From tiny mammals to giants: the rise and fall of Earth’s colossal creatures

The first mammals appeared around 225 to 200 million years ago, during the late Triassic period. This was when the first dinosaurs appeared.

Early mammals were very small, about the size of rats or badgers. This is because dinosaurs were the top predators at that time.

The rise of mammals

After the dinosaurs became extinct about 66 million years ago, mammals grew bigger and looked different. Scientists believe that after the mass extinction that wiped out dinosaurs, mammals started to fill the places that the dinosaurs used to live in.

Since there were no more dinosaurs to compete with, mammals started to change and adapt. They grew into different sizes, ate different foods, and lived differently. This period is called the “Age of Mammals”.

During the Oligocene epoch about 33.9 to 23 million years ago, most mammals remained small, but one giant, Paraceratherium, stood out. This enormous rhino, stretching over eight metres long and weighing as much as five elephants, was one of the largest land mammals ever. Over time, more giant mammals emerged (see graphic).

Vanished giants

However, at the end of the Pleistocene epoch, around 12,000 to 11,000 years ago, many large animals disappeared. Scientists are still trying to figure out why. One possible reason is that humans hunted them too much. Interestingly, many smaller relatives of these giant animals still exist. This shows that being smaller might have helped them survive and evolve.

Today, we live alongside large animals like elephants, crocodiles and ostriches.

Why do you think they survived while others died out?

Questions

• What happened to mammals after the dinosaurs disappeared?

• Can you name some large animals that still live today?