How Dinosaurs and Mammals Evolved on Planet Earth

by John Agbo, age 14

A long-held belief that dinosaurs suppressed mammals has been debunked. Recent fossil discoveries and new research show that mammals were better off than scientists originally thought.

There was a significant boom in reptile evolution during the Mesozoic Era, which was also when the earliest mammals emerged. Early mammals were small insectivores with big eyes, hinting that they were nocturnal.

It was only after the Cretaceous Period that mammals grew larger than house cats. Most scientists once considered dinosaurs “sluggish, dim-witted, and even freakish," they saw mammals as evolutionary underdogs, painting them as a superior order oppressed by the “tyranny of cold-blooded reptiles,” according to paleontologist and science writer Riley Black. Some scientists think this antagonistic relationship might have contributed to the extinction of dinosaurs.

New research shows that not all mammals feared the towering beasts. In fact, not all mammals from the Triassic, Jurassic, and Cretaceous periods were small insectivores. Some mammals, particularly ancestors of squirrels, shrews, otters, and aardvarks thrived. These early mammals lived around forests, floodplains, and deserts.

There were rivalries between dinosaurs and the mammals. If careless reptilian parents left eggs unguarded, they were easy prey for small and hard-to-see mammals.

Two fossil remains of the Repenomamus, a badger-sized mammal, have been found. One of the fossils was found in 2005, with the remains of a baby dinosaur in its stomach. The second fossil was discovered in 2023 and was found in the process of eating a baby dinosaur. A 205-million-year-old fossil was discovered in Poland, belonging to a five-ton, tusked, pig-like group called Dicynodonts. Scientists today think their diverse forms and species were an evolutionary benefit for mammals. What mammals evolved into were not signs of oppression, but a testament to their success.

The dinosaurs were just part of an ever-changing world. While they did affect the world around them, they weren’t able to alter their evolutionary fate. Mammals were neither oppressed nor ruled; however, they were part of the ever-evolving puzzle that is Planet Earth.

[Sources: Big Think; Atlas of Prehistoric Earth]