Adapted from Dinosaurs: The Very Latest Information and Hands-On Activities From the Museum of the Rockies by Liza Charlesworth and Bonnie Sachatello-Sawyer. A Scholastic Professional Book.
What Are Fossils? What Can We Learn From Them?
Fossils are the organic remains of prehistoric plants or animals. They are also the records of ancient life on Earth. Whether taking the form of bones, teeth, or eggs, fossils provide scientists with tangible links to the past. Together, these remains show changes in plant and animal life over time and provide information about shifts in climate and even geography. Fossils can also be compared to the living things of today, providing clues to present-day adaptation and behavior. For example, dinosaur bones share many commonalities with contemporary birds, leading paleontologists to believe that these long-extinct creatures were the ancestors of our feathered friends.
How Are Dinosaurs Preserved?
Dinosaur fossils are often found in layers of sedimentary rocks such as sandstone and mudstone. How do these remains, laid down in ancient rock, find their way to the Earth's surface? Here's one story:
1. The dinosaur dies. It sinks to the bottom of an ancient river or lake, or is buried in a sand dune.
2. The dinosaur's body rots or is eaten by scavengers, until only its skeleton, or a few bones, remains.
3. Layers of sediment build up over the bones. Minerals collect in the small spaces of the bones, making them harder and heavier. This process turns them into fossils.
4. The water dries up. Over millions of years, the rock erodes and the fossils are exposed.
This chain of events — occurring over the course of many million years — happens only in lucky cases. Why? So many elements can conspire to destroy dinosaur bones and fossils. For example, if the animal died on land, scavengers might have fed on its dead body, leaving the exposed bones to crack and dry up in the hot sun. And even when the bones were quickly covered by sediments, they could still be destroyed by mountain-building activity, glaciation, or chemicals leaching through the soil.
What Are Trace Fossils? What Can We Learn From Them?
Unlike a regular fossil, which is the actual remains of a prehistoric animal or plant, a trace fossil provides indirect evidence of an organism's existence. Tracks, burrows, and skin imprints are all trace fossils. How were they formed? First, an ancient organism left its impression on sand or mud before rotting away. Then, over millions of years, that impression hardened into rock, or a trace fossil. "Coprolites," the scientific term for the fossil remains of animal dung, are trace fossils, too. Are these types of remains as important to researchers as regular fossils? Absolutely. Trace fossils provide many valuable clues about the lives of the might dinosaurs. For example, tracks help paleontologists to determine:
- whether dinosaurs walked on two or four legs (absence of front footprints indicate that some species moved on two legs);
- whether dinosaurs traveled in herds (numerous and overlapping prints of the same species is a sign of herding);
- whether dinosaurs fought with one another (two sets of prints in a chaotic pattern can indicate a struggle);
- whether dinosaurs dragged their tails (the lack of tail impressions probably means that most dinosaurs held their tails erect);
- how fast dinosaurs moved (distance between consistent prints is a sign of speed);
- and of course, how big dinosaurs' feet were.
Likewise, there is much to learn from skin impressions. These imprints, which are rare, provided the first strong evidence that dinosaurs' skin was scaly. Coprolites are telling, too. Paleontologist Karen Chin has distinguished herself in this important area of dinosaur science. By studying fossilized droppings, she has learned a great deal about the diets of these enormous creatures.