Fossils are precious gifts from the geologic past: signs and remains of ancient living things preserved in the Earth's crust. The word has a Latin origin, from fossilis meaning "dug up," and that remains the key attribute of what we label as fossils. Most people, when they think of fossils, picture skeletons of animals or leaves and wood from plants, all turned to stone. But geologists have a more complicated view, so let's dig into the subject.
The Different Kinds of Fossils
Fossils can include ancient remains, the actual bodies of ancient life. These can occur frozen in glaciers or polar permafrost. They can be dry, mummified remains found in caves and salt beds. They can be preserved over geologic time inside pebbles of amber. And they can be sealed within dense beds of clay. They are the ideal fossil, nearly unchanged from their time as a living thing. But they are very rare.
Body fossils, or mineralized organisms—dinosaur bones and petrified wood and everything else like them—are the best known kind of fossil. These can include even microbes and grains of pollen (microfossils, as opposed to macrofossils) where the conditions have been right. They make up most of my Fossil Picture Gallery. Body fossils are common in many places, but on the Earth as a whole they are fairly rare.
The tracks, nests, burrows and feces of ancient living things are another category called trace fossils or ichnofossils. They are exceptionally rare, but trace fossils have special value because they are remains of an organism's behavior.
Finally there are chemical fossils or chemofossils, remains that consist of mere organic compounds or proteins found in a body of rock. Most books overlook this, but petroleum and coal, the fossil fuels, are very large and widespread examples of chemofossils. Chemical fossils are also important in scientific research into well-preserved sedimentary rocks. For instance, the waxy compounds found on modern leaves have been detected in ancient rocks, helping to show when these organisms evolved.
What Becomes Fossils?
If fossils are things dug up, then they must begin as whatever can be buried. If you look around, though, very little that is buried lasts long. Soil is an active, living mixture in which dead plants and animals are broken down and recycled. To escape this round of breakdown, the creature must be buried, and taken away from all oxygen, soon after death.
When geologists say "soon," though, that can mean years. Hard parts such as bones, shells and wood are what turn to fossils the great majority of the time. But even they need exceptional circumstances to be preserved. Usually they must be quickly buried in clay or other fine sediment. For skin and other soft parts to be preserved requires even rarer conditions, such as a sudden change in water chemistry or decomposition by mineralizing bacteria.
Despite all this, some amazing fossils have been found: 100-million-year-old ammonoids with their mother-of-pearl nacre intact, leaves from Miocene rocks showing their autumn colors, Cambrian jellyfish, two-celled embryos from half a billion years ago. There are a handful of exceptional places where the Earth has been gentle enough to preserve these things in abundance; they are called lagerstätten.
How Fossils Form
Once buried, organic remains enter a long and complex process by which their substance is changed into fossil form. The study of this process is called taphonomy. It overlaps with the study of diagenesis, the set of processes that turns sediment into rock.
Some fossils are preserved as films of carbon under the heat and pressure of deep burial. On the large scale, this is what creates coal beds.
Many fossils, especially seashells in young rocks, undergo some recrystallization in groundwater. In others their substance is dissolved, leaving open space (a mold) that is refilled with minerals from their surroundings or from underground fluids (forming a cast).
True petrification (or petrifaction) is when the fossil's original substance is gently and completely replaced with another mineral. The result can be lifelike or, if the replacement is agate or opal, spectacular. The Petrified Wood Gallery shows some examples.
Even after their preservation over geologic time, fossils may be hard to retrieve from the ground. Natural processes destroy them, principally the heat and pressure of metamorphosis. They may also disappear as their host rock recrystallizes during the gentler conditions of diagenesis. And the fracturing and folding that affects many sedimentary rocks can wipe out a large share of the fossils they may contain.
Fossils are exposed by the erosion of the rocks that hold them. But during the thousands of years it may take to unveil a fossil skeleton from one end to the other, the first part to emerge crumbles into sand. The rarity of complete specimens is why the recovery of a large fossil like Tyrannosaurus rex can make headlines.
Beyond the luck it takes to discover a fossil at the right stage, great skill and practice are required. Tools ranging from pneumatic hammers to dental picks are used to remove the stony matrix from the precious bits of fossilized material that make all the work of unwrapping fossils worthwhile.