What are Fossils?
Initially, the word "fossil" (from Latin "fossilium", "that which is dug up") included anything interesting that people pulled out of the ground: unusually-shaped stones, minerals, human artifacts, and the remains of organisms. Eventually, however, the term was restricted to organic remains.

The contemporary definition of fossil is the physical remains or trace of behavior preserved in the rock record. Specimens which were actually once part of a living thing (bones, teeth, skin, shell, leaves, trunk, pollen, egg, etc.) are body fossils; those that are marks in the sediment produced by the activities of living things are trace fossils.

Some common type of trace fossils (also called ichnofossils) include:

• Burrows, both on land and in marine sediments
• Walking traces, such as dinosaur trackways or trilobite scuttle marks
• Resting traces, such as this resting mark of a dinosaur, or this starfish trace
• Nests, such as this dinosaur nest and these fossil cocoons

Trace fossils of this sort are preserved the same way that other sedimentary structures were: the features are covered over by another layer of sediment, and the lithification process locks the shapes into place.

Other types of trace fossils are products of the action of organisms, that get preserved in the same general manner as body fossils. This includes such things as:

• Coprolites ("dung stones"), fossilized feces, such as these pieces of giant ground sloth dung and these turtle and fish coprolites
• Vomitites (also called regurgitalites), just what the name suggests, such as this mass of belemnite (squid-like cephalopod) shells horked up by a marine reptile
• Cololites (gut contents), such as these baby dinosaur bones in the belly of a mammal, or this fish with another fish in its belly (or most of a food chain, like this beetle-in-a-lizard-in-a-snake or this Jurassic Turducken: a crustacean in the gut of cephalopod in the gut of a shark)
• Feeding traces on other organisms, such as this leaf munched by insects or this bitten trilobite or this bivalve, drilled by a snail

Something to consider about trace fossils: they were produced by the organism while it was still alive. This makes them distinct from your average body fossil, which are parts of things that had died.

• Does it contain hard parts: tissues like bone or shell or wood that are likely to survive transport and burial? Some organisms do have such hard parts; others do not. Not surprisingly, organisms with abundant hard parts are common in the fossil record; those (like insects) with merely "firm" parts (tough but easily degradable) tissues are less common; those (like flatworms and jellyfish) with very few or no hard tissues are extremely rare as fossils. Major hard tissues include:
  • Carbonate shells: from everything from coccolithophorids (chalk-forming plankton) to foraminferans (armored amoebas) to some algae to corals to mollusks to sea urchins and many forms in between. These are made of the same mineral as limestone: indeed, many limestones are made of their bodies.
  • Silica shells: glassy/opaline hard parts, in microscopic diatoms and radiolarians to glass sponges.
  • Calcium phosphate (hydroxylapatite) bones, teeth, and scales: the hard parts of vertebrates.
  • Chitin: the firm parts of most arthropods (insects, crustaceans, etc.)
  • Keratin: a common firm part in fungi and animals, including hair, feathers, nails.
  • Collagen: the connective tissue of animals, found in bone, ligaments, cartilage, etc.
  • Cellulose and lignin: the hard parts of plants
  • Spongin: the firm parts of "common sponges", like the bath sponge
• Did it live in (or at least could wind up in) a depositional environment? If so, great: chances are good that at least some individuals might wind up getting incorporated into the sediment. But organisms that live in erosive environments (mountains; rocky deserts; rocky shorelines; etc.) are much less likely to wind up in the record.
  • Of special note are different ways of life of marine organisms:
    • Planktonic organisms float above the sea floor, normally in the shallow layers of the water (regardless of how deep it is). Since they don't care about the conditions underneath, some of them will settle down onto depositional environments when they die, but some might not.
    • Nektonic organisms swim around. Some may live in many environments, so some of their bodies might settle down onto accumulating surfaces. Others might prefer particular habitats: if so, those that live over sandy/muddy bottoms would be expected to become fossils, while those which preferred rocky ones would not.
    • Benthic organisms are bottom dwellers. Some are infaunal (burrow in the sediment) are already IN the sediment, so have a good chance of preservation. Others are epifaunal (crawl, scuttle, squidge, or just sit around on the surface): these will be preserved if the overall environment is one of deposition.
• How big is the organism? Microscopic organisms will be easily buried in muddy environments, and have a good chance of winding up in other ones as well. Big organisms need lots of rapid deposition to cover them. Medium-sized ones are in between.
• How many hard parts are in the organism, and how well integrated are they? Some organisms (snails, cephalopods, etc.) have only a single major shell, and bivalves and brachiopods just two: it is easy to find complete specimens of these. Sea urchins have many small parts which are highly integrated, and they are often all preserved together. Some sponges and vertebrates and plants and sea cucumbers have only loosely-joined hard parts: although individual parts preserved easily, conditions have to be right (and the body relatively fresh and still hanging together) for all the parts to be found as one.

Necrolysis: How long was the body exposed before burial? Did scavengers get a chance to eat it (or parts of it)? How about decay organisms, or the weather? Consider that the VAST majority of dead bodies never get a chance to get buried: they wind up in the guts of other organisms, or decayed into organic material that becomes part of the soil or sediment.

Biostratinomy: In general, the faster the rate of burial after death, the more complete the fossil will be. (This is why some of the best fossils are those from bodies trapped in some form of sediment such as tree resin (future amber) or tar or mud: the biostratinomy process began before the individual was dead!).

