Fossils: Keys to the Past
Fossils reveal the secrets of Earth’s past: the animals and plants from long ago, how they lived, and what their world was like. For scientists, especially paleontologists who study the history of life on Earth (including dinosaurs!), finding a good fossil is like finding hidden treasure.
Fossils are the preserved remains, impressions, or traces of animals and plants that lived long ago. On rare occasions, they include the actual remains, or bodies, of ancient animals or plants, which managed to survive the passage of time pretty much unchanged — maybe the hard shell of a scallop, the armored exoskeleton of a beetle, the tooth of a woolly mammoth, or a little lizard entombed in amber (fossilized tree resin).
The vast majority of the time, however, fossils are rocks. Paleontologists divide fossils into two main groups. Some fossils, called body fossils, show the structure of the plant or animal. They form directly from the remains of plants and animals. These fossils include such things as hardened dinosaur bones, petrified wood, or a dark film that a leaf or insect left behind. Other fossils, called trace fossils, record signs of animal or plant activities, such as walking, burrowing, feeding, scratching, or even resting. Trace fossils do not include the actual bodies. Instead, they form from impressions left behind by plants and animals, such as bird tracks, an insect’s burrow, or even dinosaur poop.
Most animals and plants don’t become fossils after they die. They simply decompose, decaying or breaking down into little bits, or get gobbled up by a scavenger. But some remains get buried too fast for those things to happen. An animal might die near a body of water and sink to the bottom, where its remains get covered in sediment. The animal’s soft parts usually still decay, leaving only hard bits like bones and teeth. As sediment builds up, mineral-rich water seeps into the remains, depositing minerals in the tiny spaces of the bones and even replacing the original bone. The newly deposited minerals react with those in the animal’s remains and, thanks to that chemical reaction, harden into fossils. These hard fossils, called cast fossils, take the shape of the original animal parts, like dinosaur bones.
Sometimes fossils form in a different way. As sediment builds up, it hardens around the animal’s remains. Water seeps into the area where the animal’s remains are decomposing. But instead of leaving behind mineral deposits that harden in the shape of the animal, the water washes the animal’s decaying remains away. The animal’s shape is left in the surrounding, fossilized rock. These fossils are called mold fossils.
Both cast and mold fossils are 3D and can show all the details of the original animal (or plant). The big difference is how they form. An easy way to remember the difference is to think about making Jell-O! You fill a fancy pan with liquid Jell-O and let it get firm. The Jell-O ends up shaped like the fancy pan; it is cast in the same shape. The fancy pan itself is a mold. Both body and trace fossils can form as either cast or mold fossils.
Most fossils are buried deep in the Earth. But over time, land changes in ways that bring fossils closer to the surface. The minerals in rock break down through a process called weathering. And rock, sediment, and soil may be cleared away by wind, water, and ice — processes known as erosion. As the Earth’s surface changes, scientists can dig up new fossils and learn more about past life and Earth’s history. (In fact, the word “fossil” comes from the Latin word fossilis, which means “dug up.”)
Over hundreds of millions of years, layers of sediment built up on the Earth, burying animals and plants specific to certain time periods. As scientists study older and older layers, they find fossils from different times in Earth’s history. In rock that formed before a certain time, roughly 2.8 million years ago, scientists will not find human fossils.If they keep digging back further, they will find layers with no modern birds, then no mammals, then no reptiles and so on. The layers represent a timeline of life on Earth.
Knowing what lived when helps scientists identify the age of new discoveries. Scientists know, for example, that ammonites, cool-looking spiraled marine animals, lived in the Paleozoic and Mesozoic Eras roughly 408 to 65 million years ago. So if they dig up an ammonite, they know the surrounding rock is that old, too. And if they discover a new fossil in that same sedimentary layer, they know it is from the same time period.
Fossils are our keys to understanding prehistoric life and Earth’s history. By studying fossils, we learn about the great variety of plants and animals that lived in the past — not just what they looked like (body fossils), but also how and where they moved and what they ate (trace fossils). By comparing fossils from different time periods, we can track the evolution of a species, see how it adapted to changes in its environment, and understand more about the climate and environment where the fossils were buried. Some of today’s deserts, for example, were lush forests or vast oceans long ago. How do we know? Paleontologists found fish fossils in the desert.