Scientists divide the past 250 million years into two eras, the Mesozoic “Age of Dinosaurs” and the Cenozoic “Age of Mammals.” There were plenty of mammals in the “Age of Dinosaurs,” but most were very small—about the size of mice—and came out only at night.
About 65 million years ago, a massive extinction killed all dinosaurs except birds. In the “Age of Mammals” that followed, many new mammal species evolved and took over ecological roles once filled by dinosaurs.
Scientists recognize mammal ancestors by the following characteristics, which evolved bit by bit over millions of years:
- Sensitive eyes, ears, and noses that helped mammalian ancestors find their way in the dark.
- Higher body temperatures and fur that let mammals stay active and warm in the chill of the night.
- A bony palate that separates the mouth and nose and allows warm-blooded animals to drink and breathe at the same time.
- A diaphragm muscle that helps pump oxygen to burn fuel for warm mammal bodies.
- Three bones in the middle ear that were once part of the jaw.
- Specialized teeth, such as nipping incisors, piercing canines, and grinding cheek teeth with ridges for chewing food.
Creatures of the Night
Many early mammals may have survived the “Age of Dinosaurs” by evolving to be active at night, while many dinosaurs slept. Even today, most mammal species are nocturnal.
Several mammalian features—such as keen hearing—are especially useful for wayfinding at night. In fact, only mammals have three bones that make up the middle ear and transmit sound in a wide range of frequencies. These bones are one of the key characteristics scientists use to identify mammal fossils.