Trace fossils are different from regular fossils. If fossils are the bones of a dinosaur, then trace fossils are all the other things that it left behind while it was alive. When a dinosaur walked on wet sand, it left footprints, which later turned into rock. Some animals dig into the ground and create holes. When a dinosaur lays an egg or goes to the bathroom that too becomes fossilized! These are all trace fossils. There are many more trace fossils than fossilized bones. This is because a dinosaur has only one skeleton but can leave hundreds of eggs and lots and lots of dung behind. Trace fossils can sometimes give scientists more information than a regular fossil of a skeleton. Trace fossils tell us how a dinosaur lived, what it ate and how it moved. The study of these traces is called ichnology.
When dinosaur eggs are found, very often unborn dinosaurs are found inside. These tiny fossils can give us clues to the way dinosaurs grew. Sometimes dinosaurs would lay their eggs as they walked and then leave the babies to find their own way. Sometimes they would come to some special place year after year, lay their eggs there, and then watch them carefully. These nesting areas are called rookeries. Usually, each nest is separated by the length of one dinosaur. Dinosaur eggs were very small compared with the size of a grown-up dinosaur. If the eggs were any larger, their shells would be too tough for the babies to break through. The largest egg ever found – the egg of a sauropod named Hypselosaurus – was only about a foot long. Once they were born, though, dinosaurs grew quickly! A baby hadrosaur was about ten inches long, but after just ten years it grew to its adult size of 26 feet.
Dinosaur dung that is fossilized is called a coprolite. These fossils are very interesting because they give us a lot of clues about what the creatures ate and the way their insides were shaped. The difficult part is trying to figure out which coprolite belongs to which dinosaur! As you can imagine, the larger the dinosaur, the larger the dung it left behind. For example, a coprolite fossil from a Brontosaurus could be almost 16 inches long! But often what we know about a dinosaur’s behavior helps us. For example, coprolite fossils from carnivores, or meat-eating dinosaurs, are more likely to survive over millions of years than those of herbivores, or plant-eating dinosaurs. This is because there are more minerals in a carnivore’s food than in an herbivore’s. Also, these dinosaurs bury their dung in little holes and cover it up with dirt.