentary bone of the lower jaw functioned in the only jaw joint as in mammals.

probainognathus probelesodon
probain 2 probainognathus 3

d) Tritylodontids

     These cynodonts possess many mammalian characteristics such as the braincase, periotic bone, the squamosal region of the skull, the infraorbital canal system, the loss of the prefrontal and postorbital bones, absence of the internarial bar of cynodonts, multirooted postcanine teeth, and the postcranial skeleton.  They are considered to be close to the mammalian ancestors although they were not directly ancestral to mammals.  They were herbivores of the Mid to Upper Triassic (Kemp, 1983).

skeleton skull

e) Tritheledonts

     These were the most mammal-like of all the cynodonts but are poorly known.  The dentary contacted the squamosal and the frontal contacted palatine as in mammals.  One small skull opening (the fenestra ovalis) was similar to the mammals in position unlike all other cynodonts.  The tooth occlusal pattern was similar to mammals and unlike all other cynodonts.  Like tritylodonts and mammals, they lacked prefrontal and postorbital bones.  They were cosmopolitan in their distribution.   (Kemp, 1982; Shubin, 1991; Kemp, 1983). 





     As pelycosaurs evolved into therapsids which evolved into cynodonts from the Carboniferous through the Triassic, there were certainly a large number of modifications to the ancestral anapsid body plan.  What do all these changes mean?  Many of them are inter-related, which perhaps can be illustrated by emphasizing two points:

1)     Cynodonts were faster, smarter, and warmer than pelycosaurs.  Their limbs longer and were held under the body for an upright posture.  Their back did not undulate as they moved as in primitive reptiles and their tails were smaller.  Expanded bone regions (such as in the vertebrae and hip) allowed for larger limb muscles.  These changes would have made them faster.  Their brains were larger and there is evidence that from the time of the therapsids, the development of some degree of endothermy had been achieved. 

2)     All of the above adaptations required more energy.  Upright body stance, larger brains, and a higher metabolism which produces excess heat are energetically expensive.  Therefore, these ancestors of mammals had to evolve modifications to produce energy more efficiently.  They evolved the ability to process their food more efficiently—they increased their jaw musculature, evolved the ability to chew, and modified their reptilian teeth to produce both tearing and grinding teeth.  Obviously, greater oxygen supplies would be needed to convert these foodstuffs into energy and so they evolved a secondary palate which ensured that they could continue to breathe, even while eating.  The loss of lumbar ribs and the evolution of the mammalian diaphragm muscle allowed them to increase the volume of their lungs when inhaling by pushing down on the digestive organs.

   By the end of this process, synapsid reptiles had evolved into primitive mammals.