Mesozoic is the name scientists give to a vast span of Earth's history stretching from about 252 million to 66 million years ago. If you've ever been fascinated by dinosaurs, you've been thinking about the Mesozoic Era, because this is when dinosaurs ruled the land.

The word comes from Greek roots meaning “middle life,” and it sits between two other eras: the Paleozoic (ancient life) and the Cenozoic (recent life, which includes us). Scientists divide the Mesozoic into three periods: the Triassic, when dinosaurs first appeared; the Jurassic, when they grew enormous and diverse; and the Cretaceous, which ended with the famous asteroid impact that wiped out the non-avian dinosaurs.

During Mesozoic times, Earth looked completely different. The continents were arranged differently, climates were generally warmer, and flowering plants didn't exist for most of the era. Besides dinosaurs, the Mesozoic saw the evolution of the first birds, the first mammals (though they stayed small while dinosaurs dominated), and massive marine reptiles like plesiosaurs swimming through ancient oceans.

When paleontologists discover dinosaur fossils, they're uncovering creatures from the Mesozoic. When you visit a natural history museum and marvel at a Tyrannosaurus rex skeleton, you're looking at bones from the final period of this extraordinary era.