Tritylodont
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.