Mammals That Lived During the Time of the Dinosaurs

While mammals didn't dominate the earth until after the dinosaurs were gone, some early mammals appeared on earth while the larger reptilian beasts still reigned. Some of these early mammals, which were small and primitive, are familiar to us today.

  1. Cynodonts: Ancestors of Mammals

    • Cynodonts flourished during the Triassic period, which was about 248 to 206 million years ago and the same period in which dinosaurs began to thrive. In appearance, these carnivorous beasts looked like medium-sized dogs but had angled legs like crocodiles. Evidence suggests that cynodonts are the ancestors of mammals, because even though they laid eggs and did not produce milk for their young, they are thought to have been warm-blooded, and they were covered in hair. They also had an evolved jaw that resembled mammalian carnivores of today and had dog-like teeth.

    Early Mammals

    • The earliest true mammals arrived during the Jurassic period, which was about 206 to 144 million years ago. Most of the early mammals were tiny, rodent-like and were marsupials. There were some types of monotremes, which are similar to the duck-billed platypus, and laid eggs. One example of the early rodent-like mammals was the Jeholodens, which fed on insects and worms. It benefited mammals to be tiny during this time period, because the dinosaurs were so large, powerful and dominant. The mammals' small size allowed them to hide in small spaces and escape the notice of those predators.

    Placental Mammals

    • During the Cretaceous period, which was about 144 to 65 million years ago, placental mammals began to evolve. Placental mammals, unlike marsupials, develop their young inside their bodies until the offspring is fully formed. The majority of mammals today, including humans, are placental. The earliest placental mammals were still small and shrew-like, and most were nocturnal. During the late Cretaceous period, mammals like the Zalabdalestes, an eight-inch-long nocturnal omnivore, began to appear. At this time, dinosaurs were beginning to struggle. This allowed the mammals to begin flourishing and diversifying.

    Carnivorous Mammals

    • As mammals started to grow in size toward the end of the Cretaceous, some species started to develop more carnivorous tendencies. The Didelphodon, for example, while technically an omnivore, fed mainly on small animals and any dinosaur eggs left unguarded. This marsupial was a larger animal, a bit bigger even, than its ancestors, the cynodonts, though it was smaller than a human. Other similar dog-like species are thought to have evolved around this same time, but their fossils are rare.