The Great Dying of the Permian Period occurred about 250 million years ago. At that time, about ninety percent of all animals on the earth died out. One group that did survive was the Cynodont Family, which then thrived before the rise of the Dinosaurs (at the end of the Triassic Period). From this group of proto-mammals (Therapsids (Synapsids)) came the ancestor of modern Mammals.
The Triassic Period featured two groups of Animals that gave rise to other well-known Animals. From the Archosaurs (“Ruling Reptiles”) came Dinosaurs, and from the Therapsids, came Mammals. The most diverse group of Therapsids were the Cynodonts, who included both meat and plant eaters. During the Triassic Extinction Event (200 million years ago), many kinds of Therapsids died out, while Archosaurs blossomed into Dinosaurs. However, a few Cynodonts did survive to be become Mammals.
Cynodonts are neither Reptiles nor Mammals, but a bridge from one group to the other. In fact, they lie at the nexus of “mammalness” and “reptileness”, and display the progression from Reptile to Mammal. In that evolution, Cynodonts have developed three bones in their ears and one for their jaws, much like Mammals of today. (Reptiles have three bones on either side of their jaws and one for their ears.) Also, Cynodonts developed a bone to separate their breathing air from eating food, which modern Mammals also have.
As They can be both Reptiles and Mammals, Cynodonts challenge our casual assumptions. To define what these Animals are, scientists have to have a deep knowledge of zoology. Moreover, the definitions of Mammal and Reptile require a precision of thought. For clarity in thinking, let Cynodonts be your guide. They can lead you into the heart of the perplexing matter, and illuminate the answer. Let Cynodonts be our bridge from fuzzy thing to concise knowledge.