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Fossil and Fossilization

The Fossil Clock, From Biosphere To Lithosphere, From Field To Laboratory, Interpreting The Fossil Record

Fossils are a significant window into Earth's history and organic evolution. The term fossil literally means something that has been 'dug up,' but its modern meaning has been restricted to evidence of past life. Such evidence may take the form of body fossils (both plant and animal), trace fossils or ichnofossils (physical features formed in rock due to animal-sediment interaction), and chemical trace fossils (chemical evidence of life processes preserved in minerals within the rocks).

Fossilization refers to the series of postmortem processes that lead to development of a body, trace, or chemical fossil. For original hard parts (e.g., shell, skeleton, and teeth), which are composed of various kinds of organic minerals, fossilization may include replacement by new minerals, permineralization (filling open spaces with minerals). Fossil shells may be represented by external or internal (steinkern) sediment molds. Soft parts of plants or animals may also be mineralized and preserved as fossils in the process of carbonization. Soft tissue can be preserved as fossil material under special conditions where bacteria and moisture are excluded (e.g., fossils buried in glacial ice, anoxic peat bogs, and amber).

Fossils and their enclosing sediment (or sedimentary rock) are carefully studied in order to reconstruct ancient sedimentary environments and ancient ecosystems. Such analysis is called paleoecology, or the study of ancient ecologic systems. Fossils occur in nearly all sediments and sedimentary rock, and some volcanic rocks (e.g., ash deposits) as well. The bulk of these fossils are invertebrates with hard parts (e.g., clam shells). Vertebrates, the class that includes reptiles (e.g., dinosaurs) and mammals (e.g., mastodons), are a relatively late development, and the finding of a large, complete vertebrate fossil, with all its parts close together, is rather rare. Microfossils, on the other hand, are extremely common. 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 microfossils 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 Upper 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 during and since Triassic pulled apart areas that were once contiguous and thus shared the same floras and faunas. In particular, Earth's tectonic 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 continental "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 Asia 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 fall occurs, thus exposing more land at the edges of continents, sediments with 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 floras and faunas in sediments and sedimentary rocks 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. Global climatic change has had a profound effect on Earth's fauna and flora over time. This is strongly reflected in the fossil record and the record of paleoecology of Earth found in sedimentary strata.

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