Fossils are the preserved remains, or traces of remains, of ancient organisms.
A fossil can preserve an entire organism, just part, or traces of one (for example, footprints). Bones, shells, fur, skin, footprints, feathers and leaves can all become fossils.
Fossils can be very large or very small. The smallest fossils are called microfossils and are only visible with a microscope. Pollen fossils are microfossils. Fossils you can see with your eyes are called macrofossils and can be several meters long and weigh several tons. An example of a macrofossil could be a petrified tree or a dinosaur bone.
Preserved remains are defined as fossils if they are older than 10,000 years old. The oldest fossils are almost 4 billion years old and are traces of ocean-dwelling bacteria. Some of the youngest fossils (10 000 years old) are, for example, the fossilized teeth of woolly mammoths.
Fossilization
The word fossil comes from the Latin word fossus, meaning "having been dug up." Fossils are often found in rock formations, which although formed deep underground, have been moved to the surface (by plate tectonics) and/or been eroded to the Earth’s surface by wind, ice, rain and water.
Fossilization is the process of remains becoming fossils. Fossilization is rare. Most organisms decompose fairly quickly after they die.
For an organism to be fossilized, the remains usually need to be covered by sediment soon after death. Sediment can include the sandy or muddy seafloor, volcanic ash, and even sticky tar.
Over time, minerals in the sediment seep into the remains. The remains become fossilized. Fossilization more commonly occurs in organisms with hard, bony body parts, such as skeletons, teeth, or shells. Soft-bodied organisms, such as worms, are rarely fossilized, because soft parts quickly decompose or are eaten.
Sometimes, however, the sticky resin of a tree can become fossilized. This is called fossilized resin or amber. Amber can preserve the bodies of many delicate, soft-bodied organisms, such as ants, flies, and mosquitoes. While fossils found in rocks made of a soft sea floor are usually compressed, amber preserves the fossils in three dimensions.
Body Fossils and Trace Fossils
The fossils of bones, teeth, and shells are called body fossils. Most dinosaur fossils are collections of body fossils.
Trace fossils are rocks that have preserved evidence of biological activity. They are not fossilized remains, just the traces of organisms. The imprint of an ancient leaf or footprint is a trace fossil. Burrows can also create impressions in soft rocks or mud, leaving a trace fossil.
Paleontologists
Paleontologists are people who study fossils. Paleontologists find and study fossils all over the world, in almost every environment, from the hot desert to the humid jungle. Studying fossils helps them learn about when and how different species lived millions of years ago. Sometimes, fossils tell scientists how the Earth has changed. Fossils also help scientists study how species have evolved over the course of millions of years.
Fossils of ancient marine animals called ammonites have been unearthed in the highest mountain range in the world, the Himalayas in Nepal. This tells scientists that millions of years ago, the rocks that became the Himalayas were at the bottom of the ocean.
Fossils of an ancient giant shark, a megalodon (Carcharocles megalodon), have been found in the landlocked U.S. state of Utah. This tells scientists that millions of years ago, the middle of North America was probably entirely underwater when this animal lived.