Cynodonts: Triassic Mammal-Like Reptiles

Cynodonts were one of the most diverse groups of therapsids, more commonly called mammal-like reptiles, that thrived in the Triassic after the Permian-Triassic mass extinction. Although these small carnivores were reptilian, they had more in common with present-day mammals than with true reptiles like lizards. Cynodonts evolved from earlier mammal-like reptiles such as the giant predatory gorgonopsians and therocephalians. Although these large carnivores died out by the end of the Permian Period, they laid the groundwork for the smaller cynodonts to carve out a niche amidst the Triassic landscapes of Pangaea. Fossil finds of cynodonts have been discovered in the Pertified Forest region of Arizona as well as sediments in South Africa. It is possible that the first true mammals evolved from creatures like the cynodonts.

Cynodonts were much smaller than their therapsid ancestors, not reaching much longer than a meter in length. It seems likely that these small creatures in the Triassic fed on small animals like insects and reptiles. Cynodonts had differentiated teeth, which is a characteristic of mammals who possess canines for shearing, incisors for biting, and molars for chewing. The cynodonts began to evolve these teeth to allow for quicker and more efficient digestion of small prey items. Reptiles have homogenous pointed teeth used for grasping prey, which are poorly suited for chewing. Most true reptiles shear off flesh with their pointed teeth and then swallow the chunks whole.

Although cynodonts laid eggs, they were most likely warm-blooded (endothermic) and covered with hair. Evidence for the hair comes from small depressions in the jaw bone that suggest whiskers, which in turn would suggest body hair. As an endotherm, they would be much more agile and regularly active than their reptile and archosaur contemporaries. Also, as cynodonts became more specialized, superfluous jaw bones began to inward toward the skull and formed the basis for our own middle-ear bones. This new sharpened sense of hearing allowed the cynodonts to both detect larger predators and their own prey. In turn, cynodonts developed a larger brain to help decipher the new auditory vibrations caused by these small inner-ear bones. This did not mean that cynodonts were more intelligent than their contemporaries, but simply had a larger brain to deal with more incoming information.

Although the cynodonts developed differentiated teeth, not all forms were predators. The Mid-Triassic saw a radiation of herbivorous forms of cynodonts which would evolve into the specialized mammal-like "rodents" that lived in the undergrowth in the Jurassic forests. Although cynodonts were a very diverse and successful group in the Triassic, the growing importance of dinosaurs caused them to retreat into the shadows. As dinosaurs were getting larger, the cynodonts were getting smaller and becoming nocturnal in order to escape larger predators. Although the dinosaurs would eclipse the cynodonts' success on Earth, they remain one of the most fascinating examples of a "missing link" between major families of organisms.