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A Record Of The Past

Fossils provide a record of the past: they identify long extinct plants and animals--from early invertebrates to dinosaurs to our ancient human relatives--and tell us where they lived. But fossils do not easily give up their secrets.

An understanding of how fossils are formed helps paleontologists know where to look for them. And careful study of local geology, along with chemical analyses, allows experts to date them accurately. Then fossils truly can reconstruct ancient history.

What Are Fossils?

  • Fossils are physical evidence of ancient life-from traces of animal activities to preserved body parts.
  • When an animal dies, predators or scavengers usually eat its remains, or wind and rain destroy the fragile bones and tissues. But sometimes an animal's remains are quickly covered and protected by sediments-usually sand and silt carried by water and wind.

Body Fossils
 Preserved or petrified bones and teeth, like these remains of an early horse relative, are the best-known types of body fossils. But imprints and casts, sometimes preserved in sediments even when biological remains decompose, are also widespread.

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© Mauro Fermariello / Photo Researchers, Inc.

Homo heidelbergensis (a European Homo erectus) tracks, Campania region of southern Italy


Trace Fossils

Trace fossils include preserved footprints, burrows, nests, tracks and even coprolites--fossilized animal dung. Here a paleoanthropologist examines fossilized footprints made by a human relative 385,000 to 325,000 years ago in southern Italy.

Rocky Remains

Bones and other biological materials embedded in sediments may be slowly replaced by minerals from the surrounding environment. Eventually they become rocks themselves! In many cases, the replacement of biological materials by minerals is so precise that even the finest details of bone structure or wear are preserved.

Halfway Home

This 30,000 year old bison leg from Alaska is a subfossil: it has lost some of its organic compounds but has not yet been fully mineralized. Blue vivianite crystals began to form on the bison's tissues when the animal, low in iron but high in phosphates, came to rest in a iron-rich but phosphate-poor bog.

Layer Lessons

Over time, sediments carried by water and wind build up and harden into layers of rock. Sometimes those layers incorporate plant and animal remains.

Full Of Fossils

Animal and plant remains usually do not become fossils unless they are quickly covered and protected by sediments. As a result, sites where sediments continually collect--ancient ocean floors, lakes, rivers and swamps, for instance--contain the most fossils.

How Do We Find Fossils?

Many fossils are destroyed before they see the light of day by intense pressure and heat generated during geological events. But others are preserved in stone for many millions of years. As rain and wind erode the surrounding rock, fossils are sometimes exposed at the surface once more. Paleontologists thus often look for fossils in areas where the ground is eroding rapidly.

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© Will Harcourt-Smith

Exposed fossil; Rusinga Island, Lake Victoria, Kenya


Get It While You Can

As a sedimentary rock erodes, fossils may emerge. But fossils unprotected by rock do not survive long. Paleoanthropologists working on Rusinga Island in Lake Victoria, Kenya in 2006 found this fossil just in time; soon it would have been destroyed by wind and water.

Fossil Layers

If you could look beneath Earth's surface, you would see many layers of dirt and rock stacked one on top of another. And sometimes those layers contain fossils!

American Museum of Natural History

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