Fossils tell us what we need to know about dinosaurs and what it was like millions of years ago. Fossils are classified remains of animals that were alive long ago. An animal dies and its skin rots away. The, its skeleton gets covered in sediments. Pressure pushes the skeleton into rock. Millions of years later, paleontologists dig the dinosaur up! There is two groups of fossils, which are mold and trace fossil. Mold fossils are the skeleton. Trace fossils are the dent/ingraving of the shape of the animal.