The first thing that comes to mind when fossils are mentioned are the complete skeletons of dinosaurs that adorn the natural history museums of the world. However, dinosaur trace fossils embody an important record of behavior and distribution that augments the information gleaned from the skeletons of dinosaurs. Dinosaur trace fossils include dinosaur footprints, dinosaur eggs, dinosaur gastroliths, dinosaur coprolites, and dinosaur skin impressions. Strictly speaking, only footprints and skin impressions are trace fossils, but eggs, gastroliths, and coprolites are also fossilized traces of dinosaurs and yield important information.
Dinosaur footprints are found on every continent except Antarctica and span from the Upper Triassic through the Upper Cretaceous. By identifying the footprints of a trackway by matching them to known dinosaur foot structures, paleontologists are able to learn about the posture, gait, foot structure, speed, and social behavior of the dinosaurs that made them. Many footprints have no match to a known dinosaur, and many footprints can only be linked to a group of dinosaurs because the distinctions between species might be in the cranial structure only.
The first dinosaur eggs were discovered in France in 1869, but the most famous eggs were found in Mongolia in 1923. The best known eggs are from the Upper Cretaceous of Asia and the western United States. Dinosaur eggs were composed of organic matter and interlocking, well-organized crystalline calcium carbonate which made them rigid. Since none of the eggs found are older than the Late Cretaceous, paleontologists believe that some primitive dinosaurs may have laid soft-shelled eggs lacking an extensive mineral matrix. The shapes of eggs vary from spherical to cylindrical, and some eggs contain well-preserved embryos or are closely associated with skeletons of hatchlings or adults.
Today, some living birds and reptiles swallow stones and hold them in a gizzard to aid digestion by grinding food. They are usually highly polished by grinding against each other and against food. Most dinosaur gastroliths are isolated polished stones from the Upper Jurassic and Lower Cretacous rocks in the western United States, and may or may not be actual gastroliths. Some highly polished stones are found in the abdominal cavities of dinosaurs and are almost certainly dinosaur gastroliths.
Coprolites are fossilized feces, and there are plenty of coprolites of many animals from crustaceans to extinct hominids that give evidence of diet. However, it is very difficult to link coprolites decisively to their producers, especially with dinosaurs. For this reason, few dinosaur coprolites have been confidently identified and analyzed. Those that are known are usually from predatory dinosaurs such as Tyrannosaurus rex because of the rich phosphatic mineral content from undigested bone that fossilized easily.
Dinosaur Skin Impressions
Fossilized skin impressions are known for ornithopod, theropod, and ceratopsian dinosaurs and reveal that these dinosaurs had scales similar to some living reptiles. Some small theropods closely related to birds had feathers and possibly feather body coverings. The color of dinosaurs is only conjecture, however, because the pigment in skin and scales did not fossilize.