Scientists Say: Dinosaur

This is a member of the group Dinosauria — and not all are extinct

This is a Tyrannosaurus rex named Sue, on display at the Field Museum in Chicago, Ill. T. rex was a dinosaur and is now extinct, but dinosaurs aren’t all gone. Modern birds are dinosaurs too.

Richard T. Nowitz/The Image Bank Unreleased/Getty Images Plus

Dinosaur (noun, “DIE-no-sore”)

This is a member of a group of both living and extinct reptiles that are in the clade Dinosauria. A clade is a group of species that share a common ancestor.

Dinosaurs appeared between 243 and 233 million years ago. Some grew to enormous size — such as the massive Dreadnoughtus (Dred-NAW-tus) schrani, which was longer than a 25-meter (82-foot) swimming pool. Others were only the size of pigeons or ducks.

Dinosaurs can be divided into two main groups, based on the direction of their hip bones. These groups are the Saurischia (Sor-ISH-ee-yah) and Ornithischia (Or-nih-THIH-shee-ah). Saurischia means “lizard hipped.” That’s because the hips of these dinosaurs point to the front, like a lizard’s. Some saurischians include Brontosaurus and Tyrannosaurus rex.

Ornithischia means “bird hipped.” The public bone of these dinosaurs points toward the back. Stegosaurs and iguanodonts were ornithischians.

Most of the big dinosaurs went extinct around 66 million years ago. They were probably killed off by a large asteroid striking the Earth near what is now Mexico.

But some dinos survived. By studying fossils — preserved remains of living things — scientists have shown that today’s birds are modern dinosaurs. Our feathered friends evolved during the Jurassic — about 165 to 145 million years ago. Oddly, they are in the “lizard-hipped,” and not the “bird-hipped,” dinosaur group. Birds are known as avian dinosaurs.

Dinosaur means “terrible lizard.” That’s right. Chickens are terrible lizards, too.

In a sentence

Very early dinosaurs may have laid leathery, turtle-like eggs.

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Bethany Brookshire was a longtime staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology and likes to write about neuroscience, biology, climate and more. She thinks Porgs are an invasive species.

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