Art by Pixeldust Studios, National Geographic December 2007

The earliest dinosaurs, small bipedal predators, appeared on Earth around 230 million years ago, spreading over the planet’s supercontinent, Pangaea. When Pangaea split into two landmasses, Gondwana in the south, and Laurasia in the north, during the Jurassic period, the dinosaurs divided into two main populations that continued to evolve, but mostly in separate ways. Eventually dinosaurs developed into the hundreds of species that have been identified by paleontologists. There may be thousands of other as yet undiscovered species, their fossilized remains trapped below the surface of the earth in rocks and sediment.

As dinosaurs evolved, they ranged in size from relatively tiny—about the same as a chicken—to giant sauropods nearly as large as today’s great whales. Over time some plant eaters, such as the sauropods, developed into behemoths, which made them less vulnerable to predators. Flesh-eating theropods, also adapted: The larger ones, including the well-known Tyrannosaurus rex, became capable of overcoming the gigantic vegetarians, while some of the smaller, light-boned ones took to the air and became birds.
The last of the nonavian dinosaurs lived around 65 million years ago, long before any mammal that resembles man appeared in the evolutionary chain on Earth. Compared to the dinosaur, man is an extreme newcomer: Homo sapiens appeared on Earth less than 200,000 years ago.


Peter Gwin, “Planet of the Dinosaurs,” National Geographic supplement, December 2007.

John Updike, “Extreme Dinosaurs,” National Geographic, December 2007.

Barrett, Paul. National Geographic Dinosaurs. NGS, 2001.

Currie, Philip J., and Kevin Padian, eds. Encyclopedia of Dinosaurs. Academic Press, 1997.

Dodson, Peter. The Horned Dinosaurs: A Natural History. Princeton University Press, 1998.

Holtz, Thomas R. Jr. and Louis V. Rey. Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. Random House Books for Young Readers, 2007.

Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Firefly Books, 2003.

Weishampel, David B., and others, eds. The Dinosauria. University of California Press, 2004.

Other Resources

“Dino Directory,”Natural History Museum

“The Dinosaurian,” University of California Museum of Paleontology,

“Dinosaurs,” Smithsonian National Museum of Natural History,

Tail Spikes

The Chinese stegosaur Tuojiangosaurus, like its well-known cousin Stegosaurus, had two rows of bony plates along its back and several stiletto-like spikes on its tail. Scientists have debated whether the spikes were used for display or in combat. A recent study of tail spikes found evidence of trauma-related damage, supporting the theory that the tail was used as a weapon.

This kind of spiky tail has become popularly known as the thagomizer, a reference to a 1982 Far Side cartoon by Gary Larson. Cavemen have been gathered for a lecture by a "professor," who points to a primitive visual aid, saying, "Now this end is called the thagomizer, after the late Thag Simmons," who evidently lost his run-in with the dino. (Of course, cavemen and dinosaurs didn't actually coexist. As Larson jokingly confessed in 1989: "Father, I have sinned—I have drawn dinosaurs and hominids together in the same cartoon.") The term has been used by paleontologist Kenneth Carpenter in a presentation at a 1993 scientific conference, in museum displays, and in popular books about dinosaurs. It has yet to appear in an official scientific journal, but perhaps it's only a matter of time. And where dinosaurs are concerned, there's all the time in the world.

Gary Larson's thagomizer cartoon. Everything Dinosaur.

Farlow, James O., and M. K. Brett-Surman, eds. The Complete Dinosaur. Indiana University Press, 1999.

Holtz, Thomas R. Jr., and Louis V. Rey. "Stegosaurs (Plated Dinosaurs)." Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages. Random House Books for Young Readers, 1997.

Larson, Gary. The Prehistory of the Far Side. Warner Books, 1992.

The Word: Thagomizer. New Scientist (July 8, 2006).

Other Resources
Tuojiangosaurus. Natural History Museum.

Carpenter, Kenneth. The Armored Dinosaurs. Indiana University Press, 2001.

Last updated: April 2, 2008