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INSTRUCTOR'S NOTES

Fossils are the remains or traces of prehistoric plants and animals. Most fossils are buried and preserved in sedimentary rock, but some are trapped in organic matter. Fossils range in age from 3.5-billion-year-old traces of microscopic cyanobacteria (blue-green algae) to 10,000-year-old remains of animals preserved during the last ice age.

Paleontologist, Ben Witt, University of Washington

Fossils are most commonly found in limestone, sandstone, and shale (sedimentary rock). Remains of organisms can also be found trapped in natural asphalt, amber, and ice. The hard, indigestible skeletons and shells of animals and the woody material of plants are usually preserved best. Fossils of organisms made of soft tissue that decays readily are more rare. Paleontologists (scientists who study prehistoric life) use fossils to learn how life has changed and evolved throughout earths history.

Many factors can influence how fossils are preserved. Remains of an organism may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form. The fossilization of an organism depends on the chemistry of the environment and on the biochemical makeup of the organism. The following are some common methods by which fossils are formed.

Carbonization

Plants are most commonly fossilized through carbonization. In this process, the mobile oils in the plants organic matter are leached out and the remaining matter is reduced to a carbon film. Plants have an inner structure of rigid organic walls that may be preserved in this manner, revealing the framework of the original cells. Animal soft tissue has a less rigid cellular structure and is rarely preserved through carbonization.

 

Petrifaction

Petrified Tree from Petrified Forest National Park Another common mode of preservation of plants is petrifaction, which is the crystallization of minerals inside cells. One of the best-known forms of petrifaction is silicification, a process in which silica-rich fluids enter the plants cells and crystallize, making the cells appear to have turned to stone (petrified). Famous examples of silicification may be found in the petrified forests of the Western United States. Petrifaction may also occur in animals when minerals such as calcite, silica, or iron fill the pores and cavities of fossil shells or bones.

Replacement

Replacement occurs when an organism is buried in mud and its remains are replaced by sulfide (pyrite) or phosphate (apatite) minerals. This process may replace soft tissue, preserving rarely seen details of the organism's anatomy. X-ray scanning of some German shells from the Devonian Period (410 million to 360 million years ago) have revealed limbs and antennae of trilobites and tentacle arms of cephalopods that have been pyritised (replaced by pyrite). Although mineral replacement is rare, fossils created in this way are important in helping paleontologists compare the anatomical details of prehistoric organisms with those of living organisms.

Recrystallization

Many animal shells are composed of the mineral aragonite, a form of calcium carbonate that breaks down over millions of years to form the more stable mineral calcite. This method of preservation, called recrystallization, destroys the microscopic details of the shell but does not change the overall shape. Snail shells and bivalve shells from the Jurassic Period (205 million to 138 million years ago) and later are still composed principally of aragonite. Most older shells that have been preserved have recrystallized to calcite.

Organic Traps

Whole organisms may become trapped and preserved in amber, natural asphalt, or peat. Amber is the fossilized remaining part of tree resin. When resin first flows from the tree, it is very thick and sticky, so as it runs down the trunk, it may trap insects, spiders, and occasionally larger animals such as lizards. These organisms can be preserved for millions of years with details of their soft tissue, such as muscles and hair-like bristles, still intact.
Prehistoric termites trapped in amber. From the cover of Scientific America, April 1996 Lizard in Amber Frog in Amber

 

 

 
Other products from ancient life:

Petroleum

Petroleum is formed under the earth's surface by the decomposition of marine organisms. The remains of tiny organisms that live in the sea.and, to a lesser extent, those of land organisms that are carried down to the sea in rivers and of plants that grow on the ocean bottoms.are enmeshed with the fine sands and silts that settle to the bottom in quiet sea basins. Such deposits, which are rich in organic materials, become the source rocks for the generation of crude oil. The process began many millions of years ago with the development of abundant life, and it continues to this day. The sediments grow thicker and sink into the sea floor under their own weight. As additional deposits pile up, the pressure on the ones below increases several thousand times, and the temperature rises by several hundred degrees. The mud and sand harden into shale and sandstone; carbonate precipitates and skeletal shells harden into limestone; and the remains of the dead organisms are transformed into crude oil and natural gas.

Coal

Coal is a solid fuel of plant origin. In remote geological times, and particularly in the Carboniferous period, between 345 and 280 million years ago, much of the world was covered with luxuriant vegetation growing in swamps. Many of these plants were types of ferns, some as large as trees. This vegetation died and became submerged under water, where it gradually decomposed. As decomposition took place, the vegetable matter lost oxygen and hydrogen atoms, leaving a deposit with a high percentage of carbon. In this way peat bogs were formed. As time passed, layers of sand and mud settled from the water over some of the peat deposits. The pressure of these overlying layers, as well as movements of the earth's crust, compress and harden the deposits, thus producing coal.

Various types of coal are classified according to fixed carbon content. Peat, the first stage in the formation of coal, has a low fixed carbon content and a high moisture content. The carbon content is greater in lignite, the lowest rank of coal. Bituminous coal has even more carbon and a correspondingly higher heating value. Anthracite coal has the highest carbon content and heating value. Coal may be transformed by further pressure and heat into graphite that is almost pure carbon. Other components of coal are volatile hydrocarbons, sulfur and nitrogen, and the minerals that remain as ash when the coal is burned.


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