An endothermic metabolism
One of the therapsid lines were the theriodonts which were small carnivorous animals (less than 1m) and were almost certainly the evolutionary line that lead to the mammals. There is some doubt as to whether theriodonts should be classified as reptiles or as very primitive mammals. An example of which is Cynognathus, an animal approximately a metre long, possessing a large dog-like skull with highly specialized and differentiated teeth (remember reptiles are generally characterized by simple, undifferentiated peg-like teeth). These teeth suggested that Cynognathus teeth were for chewing and cutting food rather than swallowing it whole. There is also a well developed secondary palate, which separates the nasal passage from the mouth, which permits continued eating while the mouth is filled with food. All these features suggest that the animal was very active and probably requiring an endothermic metabolism. To maintain such a metabolism would require some form of body insulation, possibly even fur.
Cynognathus, an animal approximately a metre long, possessing a large dog-like skull with highly specialized and differentiated teeth (remember reptiles are generally characterized by simple, undifferentiated peg-like teeth)
These fossils indicate that some theriodonts were far advanced towards the mammals in certain characters, but still remained comparatively primitive in other respects. The mixture of conservative and advanced features makes it difficult to identify the final line that evolved towards the mammals.
