251-201 million years ago



Morganucodon, Sinocodon, Megazostrodon and Dinnetherium have the tooth structure of cynodonts. Some feel that this group is ancestral to all mammals while others feel that they had diverged from ancestral stock and were the ancestors of the monotremes. Morganucodon is known from thousands of elements from 4 continents. Sinocodon may be more primitive than Morganucodon or may not even be a mammal.

The braincase of Morganucodon was larger than that of the cynodonts (and 3-4 times the same relative size of a therapsid braincase) and its head is large compared to its body. Its occipital and sphenoid bones consisted of smaller ancestral bones which fused together during development (as in modern mammals). As in reptiles and monotreme mammals, the cochlea of its inner ear was not coiled. There was a slight reduction in the size of ear bones. The lower jaw still included tiny remnants of the ancestral prearticular, angular, surangular, and coronoid bones. The two halves of lower jaw are not yet firmly joined and could move relative to each other. The dental formula was 5,1,4,4/5,1,4,4 (5 incisors, 1 canine, 4 premolars, 4 molars in both jaws) (Kemp, 1982; Carroll, 1988; Hopson).