Temporal range: Upper Triassic – mid Cretaceous
They were the longest surviving of all the non-mammalian therapsids. They appeared in the latest Triassic period, and persisted through the Jurassic until the Middle Cretaceous. This shows that the Tritylodonts were a successful group of therapsids, even though they lived right beneath the ruling dinosaurs' feet.
Chronoperates from the Palaeocene, after the Cretaceous and the K/T extinction event, may be a Tritylodont. If so, then the Tritylodonts were elusive and rare during the Upper Cretaceous, because no Tritylodonts were found by that time. However, the Chronoperates's anatomy also closely resembles to that of symmetrodonts – a mammalian lineage.