Follow a Fossil: What is a Fossil?

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 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:

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

References:
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.