Sediment is made up of solid, loose pieces of rocks (in the form of sand, silt, clay, and gravel), or the remains of living things found at the surface of the Earth. Sediment is material that has been eroded and deposited by wind, running water, waves, and ice.

Living organisms are adapted to their environment. This means that the way they look, the way they behave, how they are built, or their way of life makes them suited to survive and reproduce in their habitats. For example, giraffes have very long necks so that they can eat tall vegetation, which other animals cannot reach. The eyes of cats are like slits. That makes it possible for the cat's eyes to adjust to both bright light, when the slits are narrow, and to very dim light, when the slits are wide open.

A paleontologist collects as many fossils as possible from a rock or sediment. Once the fossils are prepared by scraping and cleaning, they are sorted by geometry. Fossils with very similar geometry are assumed to belong to a single species. Fossils with somewhat different geometry are assumed to belong to a different species. Usually the fossil species has already been studied and named. Sometimes, however, the species is a new one.

Knowing the fossil record lets a geoscientist place a particular fossiliferous rock layer into the scale of geologic time. But the time scale given by fossils is only a relative scale, because it does not give the age of the rock in years, only its age relative to other layers. Long after the relative time scale was worked out from fossils, geologists developed methods for finding the absolute ages of rocks, in years before the present. These methods involve radioactivity. Here's how one of the important ones works.

Paleontologists want to know the history of evolution and extinction of fossil species through geologic time. To do that, they try to study all of the fossils that have been preserved in sedimentary rocks. That's called the fossil record. Paleontologists have been collecting fossils from sedimentary rock layers around the world for over two hundred years. Their goal is to figure out the succession of species through all of geologic time. Once that succession is known, it serves as a scale of geologic time.

Every plant or animal belongs to a species. A species is a population of plants or animals that can breed to produce offspring that can then produce offspring themselves. Biologists believe that new species evolve from existing species by a process called natural selection. Here's how it works. Genes are chemical structures in the cells of the organism. The nature of the organism is determined by its genes. The organism inherits the genes from its parents. Occasionally a gene changes accidentally. That's called a mutation. The changed gene is passed on to the next generation.

For a fossil to form, several conditions have to be met. First of all, the animal had to live in the given area! Animals live in many environments on Earth, but not everywhere. The water above many lake bottoms and many areas of the deep ocean bottom are stagnant. The bottom water is never exchanged with surface waters, so the water contains no dissolved oxygen. Animals cannot live without oxygen, so no animals live there.

Sediments are home for many kinds of marine animals. Some animals live on the surface of the sediment, and some burrow into it. Some fossil shells are found mixed with the mud they lived in. Other fossil shells were moved by strong currents and deposited along with sand or even gravel. If the shells are buried by more sediment before they are worn away or dissolved, they become fossilized.

In certain conditions, and over a very long period of time, sediment becomes compacted and cemented into sedimentary rock. Fossils are more common in some kinds of sedimentary rocks than others. There are many factors that can contribute to the likelihood of an organism being preserved as a fossil. Fossils are most common in limestones. That is because most limestones consist partly or mostly of the shells of organisms. Sometimes, however, the shells are worn so much that they look like sediment grains rather than "real" fossils. Fossils are also common in shales, which form from muds.