September 3. Cynodonts

Triassic Dicynodont from Poland. Drawing by Dmitry Bogdanov.

A few days ago, talking about the end Permian extinction, I said something that’s wrong. I said the therapsids became extinct. That’s not true, since the therapsids include modern mammals. I should have made it clear that it was the early therapsids, dimetrodons and others, that did not survive into the Triassic. Other groups were decimated, but they did survive.

One surviving group was the cynodonts, whose name means “dog-teeth.” Cynodonts are generally seen as the early therapsids that ultimately gave rise to true mammals later in the Triassic, but as the Triassic began, only a few varieties existed. In addition to differentiated teeth, cynodonts had skull and jaw structures similar to modern mammals. It’s speculated that cynodonts were warm-blooded and laid eggs, like modern platypuses.

The recovery from the Permian extinction was slow, but cynodonts did diversify during the Triassic, increasingly through the period. The variety called dicynodonts, meaning “two dog teeth” for the pair of large tusks that they had, were perhaps the most diverse of the pre-mammalian therapsids. They started in late Permian time but were pretty common during the Triassic. Dicynodont species ranged from the size of a rat to the size of an ox, and despite the tusks they were herbivores. They probably used the tusks for digging, like modern boars. Dicynodonts were probably the most widespread large herbivores on land during the early Triassic, expanding from two families that survived the extinction to 14 or 15 families by mid-Triassic time.

The Triassic’s changing climates and arid conditions in many locations were probably both stimulating in terms of evolutionary pressure and challenging in terms of survival. The dicynodonts were essentially extinct by the late Triassic, but there is a possibility that some survived until the Cretaceous, based on fragmentary fossils from Queensland, Australia. But by the end of the Triassic, most of the terrestrial herbivores were dinosaurs, not pre-mammalian therapsids like the cynodonts and dicynodonts.

—Richard I. Gibson

Image by Dmitry Bogdanov under GFDL.