Temporal range: Middle Permian – Lower Cretaceous
265 to 100 million years ago
Descendant taxon Mammalia survives to present.
Theriodonts appeared about 265 million years ago, in the Middle Permian. Even these early theriodonts were more mammal-like than their relatives.
Theriodonts fall into three main groups: Gorgonopsia, Therocephalia and Cynodontia. Early theriodonts may have been warm-blooded. Early forms were carnivorous, but several later groups became herbivorous during the Triassic.
Several other bones that were previously on the lower jaw, moved into the ears, allowing the theriodonts to hear better and their mouths to open wider. This made the theriodonts the most successful group of synapsids.
Eutheriodonts refer to all theriodonts except the gorgonopsians (the most primitive group). They included the therocephalians, cynodonts and their descendants – the mammals. The name means "true beast tooth". The eutheriodonts have larger skulls, accommodating larger brains and improved jaw muscles.
The theriodonts (eutheriodonts) are one of the two synapsid survivors of the great Permian–Triassic extinction event, the other being the dicynodonts. Therocephalians included both carnivorous and herbivorous forms; both died out after the early Triassic.
The remaining theriodonts, the cynodonts, also included carrnivores such as Cynognathus, as well as newly evolved herbivorous Traversodonts. While Traversodonts for the most part remained medium-sized to reasonably large (length of largest species up to 2 meters), the carnivorous forms became progressively smaller as the Triassic progressed.
By the Late Triassic the small cynodonts included the rodent-like tritylodonts (possibly related to or descended from travsersodonts), and the tiny, shrew-like, trithelodonts, which evolved into the first mammals. The trithelodonts died out during the Jurassic, and the tritylodonts survived in the Cretaceous, but the mammals continued to evolve. Many mammal groups managed to survive the Cretaceous–Tertiary extinction event, which wiped out the non-avian dinosaurs, allowing the mammals to diversify and dominate the Earth.