Monotremes. Like the echidna or (spiny anteater) and platypus. These are egg laying mammals. This early mammal branch never evolved past this stage. Not all creatures evolve as fast as others.
Mammals existed before dinosaurs and was kept down By the more advanced (at this early point ) and bigger more aggressive dinosaurs.
Divergent evolution gave the platypus a flat bill and poison glands/injector in their hind legs. Early mammals were egg layers. Some lines never evolved from this method of procreation.
What are some of the things that would stop a line from evolving?