299-252 million years ago



human skull

In the early cynodonts, the maxillary and palatine bones grew until they fused to form a hard bony palate which separated the mouth from nasal cavity. This secondary palate was important for cynodonts and all modern mammals because it allows an uninterrupted flow of oxygen into the body (even while eating). This allowed cynodonts to evolve a higher metabolic rate and a larger brain.

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).

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).