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Types of Dinosaur Fossils

Astounding creatures once roamed our planet. Some were as tall as a nine-story building or longer than a city bus. Some weighed as much as ten elephants or as little as a house cat. And the only way we know about them is through fossil evidence. From tiny sea invertebrates to winged reptiles the size of airplanes, animals that lived millions of years ago became fossilized. And the most familiar of these are the dinosaurs.

Trace Fossils
- Teeth marks on bones can show who was eating whom.
  
  Also called ichnofossils, trace fossils are any that "record the movements and behaviors of the dinosaurs." These can be nests or burrows, teeth marks on fossilized bones, foot and tail prints, even fossilized droppings (coproliths) and fossilized gizzard stones (gastroliths). Trace fossils are far more common than fossilized skeletons because a creature leaves behind more footprints than bones. Missing evidence can be useful too, such as a lack of tail marks found with footprints, which would indicate the dinosaur walked upright.
Mold Fossils
- Footprints tell paleontologists much about the animal's biology.
  
  Mold fossils are a kind of trace fossil. Because they lived on land, dinosaurs left tracks behind. A mold fossil is a negative impression made by an object --- perhaps a prehistoric foot --- in a soft substance like mud or clay, called a substrate. This is similar to making a foot or hand print in plaster. Mold fossils can also form when the original object that made the impression --- such as a bone or shell --- wears away, leaving the impression.
Cast Fossils
- A cast footprint creates the reverse of a mold print.
  
  Another type of trace fossil is a cast fossil. Cast fossils form when an impression fills with mud or other mineral sediment and then fossilizes. It can happen as soon as the impression is made, like when a foot presses into the ground, or it can happen long afterward, such as when the object that made the original impression wears away and the resulting mold fills with sediment. Like mold fossils, cast fossils are evidence of dinosaur behavior and biology.
True Form Fossils
- Few intact skeletons are ever found.
  
  True form fossils are some of the most exciting and certainly the ones that come to mind when thinking of dinosaurs. True form fossils are also called body fossils, and they can include everything from bones to eggs to teeth and claws to skin and organs. These last are extremely rare, as the soft tissues tend to rot away or be eaten before they ever get the chance to become fossilized.

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