Mammals first appear in the fossil record in the upper Triassic (200 million years ago). Through the rest of the Mesozoic Era, up to 70 million years before present (MBP), mammals remained small, and were probably nocturnal. It is difficult to estimate their abundance, since remains are scanty and minute, but they were certainly outnumbered by dinosaurs and other reptiles.

Mammals did not begin to radiate into the varied forms we recognize today until after the dinosaurs disappeared. It is interesting to consider whether mammals were kept to a minor role by competition from the dinosaurs, or whether mammals prevailed and caused the dinosaurs' extinction.