However, the higher the energy of the environment of burial, the greater the chance the body will be torn apart by current action. Very quiet water (like the bottom of lakes, lagoons, etc.) is very good for preserving fine details (especially of small-bodied organisms), but only if that bottom environment is not populated by worms, clams, and other sediment-churning animals: for example, if the bottom-water is anoxic.

Sometimes specimens are buried in place or in situ (also called autochthonus); other times the specimen was transported before burial (allochthonous).

Diagenesis: Once the prevailing wisdom was that most fossils were "petrified": turned to stone by chemical action. In fact, it turns out the situation is more complicated than that.

For a great many fossils the original hard parts and even some of the original organic material remains. This was not appreciated until chemical analyses began to show that the cellulose was still there in most fossil plants, the hydroxylapatite and collagen fibers in fossil bone, even the chemical residues of feathers and hair and leaf. These are definitely NOT pristine material, and they have been subject to some decay and alteration by time, but they are still there.

In some cases the original material is still there, nothing else has been added to it, and the tissue and hard parts are still basically in their original dead condition, as in this freeze-dried mammoth. Such unaltered material becomes increasingly rare the farther back in time you go.

For shells made of carbonate material, it is common to find them where the original material is still there, nothing has been added to it, but the crystals have regrown at the temperature and pressures of burial. These recrystallized fossils will lose the microscopic details of more pristine ones.

The most common mode of preservation for vertebrate bones and teeth and wood (and pretty common for shelly fossils, too) is permineralization: the original hard parts and even some organics are present, but the pore space of the bone, wood, or shell has been filled in by minerals from the pore water. This is exactly comparable too (indeed, happens at the same time as) the cementation of the sediment. Because of this, fossil bone or wood is much denser than living bone or wood; breaks along cracks; and otherwise acts like rock. But the original microscopic details -- and even chemical details -- are often still there.

Carbonized fossils are ones where compression after burial has squeezed some of the tissues (chitin; hair and leaves; cellulose and lignin) to concentrate the carbon and remove some of the other chemical components. These are preserved as thin black surfaces (in the case of wood, they can be much thicker: that is, as coal). Often the original microscopic structures of these tissues can still be found. The presence of carbonized keratin (beaks, scales, etc.) is more common than once thought: older generations of paleontologists dug right through these to get to the bones.

However, there are also situations when original fossil material is lost to some degree or other. Actual replacement of the original hard or soft parts by some other mineral does occur, as in this coiled ammonoid where the carbonate was replaced by pyrite. Recent experimental evidence suggest that chemical replacement by such minerals as pyrite, silica, phosphates, and the like actually begins quite quickly after burial if the diagenetic conditions (particular combinations of acidity or alkalinity; pressure; temperature; oxygen level; etc.) are correct.

Silicified fossils are particularly useful for understanding fine details: because silica is so chemically and mechanically resistant, it is possible to take silicified specimens and basically dissolve the rock around them, and find little projections, spikes, and other features that might normal be lost through preparation.

Sometimes the fossil itself is actually lost during diagensis: acidity in the pore water dissolved the shell or bone or wood, leaving a void space or mold. Molds are hollow spaces showing the fossil in reverse. Sometimes molds are of just one surface (an impression); other times they are the complete outside surface in reverse (external mold); or sometimes an inner space filled in (an internal mold).

These can be filled in (artificially by humans, or naturally by sediment or mineral from ground water) to produced a cast: a 3-D replica of the original fossil, where not a single atom of the cast was part of the original organism. Here is a complementary set of a mold (left) and natural cast (right) of a coiled ammonoid.

Some Lagerstätten occur because they trapped a great number of individuals, and thus concentrated the amount of specimens in one area (such as the La Brea Tar Pits of Los Angeles). Others are famous because they preserved otherwise rare soft-bodied organisms, and thus give us a much more complete picture of the biodiversity of that time and space (such as the Burgess Shale of British Columbia.)

Lagerstätten are thus of great importance for our better understanding of the ancient world.

• Isotopes: not radioactive ones in this case, but stable isotopes of certain elements like oxygen, carbon, and nitrogen. Different physical and biological phenomena shift the ratios between isotopes before they are incorporated in the tissues of living things (we will explore some of these in later lectures). We can used geochemical techniques to recover the shifted (fractionated) ratios as markers of different physical and biological factors.
• Proteins & Other Polymers: some of the firm tissues of organisms are composed of materials which can have long term stability in particular diagenetic settings. We can recover some information from these materials (for example, they are source for some of the isotopic ratios, and some genetic information.)
• Biomarkers: There are some substances that are unique byproducts of certain groups of organisms. Geologists can find these substances in sedimentary rocks, indicating the presence of these groups in that ancient environment. There needn't even be any other part of the organism present: the biomarker itself is enough to show that the organism was there.
• DNA: Yes, DNA molecules CAN be preserved in the geologic record. However, it tends to degrade relatively quickly. There are a few dozen species of fossil animals for which we have preserved DNA, all from the last few 10s of thousands of years. It is very unlikely that we will ever be able to directly read the DNA from species millions of years old, but for the younger fossils they have provided important information (often about their sometimes-unexpected evolutionary relationships!).

On the "unknown unknown" of fossils:

Uncovering the nature of Daemonelix trace fossils:

The "Odd Couple" fossil pair:

Last modified: 24 January 2024