Fossils and Fossilization
Fossils and fossilization
A fossil is the remains of an ancient life form—plant or animal—or its traces, such as nesting grounds, footprints, worm trails, or the impressions left by leaves, preserved in rock . Fossil traces are called ichnofossils. Fossilization refers to the series of postmortem changes that lead to replacement of minerals in the original hard parts (shell, skeleton, teeth, horn, scale) with different minerals, a process known as remineralization. Infrequently, soft parts may also be mineralized and preserved as fossils. A new category of subfossil—a fossil that has not yet begun to mineralize—is increasingly recognized in the scientific literature. Many subfossils originated in the Holocene or Recent, the period that we now live in, and cannot be dated with any greater accuracy. In addition, the term "fossil" is applied in other ways, for example, to preserved soils and landscapes such as fossil dunes . Through the study of fossils, it is possible to reconstruct ancient communities of living organisms and to trace the evolution of species.
Fossils occur on every continent and on the sea floors. The bulk of them are invertebrates with hard parts (for example, mussels). Vertebrates, the class that includes reptiles (for example, dinosaurs) and mammals (mastodons, humans), are a relatively late development, and the finding of a large, complete vertebrate fossil, with all its parts close together, is rare. Microfossils, on the other hand, are extremely common. The microfossils include very early bacteria and algae; the unicellular organisms called foraminiferans, which were common in the Tertiary Periods; and fossil pollen. The study of micro fossils is a specialized field called micropaleontology.
Fossils of single-celled organisms have been recovered from rocks as old as 3.5 billion years. Animal fossils first appear in late Precambrian rocks dating back about a billion years. The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, reflects both shifting of continental position by plate tectonics and environmental changes over time. The breakup of the supercontinent Pangaea in the Triassic Period pulled apart areas that were once contiguous and shared the same flora and fauna. In particular, the plates carrying the southern hemisphere continents—South America, southern Africa , the Indian subcontinent, Australia , and Antarctica—moved in different directions, isolating these areas. Terrestrial vertebrates were effectively marooned on large islands. Thus, the best explanation for dinosaurs on Antarctica is not that they evolved there, but that Antarctica was once part of a much larger land mass with which it shared many life forms.
An important environmental factor influencing the kinds of fossils deposited has been radical and episodic alteration in sea levels. During episodes of high sea level, the interiors of continents such as North America and Australia are flooded with seawater. These periods are known as marine transgressions. The converse, periods of low sea level when the waters drain from the continents, are known as marine regressions. During transgressions, fossils of marine animals may be laid down over older beds of terrestrial animal fossils. When sea level falls, exposing more land at the edges of continents, fossils of terrestrial animals may accumulate over older marine animals. In this way plate tectonics and the occasional marine flooding of inland areas could result in unusual collections of fossil flora and fauna where the living plants or animals could not exist today—such as fishes on the Siberian steppes.
Changes in sea level over the past million years or so have been related to episodes of glaciation . During glaciation, proportionately more water is bound up in the polar ice caps and less is available in the seas , making the sea levels lower. It is speculated, but not certain, that the link between glaciation and lower sea levels holds true for much of Earth's history. The periods of glaciation in turn are related to broad climatic changes that affect the entire Earth, with cooler weather increasing glaciation and with warmer temperatures causing glacial melting and a rise in sea levels. A change in temperature would also affect the availability of plants for herbivores to eat, and the availability of small animals for carnivores to eat. Thus, even modest temperature changes, if long-lasting enough, could produce large changes in the flora and fauna available to enter the fossil record in any given locale.
The principal use of fossils by geologists has been to date rock strata (layers) that have been deposited over millions of years. As different episodes in Earth's history are marked by different temperature, aridity, and other climatic factors, as well as different sea levels, different life forms were able to survive in one locale or period but not in another. Distinctive fossilized life forms that are typically associated with given intervals of geologic time are known as index fossils, or indicator species. The concepts that different fossil species correlate with different strata and that, in the absence of upheaval, older strata underlie younger ones are attributed to the English geologist William Smith , who worked in the early nineteenth century.
The temporal relationship of the strata is relative: it is more important to know whether one event occurred before, during, or after another event than to know exactly when it occurred. Recently geologists have been able to subdivide time periods into smaller episodes called zones, based on the occurrence of characteristic zonal indicator species, with the smallest time slices about one-half million years. Radiometric dating measures that measure the decay of radioactive isotopes have also been used to derive the actual rather than relative dates of geological periods; the dates shown on the time scale were determined by radiometry. The relative dating of the fossil clock and the quantitative dating of the radiometric clock are used in combination to date strata and geological events with good accuracy.
The fossil clock is divided into units by index fossils. Certain characteristics favor the use of one species over another as an index fossil. For example, the ammonoids (ammonites), an extinct mollusk, function as index fossils from the lower Devonian through the upper Cretaceous—a period of about 350 million years. The ammonoids, marine animals with coiled, partitioned shells, in the same class (Cephalopoda) as the present-day Nautilus, were particularly long lasting and plentiful. They evolved quickly and colonized most of the seas on the planet. Different species preferred warmer or colder water, evolved characteristically sculpted shells, and exhibited more or less coiling. With thousands of variations on a few basic, easily visible features—variations unique to each species in its own time and place—the ammonoids were obvious candidates to become index fossils. For unknown reasons, this group of immense longevity became extinct during the Cretaceous-Triassic mass extinction. The fossils are still quite plentiful; some are polished and sold as jewelry or paperweights.
Index fossils are used for relative dating, and the geologic scale of time is not fixed to any one system of fossils. Multiple systems may coexist side-by-side and be used for different purposes. For example, because macrofossils such as the ammonoids may break during the extraction of a core sample or may not be frequent enough to lie within the exact area sampled, a geologist may choose to use the extremely common microfossils as the indicator species. Workers in the oil industry may use conodonts, fossils commonly found in oilbearing rocks. Regardless of which system of index fossils is used, the idea of relative dating by means of a fossil clock remains the same.
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 to a fine dust, 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 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 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 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 mollusks 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.
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 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 very difficult to date a fossil with precision based on a presumed association with nearby fossils. Nevertheless, geologists do 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.
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 the geological eras: the Permian-Triassic extinction separates the Palaeozoic Era from the 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 the Mesozoic from the 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.
The fossil record—the sum of all known fossils—has been extremely important in developing the phylogeny, or evolutionary relations, of ancient and living organisms. The contemporary understanding of a systematic, phylogenetic hierarchy descending through each of the five kingdoms of living organisms has replaced earlier concepts that grouped organisms by such features as similar appearance. It is now known that unrelated organisms can look alike and closely related organisms can look different; thus, terms like "similar" have no analytical power in biology. Charles Darwin, working in the mid-1800s, was the chief contributor to the systematic approach to biological relationships of organisms.
In addition to providing important information about the history of Earth, fossils have industrial uses. Fossil fuels (oil, coal , petroleum , bitumen, natural gas ) drive industrialized economies. Fossil aggregates such as limestone provide building material. Fossils are also used for decorative purposes. This category of functional use should be distinguished from the tremendous impact fossils have had in supporting evolutionary theory.
See also Evolution, evidence of; Geologic time; Marine transgression and marine regression
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