Fossils: From Organic to Rock in a Matter of Millennia

China has provided a large number of fossils to the fossil record -- the natural history of Earth that's painted by all of the preserved remains discovered to this point. This skeleton was displayed in Shanghai in July 2007. See another complete dinosaur skeleton on the next page.

Theropods, like this one in the Evolving Planet exhibit at Chicago's Field Museum, were carnivorous dinosaurs that walked on two legs. Next, see how large these dinosaur bones can get.

They're the world's biggest, most difficult puzzles. After piecing together thousands of fossilized bones, paleontologists are dwarfed by the towering skeleton. Next, see how they uncover the bones in the first place.

Chinese scientists excavate a dinosaur fossil discovered at a site on the shore of the Jialing River near southwestern Chongqing municipality in 2004. Next, see a fossilized dinosaur embedded in stone.

Check out that fossil of a Microraptor from a 130-million-year-old forest that existed in what's now Liaoning province, China. The Microraptor was thought to be one of the smaller dinosaurs. Next, see the famous fossil known as Leonardo.

Embedded in a canyon wall in Montana, Leonardo, the 77-million-year-old Brachylophosaurus, was the first dinosaur fossil found with its skin and organs still intact.

A fossil trackway crosses the slope of a hill in Yongjing County in northwest China's Gansu province. At this site, scientists found more than 100 dinosaur footprints composed of 10 groups and estimated to have been made at least 100 million years ago. Next up: a single three-toed dinosaur print.

This fossilized track of a lower Jurassic theropod dinosaur was found on a Navajo reservation in Arizona. Millions of years ago, the soil where the dino stepped was wet, and now its tracks are embedded in sedimentary rock. Next up, proof that dinosaurs traveled in packs.

English paleontologist Martin Lockley stands with a series of parallel dinosaur tracks along Purgatoire River, Colo. How did dinosaurs reproduce? Find out next.

Which came first: the dinosaur or the egg? Fossilized eggs sit on display at the Inner Mongolia Museum in the regional capital of Hohhot. Next, see the inside of a dinosaur egg.

Before the long process of fossilization began, this egg shows signs of being eaten by tunneling beetles (on the left). Eggs are one sign that dinosaurs were related to birds. Their skeletal structures may also support this theory.

Paleontologists often disagree on whether birds are directly descended from dinosaurs. Here, Professor Fernando Novas demonstrates similarities between the skeletons of a 70-million-year-old fossil and modern flightless birds during a press conference at the National Geographic Society in Washington, D.C. Of course, dinosaurs aren't the only animals whose bits and bones have been fossilized.

Children love to search for fossilized shark teeth on the beach. All those sharp, pointy choppers are pretty terrifying, but see a giant one on the next page.

This fossilized tooth once belonged to a prehistoric shark known as the megalodon. Kind of makes you glad they're now extinct. Want to meet another extinct sea creature?

Museum Victoria Research Associate and Monash University Ph.D. student Erich Fitzgerald inspects the skull of a 25-million-year-old fossil from southeast Australia identifying a new family of small, highly predatory, toothed baleen whales with enormous eyes. Next, see the skull of an extinct land-dwelling mammal known for its teeth.

Fossilized bones of saber-tooth cats are a primary source of information about how they may have behaved. The shape of the cats' teeth supports the theory that they ripped through the throats or abdomens of their prey, leading to death through loss of blood.

These pieces of petrified wood, found in Petrified Forest National Park in Arizona, used to be trees. Now they're rocks -- good for paperweights but bad for paper. Next, see what evidence the fossil record includes in support of the evolution of man.

This collection of fossils on display in Nairobi National Museum is believed to be the evolutionary series of man from his earliest existence millions of years ago. Next, see the fossilized skull of a young hominid child.

Ethiopian paleontologist Zeresenay Alemseged holds the skull of a hominid child known as Australopithecus afarensis during a press conference in Addis Ababa in September 2006. On the next page, see the fossilized remains of Ida.

The 47-million-year-old fossilized remains of primate skeleton named Ida are shown during a news conference. Ida may help illuminate the early evolution of monkeys, apes and humans.