Fossilization

Fossilization

What Kinds of Rocks Contain Fossils? 

Most fossils are found in sediments or sedimentary rocks. Fossils form when organisms die and become buried by sediment, or when organisms travel over or through sediment and leave imprints or debris. The degree of preservation of a fossil reflects the context of burial. For example, rocks formed from sediments deposited under anoxic (oxygen-free) conditions in quiet water (such as lake beds or lagoons) can preserve particularly fine specimens. In contrast, rocks made from sediments deposited in high-energy environments where strong currents tumble shells and bones and break them up contain at best only small fragments of fossils mixed with other clastic grains. Fossils sometimes occur in volcaniclastic rocks, but they are not found in intrusive igneous rocks and tend to be destroyed by metamorphism.

Forming a Fossil 

How a dinosaur eventually becomes a fossil. This takes many steps, over a long period of time.

Paleontologists refer to the process of forming a fossil as fossilization. To see how a typical fossil develops in sedimentary rock, let’s follow the fate of an old dinosaur as it searches for food along a riverbank (figure above). On a scalding summer day, the hungry dinosaur, plodding through the muddy ground, succumbs to the heat and collapses dead into the mud. Soon after, scavengers strip the skeleton of meat and scatter the bones. But before the bones have had time to weather away, the river floods and buries the bones, along with the dinosaur’s footprints, under a layer of silt. More sediment from succeeding floods buries the bones and prints still deeper, so that the bones cannot be reworked by currents or disrupted by burrowing organisms. Later, sea level rises and a thick sequence of marine sediment buries the fluvial sediment. 

Eventually, the sediment containing the bones and footprints turns to rock (siltstone and shale). The footprints remain outlined by the contact between the siltstone and shale, while the bones reside within the siltstone. Minerals  precipitating from groundwater passing through the siltstone gradually replace some of the chemicals constituting the bones, until the bones themselves have become rock-like. The buried bones and footprints are now fossils. One hundred million years later, uplift and erosion expose the dinosaur’s grave. Part of a fossil bone protrudes from a rock outcrop. A lucky paleontologist observes the fragment and starts excavating, gradually uncovering enough of the bones to permit reconstruction of the beast’s skeleton. Further digging uncovers the footprints. The dinosaur rises again, but this time in a museum. In recent years, bidding wars have made some fossil finds extremely valuable. For example, a skeleton of a Tyrannosaurus rex, a 67-million-year-old dinosaur, sold at auction in 1997 for $7.6 million. The specimen, named Sue after its discoverer, now stands in the Field Museum of Chicago. 

Similar tales can be told for fossil seashells buried by sediment settling in the sea, for insects trapped in hardened tree sap (amber), and for mammoths drowned in the muck of a tar pit. In all cases, fossilization involves the burial and preservation of an organism or the trace (a footprint or burrow) of an organism. 

The Many Different Kinds of Fossils 

Perhaps when you think of a fossil, you picture either a dinosaur bone or the imprint of a seashell in rock. In fact, paleontologists distinguish many different kinds of fossils, according to the specific way in which the organism was fossilized. Let’s look at examples of these categories. 

Examples of different kinds of fossils.

• Frozen or dried body fossils: In a few environments, whole bodies of organisms may be preserved. Most of these fossils are fairly young, by geologic standards their ages can be measured in thousands, not millions, of years. Examples include woolly mammoths that became incorporated in the permafrost (permanently frozen ground) of Siberia  (figure above a). In desert climates, organisms can become desiccated (dried out). Such “mummified” corpses can survive for millennia in caves. 
• Body fossils preserved in amber or tar: Insects landing on the bark of trees may become trapped in the sticky sap or resin the trees produce. This golden syrup envelops the insects and over time hardens into amber, the semiprecious “stone” used for jewellery. Amber can preserve insects, as well as other delicate organic material such as feathers, for 40 million years or more (figure above b). Tar similarly acts as a preservative. In isolated regions where oil has seeped to the surface, the more volatile components of the oil evaporate away and bacteria degrade what remains, leaving behind a sticky residue of tar. At one such locality, the La Brea Tar Pits in Los Angeles, tar accumulated in a swampy area. While grazing, drinking, or hunting at the swamp, animals became mired in the tar and sank into it. Their bones have been remarkably well  preserved for over 40,000 years (figure above c).
• Preserved or replaced bones, teeth, and shells: Bones (the internal skeletons of vertebrate animals) and shells (the external skeletons of invertebrate animals) consist of durable minerals, which may survive in rock (figure above d). Some bone or shell minerals are not stable, and they recrystallize. But even when this happens, the shape of the bone or shell can be preserved in the rock. 
• Molds and casts of bodies: As sediment compacts around a shell, it conforms to the shape of the shell or body. If the shell or body later disappears, because of weathering and dissolution, a cavity called a mold remains (figure above e). (Sculptors use the same term to refer to the receptacle into which they pour bronze or plaster.) A mold preserves the delicate shape of the organism’s surface; it looks like an indentation on a rock bed. The sediment that had filled the mold also preserves the organism’s shape; this cast protrudes from the surface of the adjacent bed.
• Carbonized impressions of bodies: Impressions are simply flattened molds created when soft or semisoft organisms (leaves, insects, shell-less invertebrates, sponges, feathers, jellyfish) get pressed between layers of sediment. Chemical reactions eventually remove the organic material, leaving only a thin film of carbon on the surface of the impression (figure above f). 
• Permineralized organisms: Permineralization refers to the process by which minerals precipitate in porous material, such as wood or bone, from groundwater solutions that have seeped into the pores. Petrified wood, for example, forms by permineralization of wood, causing the wood to become chert. In fact, the word petrified literally means turned to stone. When the wood dries out, open spaces form inside the cells because the protoplasm of cells is 80% water. The chert precipitates inside these spaces and the organic matter of the cell walls becomes carbonized. The fine detail of the wood’s cell-wall structure can be seen in a petrified log (figure above g).
• Trace fossils: These include footprints, feeding traces, burrows, and dung (coprolites) that organisms leave behind in sediment (figure above h, i).
• Chemical fossils: Living things consist of complex organic chemicals. When buried with sediment and subjected to diagenesis, some of these chemicals are destroyed, but some either remain intact or break down to form different, but still distinctive, chemicals. A distinctive chemical derived from an organism and preserved in rock is called a chemical fossil. (Such chemicals may also be called molecular fossils or biomarkers.) 

