Fossil and Fossilization

The likelihood that any living organism will become a fossil is quite low. The path from biosphere to lithosphere—from the organic, living world to the world of rock and mineral—is long and indirect. Individuals and even entire species may be 'snatched' from the record at any point. If an individual is successfully fossilized and enters the lithosphere, ongoing tectonic activity may stretch, abrade, or pulverize the fossil, or the sedimentary layer housing the fossil may eventually be subjected to high temperatures in Earth's interior and melt, or be weathered away at the Earth's surface. A fossil that has survived or avoided these events may succumb to improper collection techniques at the hands of a human.

Successful fossilization begins with the conditions of death in the biosphere. Fossils occur in sedimentary rock and are incorporated as an integral part of the rock during rock formation. Unconsolidated sediments such as sand or mud, which will later become the fossiliferous (fossil-bearing) sandstone or limestone, or shale, are an ideal matrix for burial. The organism should also remain undisturbed in the initial phase of burial. Organisms exposed in upland habitats are scavenged and weathered before they have an opportunity for preservation, so a low-lying habitat is the best. Often this means a watery habitat. The fossil record is highly skewed in favor of organisms that died and were preserved in calm seas, estuaries, tidal flats, or the deep ocean floor (where there are few scavengers and little disruption of layers). Organisms that died at altitude, such as on a plateau or mountainside, and are swept by rivers into a delta or estuary may be added to this death assemblage, but are usually fragmented.

A second factor contributing to successful fossilization is the presence of hard parts. Soft-bodied organisms rarely make it into the fossil record, which is highly biased in favor of organisms with hard parts—skeletons, shells, woody parts, and the like. An exception is the Precambian Burgess Shale, in British Columbia, where a number of soft-bodied creatures were fossilized under highly favorable conditions. These creatures have few relatives that have been recorded in the fossil record; this is due to the unlikelihood of the soft animals being fossilized.

From the time of burial on, an organism is technically a fossil. Anything that happens to the organism after A fossil trilobite from the Mid-ordovician period. Photograph by Neasaphus Rowalewkii. JLM Visuals. Reproduced by permission. burial, or anything that happens to the sediments that contain it, is encompassed by the term diagenesis. What is commonly called fossilization is simply a postmortem alteration in the mineralogy and chemistry of the original living organism.

Fossilization involves replacement of minerals and chemicals by predictable chemical means. For example, the shells of molluscs are made of calcium carbonate, which typically remineralizes to calcite or aragonite. The bones of most vertebrates are made of calcium phosphate, which undergoes subtle changes that increase the phosphate content, while cement fills in the pores in the bones. These bones may also be replaced by silica.

The replacement of original minerals and chemicals takes place according to one of three basic schemes. In one scheme; the skeleton is replaced one to one with new minerals. This scheme is known as replacement. In a second scheme, the hard parts have additional mineral material deposited in their pores. This is known as permineralization. In a third scheme, both hard and soft parts dissolve completely and a void is left in the host rock (which may later be filled with minerals). If in the third scenario, the sediments hardened around the hard part and took its shape before it dissolved, and the dissolved hard part was then not replaced (i.e., there is a void), a thin space remains between two rock sections. The rock section bearing the imprint of the interior face of the shell, let us say, is called the part, or internal mold, and the rock section bearing the imprint of the exterior of the shell is called the counterpart, or external mold. External molds are commonly but mistakenly discarded by amateur fossil collectors.

Because of the nature of fossilization, fossils are often said to exist in communities. A fossil community is defined by space, not time. Previously fossilized specimens A fly in amber, 35 million years old. JLM Visuals. Reproduced by permission. of great age may be swept by river action or carried by scavengers into young sediments that are just forming, there to join the fossil mix. For this reason, it may be difficult to date a fossil with precision on the basis of a presumed association with nearby fossils. Nevertheless, geologists hope to confirm relationships among once living communities by comparing the makeup of fossil communities.

One of the larger goals of paleontologists is to reconstruct the prehistoric world, using the fossil record. Inferring an accurate life assemblage from a death assemblage is insufficient and usually wrong. The fossil record is known for its extreme biases. For example, in certain sea environments over 95% of species in life may be organisms that lack hard parts. Because such animals rarely fossilize, they may never show up in the fossil record for that locale. The species diversity that existed in life will therefore be much reduced in the fossil record, and the proportional representation of life forms greatly altered.

To gain some idea of the likelihood of fossilization of an individual or a species, scientists have sampled the death assemblages—decaying plants and animals that have gained the security of undisturbed sediments—in modern-day harbor floors and offshore sediments, and compared those death assemblages with actual life assemblages in the overlying waters. It seems that no more than 30% of species and 10% of individuals are preservable after death. The death assemblage is still millions of years away from becoming a fossil community, however, and once such factors as consumption of the decaying organisms by scavengers, transport of the organisms out of the area, disturbance of sediments, reworking of the rock after it has formed, and erosion are added to the picture, the fossilization rate falls well below the preservation rate.

In some cases, however, a greater than usual proportion of preservable individuals in a community has fossilized in place. The result is a bed of fossils, named after the predominant fossil component, "bone bed" or "mussel bed," for example. Geologists are divided over whether high-density fossil deposits are due to reworking and condensation of fossiliferous sediments or to mass mortality events. Mass mortality—the contemporaneous death of few to millions of individuals in a given area—usually is attributed to a natural catastrophe. In North America, natural catastrophe is thought to have caused the sudden death of the dinosaurs in the bone beds at Dinosaur National Park, Colorado, and of the fossil fishes in the Green River Formation, Wyoming. These are examples of local mass mortality. When mass mortality occurs on a global scale and terminates numerous species, it is known as a mass extinction. The greatest mass extinctions have been used to separate strata formed during different geological eras: the Permian-Triassic extinction separates the Paleozoic era from Mesozoic; the Cretaceous-Tertiary extinction, which saw the demise of the dinosaurs and the rise of large mammalian species to fill newly available biological niches, separates Mesozoic from Tertiary. Thus, mass extinctions are recorded not only in the high-density fossil beds, but in the complete disappearance of many species from the fossil record.