Fossil

I

Introduction

Fossil, remains or traces of prehistoric plants and animals, buried and preserved in sedimentary rock, or trapped in organic matter. Fossils representing most living groups have been discovered, as well as many fossils representing groups that are now extinct. 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.

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 earth’s history (see Paleontology).

II

Processes of Fossilization

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. As a result, not all organisms in a community will be preserved.

A

Carbonization

Plants are most commonly fossilized through carbonization. In this process, the mobile oils in the plant’s 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. Although paleontologists have found the carbonized skin of some ichthyosaurs, marine reptiles from the Mesozoic Era (240 to 65 million years before present), the microscopic structure of the skin was not preserved.

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B

Petrifaction

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 plant’s 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 (see Petrified Forest National Park). Petrifaction may also occur in animals when minerals such as calcite, silica, or iron fill the pores and cavities of fossil shells or bones.

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