Most of the Earth's surface is covered by a thin veneer of sediment, which preserves the vestiges of past life. Under special geologic conditions, with minimal decomposition and rapid burial, the bodies of dead organisms can be preserved and withstand the rigors of time.
Fossil Family Tree
Fossils of extinct organisms are classified by the same system used for classifying living organisms. The first classification scheme was developed by the 18th-century Swedish botanist Carl von Linné, better known by his Latinized name Carolus Linnaeus. He gave Latin names to organisms because Latin was the universal language of science during his time. His naming was based on the number of characteristics organisms had in common.
Linnaeus realized that some organisms had a greater similarity than others because they were more closely related. Later, as evolution was recognized as the process by which organisms develop into new species, and by examination of fossils, classification schemes were developed to describe these evolutionary patterns, demonstrating how groups of organisms were related in both space and time.
In the classification scheme used by biologists and paleontologists, each organism is assigned an italicized two-part species name. The first word, which is capitalized, is the generic name and is shared with other very closely related species. The second word, written in lowercase, is the species name and is unique to a particular genus: for example, Homo sapiens, which is who we are, as distinct from Homo habilis, our extinct relative known only from the fossil record.
Sedimentary Conditions
Sedimentary rocks are encountered more frequently than any other rock type. The sedimentary environment provides the conditions necessary for the preservation of fossils. The constant shifting of sediments on the surface and the accumulation of deposits on the ocean floor assure that the face of the Earth will continue to change with time, while preserving ancient sedimentary layers.
Fossils of extinct organisms can be found in most sedimentary rocks, especially limestones and shales. Outcrops of marine sediments are generally the best sites for prospecting for fossils. In most parts of the world, the central portions of the continents were inundated during various times in the geologic past, and thick deposits of marine sediments accumulated in the basins of inland seas. When the seas departed and the land was uplifted, erosion exposed many of these marine sediments.
Fossil Preservation
Plants and animals must be buried under certain specific conditions to become fossilized. Given enough time, the remains of an organism are modified, often becoming petrified, literally turned to stone.
The story of ancient life is written in the fossil record of extinct organisms, which leave their epitaphs inscribed on stone. The term fossil comes from the Latin fossilis, meaning "to dig," as paleontologists and amateur fossil hunters often have to do to find them. The rarest as well as the most interesting fossils are the actual remains of organisms, which include bones, shells, and other well-preserved body parts.
Fossils are formed in a variety of ways under different environmental conditions. In rare circumstances, the complete animal is preserved. More frequently, the durable parts of the organism are preserved in a relatively unaltered condition. But mostly, fossils are extensively altered so that little of the original material remains, although the shapes and textures are recognizable. The greatest number of fossils comprise not the remains of organisms themselves but indirect evidence of their existence in preserved tracks, trails, burrows and imprints.
Tracks and Trails
The bones of extinct animals are much rarer than their footprints, and many animals are known only by their tracks. The formation of clear foot impressions requires a moist, fine-grained and cohesive sediment bed for the animal to walk on. If the animal walks slowly, it will leave a detailed impression of its feet. Even clear outlines of claws or nails, the shape of the footpad, and the pattern of scales can be discerned.
Unfortunately, few such high-quality fossil footprints are found, and most are partially destroyed during the sedimentary process that buries and preserves them. The most favorable conditions for the preservation of footprints exist after high tide waters have receded, when the tracks are allowed to dry and harden and eventually fill with a different type of sediment. The weight of the animal is also important; large animals, such as dinosaurs, leave deeper tracks that are less readily destroyed and more likely to be preserved.
Plants
In the plant kingdom, the thallophytes include algae, fungi and lichens. They have a soft, nonwoody structure that lacks a vascular circulatory system and grow in or near water or in moist places. Only algae possess chlorophyll, which enables them to manufacture their own energy from water, carbon dioxide and sunlight. The thallophytes generally reproduce by the fusion of male and female single-celled gametes. The earliest fossils are one-celled bacteria and blue-green algae in Precambrian rocks.
Fungi and lichens comprise as many as 100,000 living species. The bryophytes, including mosses and liverworts, were the first plant phylum to become well established on land. They have stems and simple leaves but lack true roots or vascular tissues to conduct water to the higher extremities and therefore are required to live in moist environments. They reproduce by spores that are carried by the wind for wide distribution. The earliest species occupied freshwater lakes in the late Precambrian. Because of their dominance over vast periods of time, many fossil imprints of these early plants have survived.
Tags: extinct organisms, marine sediments, classification scheme, closely related, fossil record, Fossils extinct
