the mammalian traits you speak of were the default mamalian due to pressure from the dinosaurs existence, they could not compete so survied in a compressed nich

the mammalian traits you speak of were the default mamalian due to pressure from the dinosaurs existence, they could not compete so survied in a compressed niche. when the dinosaurs failed to survive the bottleneck at the KT boundry, the mammals were in a position to weather it out and expand niche into a landscape devoid of most large dinosaur lineages, this was followed up with mammalian diversification founded on the small cynodont plan. this is all a tale of the fossil record and the principle of superposition