What is a Fossil?
Fossil palm: Click to enlarge
Fossil palm frond from Wyoming
If you're like most people, when you think about fossils, you think of dinosaur or mammoth bones. While those images are accurate, they don't include the whole range of possibilities. A fossil can be as young as 10,000 years, or as old as 3.5 billion years. It can be as big as a Seismosaurus dinosaur, or so small it can't be seen with the naked eye. It can be a frozen Siberian mammoth, complete with hair, or a track left behind by a long-gone animal. It can contain material from the original organism, or no trace of organic material at all. So what is a fossil?

A fossil is any evidence of ancient life. What is common to all fossils is that they give us clues to past worlds. Thanks to fossils, we know that a wide variety of life forms have occupied this planet. We know life has evolved over time. We also know that the vast majority of life forms (over 99 percent) have gone extinct. In fact, almost all fossils are of organisms that are now extinct.

Crinoids: Click to enlarge
Crinoids from Indiana
Fossils are all around us. Currently, we derive most of our energy from fossil fuels, including coal (remains of ancient swamps) and oil (remains of ancient marine organisms). In our commutes to school and work, we drive over dinosaur tracks every day. Amazing as it sounds, there are an estimated 1 billion Cretaceous dinosaur tracks in the Denver metro area alone! Yet as common as fossils are, they are a finite resource, and the choicest ones can be rare indeed. Earth's fossil record has given us a telling picture of the past, yet it's still an incomplete picture. Most of the life forms that have lived on this planet haven't become fossils, and most fossils haven't been found. What's more, not all parts of an organism are likely to become fossils. Shells, bones, and teeth are likely to fossilize, but soft tissue almost never does.

Fossils come in a variety of forms, such as:

    Diplocaulus: Click to enlarge
    Fossil amphibian from Texas
  • Compressions, where the organic material is flattened by burial
  • Impressions, images of ancient life forms where no organic material is left
  • Casts and molds, where sediment fills the space that was once occupied by the organism (which has since decayed), leaving a three-dimensional replica
  • Permineralization, in which mineralized water has percolated through the material and gradually replaced some or all of the organic material
  • Trace fossils, such as tracks or burrows (These fossils are especially useful in that they record behavior.)

So what is the big picture left by fossil remains? Since the early 19th century, paleontologists and geologists have marked the ages of the Earth by the appearance and disappearance of certain types of fossils. Twentieth-century techniques of radiometric dating (based on measurable rates of radioactive decay) have enabled scientists to attach approximate dates to these ages of life. This chart illustrates youngest to oldest.
Key:  ya=years ago  mya=million years ago  bya=billion years ago
Phanerozoic Eon Cenozoic Era Quaternary Period Holocene Epoch 10,000 ya - now
Pleistocene Epoch 1.6 mya - 10,000 ya
Tertiary Period Pliocene Epoch 5.3 mya - 1.6 mya
Miocene Epoch 23.5 mya - 5.3 mya
Oligocene Epoch 34 mya - 23.5 mya
Eocene Epoch 55 mya - 34 mya
Paleocene Epoch 65 mya - 55 mya
Mesozoic Era Cretaceous Period 145 mya - 65 mya
Jurassic Period 205 mya - 145 mya
Triassic Period 251 mya - 205 mya
Paleozoic Era Permian Period 290 mya - 251 mya
Pennsylvanian Period 320 mya - 290 mya
Mississippian Period 360 mya - 320 mya
Devonian Period 410 mya - 360 mya
Silurian Period 440 mya - 410 mya
Ordovician Period 505 mya - 440 mya
Cambrian Period 545 mya - 505 mya
Precambrian Proterozoic Eon 2.5 bya - 545 mya
Archean Eon 4.6 bya - 2.5 bya

Johnson, Kirk R. and Richard K. Stucky. 1995. Prehistoric Journey: A History of Life on Earth. Boulder: Roberts Rinehart Publishers.
Lockley, Martin and Adrian Hunt. 1994. Fossil Footprints of the Dinosaur Ridge Area. Denver: Friends of Dinosaur Ridge/University of Colorado at Denver.
Rudwick, Martin J.S. 1976. The Meaning of Fossils: Episodes in the History of Paleontology. Chicago: University of Chicago Press.


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