there are quite a few parallels between these tritylodonts (which are outside the mammal line) and multituberculates, which are mammals. the tritylodonts had no competition in the jurassic, because all the other mammal-like types were either carnivores or insectivores. kermack's conclusion is that when the multituberculates appeared (middle jurssic) they were in direct competition, and eventually they out-competed the earlier group, and replaced it.