The theriodonts or Theriodontia are a major group of therapsids. They can be defined in traditional, Linnaean terms, in which case they are a suborder of synapsids that lived from the Middle Permian to the MiddleCretaceous, or in cladistic terms, in which case they include not only the traditional theriodonts but also their descendants the mammals as well.
Theriodonts appeared at the same time as their sister group within the Neotherapsida, the Anomodontia, about 265million years ago, in the Middle Permian. Even these early theriodonts were more mammal-like than their anomodont and dinocephalian contemporaries. ''. Early theriodonts may have been warm-blooded. Early forms were carnivorous, but several later groups became herbivorous during the Triassic. Theriodont jaws were more mammal-like than was the case of other therapsids, because their dentary was larger, which gave them more efficient chewing ability. Furthermore, several other bones that were 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.
Eutheriodontia
Eutheriodontia refers to all theriodonts except the gorgonopsians. They included the therocephalians and the cynodonts. The cynodonts include the mammals. The name means "true beast tooth". The eutheriodonts have larger skulls, accommodating larger brains and improvedjaw muscles. The eutheriodontian theriodonts 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 carnivores, such as Cynognathus, as well as newly evolved herbivores. While traversodontids for the most part remained medium-sized to reasonably large, the carnivorous forms became progressively smaller as the Triassic progressed. They "miniaturised". By the Late Triassic, the small cynodonts included the rodent-like Tritylodontidae, and the tiny, shrew-like, Trithelodontidae, related to the Mammaliaformes. The trithelodontids died out during the Jurassic, and the tritylodontids survived into the Cretaceous, but their relatives, the mammals, continued to evolve. Many mammal groups managed to survive the Cretaceous–Paleogene extinction event, which wiped out the non-avian dinosaurs, allowing the mammals to diversify and dominate the Earth.
