Fossils are the remains or traces of organisms that were once alive. From the massive bones of dinosaurs to the delicate impression of a fern frond, fossils come in all shapes and sizes. Fossils that are the remains of an actual organism, such as a shell, leaf, or bone, are known as body fossils. Those fossils that record the action of an organism, such as a footprint or burrow, are known as trace fossils. Paleontologists study fossils to help understand the evolution and the history of life on Earth. Fossils also tell us about ancient ecosystems and climates, and how climate change can affect life.
How do fossils form?
Most organisms die and decay to leave no remains at all, but on very rare occasions a dead organism can become fossilized. The most important step to become a fossil is to get buried quickly after death. Once buried, water trickling through the ground penetrates the shell or bone, and the minerals in that ground water can begin replacing the original shell or bone. After a very long time, the shell or bone is turned to stone, making a fossil. Of course, some more recent fossils—such as the frozen mammoths found in Siberia—are not altered but contain the original bone and tissue.
How do we know the age of fossils?
There are two different methods to age-date fossils. Relative dating simply tells us which of two fossils is older, and which is younger. Fossils are most commonly found in sedimentary rock, which forms in layers. The layers at the bottom formed first and, therefore, are older than those above them. Dinosaurs are found in layers that are below those containing the first humans, so we can conclude that dinosaurs and humans did not live at the same time; the big dinosaurs like Tyrannosaurus and Triceratops lived and died long before humans first evolved.
Absolute dating allows scientists to determine exactly how long ago an organism lived. Certain elements in rocks are radioactive. These elements are unstable and decay, or change, over time into a stable element (the original, unstable element is known as the parent, while the new, stable element is known as the daughter). This decay occurs at a steady, constant rate; it's like a clock that starts running as soon as a rock formed. So, by first knowing the rate of decay and then measuring the amount of the parent element left compared to amount of daughter element, scientists can determine the age of a rock. The age of fossils found in those rocks (or just below or above them) can then be determined!
What is paleontology?
Paleontology is the science that investigates the history of life on Earth. The term is derived from the Greek words palaios ("ancient") and -ology ("study of").
What is the difference between paleontology and archaeology?
Although the two terms are often confused, paleontology and archaeology are two very different fields in science. Archaeology is the study of peoples and their cultures. While paleontology is based on fossils, the science of archaeology is based on artifacts. Artifacts are objects made by humans, such as hunting points or pottery, while fossils are naturally occurring remains or traces of organisms.
How do I become a paleontologist?
To study paleontology, it is useful to have knowledge in biology and geology—both provide clues to help understand ancient worlds. Knowledge of anatomy and evolutionary theory will help understand the fossils themselves, while the rocks in which fossils are found provide clues to ancient environments.
Some paleontologists work in a museum, while others work at a university or with organizations like the National Park Service. There are many different roles that a paleontologist can play: some work in a laboratory and extract the fossils from the surrounding rock, while others focus on research and study the fossils once they are freed from the rocks.
Can I find a dinosaur bone in my backyard?
That depends. If a paleontologist wants to find dinosaur fossils, he or she must first go to a place where the rocks are the right age. In other words, the rocks at the surface must have first formed during the age of dinosaurs. In the United States, rocks of this age are very common throughout the Rocky Mountain region. So, the rocks found in any area depend on the rocks at the surface. If the rocks were first deposited in a shallow sea, you might find fossils of corals and other sea creatures. If the rocks formed in a river during the age of dinosaurs, you might find a dinosaur bone!
Why do we study fossils?
Fossils help us understand life's past. Without fossils, we'd know nothing of the mighty dinosaurs, tiny three-toed horses, and thousands of other prehistoric forms that are long extinct. Fossils also provide clues to the interrelationships of all species. The life that we see today is just a snapshot in time, but that life has a history. One of the ways we learn about the ancestors of today's species (and their ancestors, and their ancestors, and so on) is by studying fossils. Fossils of walking whales, for example, tell us that whales evolved from four-footed mammals that lived on land, and fossils of ancient humans show us our own roots in Africa.
Finally, fossils can also provide more information regarding changing climates. Fifty million years ago, areas of Wyoming were covered with large lakes that were inhabited by fishes and crocodiles, with tall palms along the shore. Together, the occurrence of these plant and animal-types tell us that Wyoming was much warmer in the past than it is today. Fossils can therefore be used as "thermometers" for determining ancient climates.
Not long after the tropical lakes covered areas of Wyoming, global temperatures cooled. Grasses flourished in these cooler and drier conditions, while thick tropical forests shrank. This climate-change induced floral turnover had an incredible effect on mammals; many mammal types evolved to possess a diet of grasses instead of leaves (such as horses and rhinos), while others became extinct (such as titanotheres).
With concerns of climate change today, we can look to the fossil record to see not only how climates change over time, but the effects of climate change on Earth's life.
[ Thank you to R. Kissel, PhD; PRI, for preparing these answers ]
Last updated: December 14, 2016