The Cynodonts

a cynodont in profile

The ‘dog teeth’ cynodonts were therapsids (mammal-like reptiles) that appeared in the Late Permian around 260 million years ago and survived into the Early Jurassic around 175 million years ago. Cynodonts were one of the most diverse groups of therapsids, and they were ancestors of the mammals – and thus of you and me! The earliest known cynodont, Charassognathus, appears during the Late Permian. Cynodonts survived the Permian-Triassic Extinction Event and flourished in the Triassic. They barely weathered the Triassic-Jurassic extinction and went extinct in the Early Jurassic. This may seem odd, given that their mammalian ancestors continue to this day, but the extinction of the primitive cynodonts didn’t eradicate the off-shooting mammalian lines that continued through the Mesozoic and into the Cenozoic.

Cynaognathus, perhaps the first mammal - and our ancestor

Some cynodonts were small and mammal-like; others were large, carnivorous predators; and still others were ponderous herbivores. Studies of cynodont remains show that even from an early stage they were showing mammalian characteristics. Marks in the upper and lower jaws may have been channels that supplied blood vessels and nerves to whiskers. The presence of respiratory turbinates indicates rapid metabolisms and warm-bloodedness. The idea that cynodonts were endothermic has prompted numerous reconstructions of them having fur coats; if they were warm-blooded, they may have needed fur for thermoregulation. Unfortunately, fossil evidence of cynodont fur (or even lack thereof) hasn’t been easy to find. Modern mammals have special glands that secrete lipids to coat their fur, but the telltale marks of this structure is found at the earliest during the Late Triassic and onwards. However, recent studies on Permian synapsids coprolites suggest that primitive therapsids had fur; cynodonts, being derived therapsids, may have carried on this trait. Thus, though we have no hard evidence of cynodonts being furred, it’s a justifiable speculation. 

Botucaraitherium: a carnivorous cynodont slightly larger than a rat

There remain more similarities between primitive cynodonts and mammals: during cynodont evolution, the number of their jawbones reduced; this trend towards a single bone for the mandible opened space for other bones in the jaw to migrate to the cranium where they function today as parts of the mammalian hearing apparatus (the ‘inner ear’). Cynodonts also developed a secondary palate in the roof of the mouth, a development that caused airflow from the nostrils to travel to a position in the back of the mouth instead of directly through it; this enabled cynodonts to chew and breathe at the same time, a characteristic present in all mammals (most reptiles don’t chew their food but swallow it in large pieces to slowly digest in their bowels). The ability to breathe while chewing prompted the development of specialized chewing teeth, such as molars, that were designed for chewing; thus cynodonts gave up teeth designed for catching and holding prey and took to savoring their food. Though cynodont brains were small, their braincases had a mammalian appearance (notably it bulged at the back of the head). Paleontologists speculate that the first true mammals derived from small carnivorous cynodonts sometime in the Middle Triassic. 

Trucidocynodon of the Late Triassic

Despite their mammalian offshoots, cynodonts remained reptile-like in a lot of ways. Most cynodonts probably laid eggs, though they may have done so in a manner more in line with modern monotremes than with reptiles. Mammalians are divided into three classes depending on how they give birth: placentals (such as human beings) given birth to mostly developed young who still require nursing to reach full development; marsupials give birth to live young at an earlier stage in the developmental process than placentals, so the young need to continue their development in a pouch; and monotremes lay eggs. Monotreme young hatch early and require nursing towards full development; it’s likely, based on skeletal analysis of cynodont remains, that cynodonts gave birth in a monotreme fashion. Paleontologists have discovered epipubic bones that strengthened the cynodont torso and supported abdominal and hind limb muscles; while this aided in the development of an erect gait, it did so at the expense of a prolonged pregnancy. The end result is that they would’ve been forced to reproduce like modern monotremes. The cynodont skeleton, except for the major mammalian characteristics outlined above, remained largely reptilian. Unlike mammals, they had a sprawling posture and gait.