Examples of fossil plankton shells. Because of their size, geologists refer to these as microfossils or nannofossils.

Paleontologists also find it useful to distinguish among different fossils on the basis of their size. Macrofossils are large enough to be seen with the naked eye. But some rocks and sediments also contain abundant microfossils, which can be seen only with a microscope or even an electron microscope (figure above). Microfossils include remnants of plankton, algae, bacteria, and pollen.

Fossil Preservation 

Not all living organisms become fossils when they die. In fact, only a small percentage do, for it takes special circumstances namely, one or more of the following three to produce a fossil and allow it to survive. 

• Death in an anoxic (oxygen-poor) environment: A dead squirrel by the side of the road won’t become a fossil. As time passes, birds, dogs, or other scavengers may come along and eat the carcass. And if that doesn’t happen, maggots, bacteria, and fungi infest the carcass and gradually digest it. As the flesh rots, some reacts with oxygen in the atmosphere and transforms into carbon dioxide gas. The remaining skeleton weathers in air and turns to dust. Thus, before roadkill can become incorporated in sediment, it has vanished. If, however, a carcass settles into an oxygen-poor environment, oxidation reactions happen slowly, scavenging organisms aren’t abundant, and bacterial metabolism takes place very slowly. In such environments, the organism won’t rot away before it has a chance to be buried and preserved, so the likelihood that the organism becomes fossilized increases. 
• Rapid burial: If an organism dies in a depositional environment where sediment accumulates rapidly, it may be buried before it has time to rot, oxidize, be eaten, or undergo complete weathering. For example, if a storm suddenly buries an oyster bed with a thick layer of silt, the oysters die  and become part of the sedimentary rock derived from the sediment. 
• The presence of hard parts: Organisms without durable shells or skeletons, or other “hard parts,” usually won’t be fossilized, for soft flesh decays long before hard parts do under most depositional conditions. For this reason, paleontologists have learned more about the fossil record of bivalves (a class of organisms, including clams and oysters, with strong shells) than they have about the fossil record of jellyfish (which have no shells) or spiders (which have very fragile exoskeletons). 
• By carefully studying modern organisms, paleontologists have been able to provide rough estimates of the preservation potential of organisms, meaning the likelihood that an organism will be buried and eventually transformed into a fossil. For example, in a typical modern-day shallow-marine environment, such as the mud-and-sand sea floor close to a beach, about 30% of the organisms have sturdy shells and thus a high preservation potential, 40% have fragile shells and a low preservation potential, and the remaining 30% have no hard parts at all and are not likely to be fossilized except in special circumstances. Of the 30% with sturdy shells, though, only a few happen to die in a depositional setting where they actually do become fossilized. Thus, fossilization is the exception rather than the rule.

Extraordinary Fossils:  A Special Window to the Past 

Extraordinary fossils. These fossils are particularly well preserved.

Though only hard parts survive in most fossilization environments, paleontologists have discovered a few special locations where rock contains fossils of soft parts as well; such fossils are known as extraordinary fossils (figure above a, b). We've already seen how extraordinary fossils such as insects and even feathers can be preserved in amber, and how complete skeletons have been found in tar pits. In a few cases, such finds include actual tissue, a discovery that has led to a research race to find the oldest preserved DNA. Extraordinary fossils may also be preserved in sediment that accumulates on the anoxic floor of lakes or lagoons or the deep ocean. Here, oxidation cannot occur, and flesh does not rot before burial. 

Credits: Stephen Marshak (Essentials of Geology)