What is a fossil?
A fossil is what is left of a living thing from long, long ago, kept in rock. Some fossils are parts of the body: bones, teeth, shells, and once in a while skin or feathers pressed into the stone. Scientists call these body fossils.
Other fossils are signs that an animal was there: a footprint in what was once mud, a nest of eggs, even a lump of dung that turned to stone. These are called trace fossils. A trace fossil can tell you what an animal did, not just what it looked like.
How a bone becomes stone
Most animals never become fossils. When an animal dies, its body is usually eaten or rots away, and the bones break up in the wind and rain. A fossil needs luck.
The lucky ones are covered quickly, by mud at the bottom of a river or a lake, by sand, or by ash from a volcano. Over a very long time, more and more layers pile on top and slowly press down into rock. Water creeping through the ground carries tiny bits of minerals, and they fill the spaces in the bone and take its place, little by little. The bone keeps its shape, but now it is made of stone.
Much later, rain, rivers and wind wear the rock away, and a fossil can poke out at the surface again. That is why dinosaur hunters search in dry, bare places like badlands and deserts, where the rock is easy to see.
Digging one out
The people who study fossils are called paleontologists. When one spots a bone in the rock, the work goes slowly: small picks, brushes and even dental tools clear the rock away a little at a time, and every piece is photographed and mapped before it moves.
A big bone is too fragile to lift bare, so the team wraps it in cloth soaked in plaster, like the cast for a broken arm. The hard jacket keeps it safe on the trip to the lab, where workers called preparators spend weeks or months cleaning away the last of the rock.
A wall of bones you can visit
At Dinosaur National Monument, on the border of Utah and Colorado, a building has been put up right over a cliff full of dinosaur bones. Inside, visitors can see the bones still sitting in the rock, just as they were found, and even touch a few. The bones belong to dinosaurs that lived in the Jurassic, such as Allosaurus, Stegosaurus, Diplodocus, Camarasaurus and Apatosaurus, the close cousin of Brontosaurus.
What fossils can tell us
Bones show how big an animal was and how it was built. Teeth tell what it ate: sharp, curved teeth for meat, flat or peg-shaped ones for plants. Footprints show how it walked and how fast, and a long line of prints side by side means some dinosaurs walked in groups.
Eggs and nests show how dinosaurs raised their young. In Montana, scientists found whole nesting grounds of the duck-billed Maiasaura, with eggs, babies and young ones together, and named it the good mother lizard.
What fossils can't tell us
Most skeletons are found with pieces missing, so many dinosaur sizes are scientists' best estimates from the bones they have. That is why Peabody marks those numbers as estimates and says where each one came from, and why two books can give different sizes for the same dinosaur.
Color is the hardest of all. Skin and feathers almost never last. In a few feathered fossils, scientists have found tiny color-making parts, so for a handful of dinosaurs they have a good idea of the colors. For most, the colors in pictures and games are an artist's guess.
If you find a fossil
In national parks and on most public land in the United States, fossils are protected: you may look and take a photo, but not take them home. Leave it where it is, take a picture with something beside it to show the size, and tell a ranger. Your photo could help a scientist find something new.
For Big readers: how old is that rock?
Rock is laid down in layers, and unless something has turned them over, the layers lower down are older than the ones on top. So a fossil in a lower layer lived before one in a higher layer. That tells you the order, but not the age.
For the age, scientists look for layers of volcanic ash above and below the fossil. Ash holds tiny crystals with a few unstable atoms that change into other atoms at a steady rate, like sand running through an hourglass. By measuring how much has changed, scientists can work out when the ash fell, and so how old the fossil between the layers must be.
Pick Big at the top to see the part for older readers.