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Therocephalians ("beast-heads") were an extinct suborder of theriodont therapsids (Permian/Triassic ancestors of mammals) that were among the dominant animals on Earth during the mid-to-late Permian and the early Triassic, until archosaurs (the ancestors and relatives of dinosaurs) took over about 235 million years ago. Therocephalian fossils are dated to between about 275 and about 235 million years ago. The large skulls and teeth of therocephalians indicates they were successful carnivores. Therocephalians had some early mammal-like characteristics, including fur and whiskers. At least some species were warm-blooded. At least one species (Euchambersia) had a maximal pit and grooved caniform teeth, indicative of a venom gland, which would make therocephalians the earliest known venomous vertebrates.
The therocephalians were part of the second wave of development in synapsid (mammalian ancestor) life, taking over the land in the mid-to-late Permian, competing with and replacing relatives that had branched off earlier, such as other synapsids, including the famous pelycosaurs, which became extinct in the mid-Permian. Therocephalians evolved on Earth during a time when synapsids had been the dominant terrestrial animals for several tens of millions of years, displacing the amphibian-dominated fauna of the Carboniferous.
Therocephalians are unique in being synapsids that started their career as the dominant predators, topped ecosystems for about twenty million of years, barely survive the greatest mass extinction in the history of the planet (the Permian-Triassic extinction), then go on to see reptilian rivals (archosaurs) overcome them in an event known as the "Triassic Takeover," the exact causes of which are still unknown. It may have something to do with the increasing aridity of the planet during that time, favoring reptiles. Reptiles excrete uric acid instead of urea, a form of excretion that conserves water more effectively urea, which is used by mammals today and synapsids (including therocephalians) at the time.
Cynodonts, the Triassic ancestors of mammals, were another suborder of theriodonts related to therocephlians. Together, both groups constituted most powerful Late Permian/Early Triassic predators and omnivores, corresponding to today's carnivorans. Therocephalians would have fed on pareiasaurs, herbivorous anapsids (reptiles without skull holes, like turtles), which ranged in size from 60 cm (22 in) to 3 m (10 ft), tapinocephalians (ton-sized synapsids with thick heads), and large dicynodonts (rat-to-ox-sized therapsids that were among the most successful animals in the immediate wake of the Permian-Triassic extinction, in some places accounting for 99% of all vertebrate faunas).