The Mammals of the Dinosaur Age

Many different types of mammals lived during the Dinosaur Age. After the Cretaceous-Paleogene mass extinction, they dominated the world.

Jul 25, 2023bySara Payne
mammals dinosaur age

When the KT extinction event took out 80% of life on Earth, it left open a wide variety of environments for the relatives of modern mammals to take over. Eventually, these mammals dominated the Earth becoming diverse and adaptable.

But, what were mammals like while dinosaurs roamed the earth? There were many, rodent-like mammals living alongside dinosaurs. Their unique and diverse characteristics gave them a leg up on surviving the changing climates of Earth.

What were Mammals Like when they Lived Alongside Dinosaurs?

ancient mammals

Mammals are on every continent in the world. They are large, small, carnivorous, herbivorous, and every variation in between. These mammals are able to dominate the world now, but millions of years ago, they were small, rodent-like creatures. They had to constantly hide to keep from being prey to the larger and more domineering animals at that time.

Mammals had existed in some form since the Pennsylvanian sub-period alongside dinosaurs. They were divided into two main groups in the Jurassic period: Prototheria and Theria.

The Prototheria laid eggs while Therians produced live young. Therians, today, are further broken into three subcategories, monotremes (egg-laying mammals), marsupials (mammals with pouches), and eutherian mammals (creatures with placentas). Prototherians went extinct with the dinosaurs during the KT extinction event.

There are few completely intact fossils of early mammals. Most classifications and studies are conducted using only a handful of teeth and some jawbones.

So, there is still a lot to learn about these early mammals.

Filikomys Primaevus

Filikomys primaevus

These social, rodent-like mammals were found at an excavation site in Montana in 2020. Their discovery revealed much about how mammals lived during the Mesozoic Era, nearly 71 million years ago. This finding was exciting because it revealed a large group of mammals completely intact in the rock.

These fossils indicate that mammals lived in groups and were social. They also showed that these mammals burrowed. They were probably omnivorous, living in the Jurassic to late Eocene periods.



This shrew-like creature was one of the earliest known primates. They were alive before the dinosaurs became extinct. They were about 6 inches long and weighed only a few ounces. They are distinct because of their primate-like teeth and jawbones.

Only 100,000 years after the mass extinction, they had changed dramatically. Studies by the University of California Museum of Paleontology found were taking to the trees, eating up the fruits and insects in the forest canopy. This likely saved the species because, on the ground, other animals were starving.

Many scientists propose that these ancestral primates are directly related to modern monkeys, apes, and humans.



Didelphodon was one of the largest mammals in the world when dinosaurs roamed the Earth. They were about a foot long. They ate small animals and insects. They were opossum-like and lived in swamps, lakes, and rivers in what is now North America. These early marsupials had a strong bite.

They were not like many other mouse-like mammals. According to the Burke museum, they were fearsome predators for small dinosaurs and other small animals. Although most marsupials live in Australia and South America, the opossum likely originated from the didelphodon.



These early mammals were herbivores and omnivores. They had sturdy builds and hooved feet. Fossil remains of this extinct species were found in North America. The periptychus moved slowly over the ground. It could probably climb and dig. After the KT mass extinction, these creatures were able to thrive.

How did Mammals end up Ruling the Animal Kingdom?


Prototherians thrived during dinosaur times. Unlike the Therians, these creatures could swim, climb, and burrow. They also grew much larger than their therian counterparts. These early mammals competed greatly with the therians.

However, most of the prototherians were terrestrial, living on the surface of the Earth alongside dinosaurs. When the mass extinction took out all dinosaurs besides the flying dinosaurs, most other terrestrial animal species died out, as well.

Although scientists don’t know for sure what caused this major extinction event, the major theory today is that a large asteroid hit the earth, triggering a series of events that led to rock debris in the atmosphere. This made sunlight scarce, killing many plants and interrupting the food chain.

Therians, however, mostly burrowed underground. They were able to avoid many of the ravages of the mass extinction, although their numbers decreased.

Once there was no competition and fewer predators, they were able to expand, adapt, grow larger, and take over the world.



Early mammals are the ancestors of many of the creatures living on Earth today–including humans. There is so much more to learn about and discover about these ancient creatures.

Learning about how they survived the mass extinction, adapted, and evolved can shed light on our own origins and the future of the world should another event happen.

Sara Payne
bySara Payne

Sara is a mother of two and a high school English teacher who rediscovered her love of writing during the pandemic. She has 5 rescue cats: Neville and Luna, who are white cats with black and grey spots, and Ginny, Blue, and Fairy, who are calicos. Besides taking care of humans and fur babies, Sara enjoys gardening, crafting, and spending time in nature.