Methods Of Fossilization

When bones themselves are not fossilized, a fossil animal can be found in the form of a cast (top), or a mold (below). A mold is created when an entombed animal is dissolved away, leaving behind a cavity. Usually, the mold is of the hard parts of the body. Where these cover the outside of a creature, as in the bony fish illustrated, the whole body shape can be discerned. A cast is formed when a mold becomes filled with a mineral deposit. In such circumstances, no traces remain of internal body structure.

Cast And Mold Dinosuars

inhabited Australia during the Early Cretaceous?" Such questions lead them, in turn, to study geological maps to find out where suitable, accessible rocks can be found. Even then, the costs of an expedition are so high that this would normally be undertaken only if some promising remains had already been found there.

Once "on location," the scientists can begin to search for fossils. There is no magical method by which these can be found. Long hours tramping across the exposed rocks under a hot sun, and returning, usually empty-handed, to a fairly spartan camp normally precede any discovery. Rocks are searched methodically, layer by layer, or area by

Layer Fossilization

The forces of erosion wear away the land surface. The eroding action of a stream has created a sheer cliff face (right). As a result, the tail of the ichthyosaur is beginning to be exposed and bones drop into the bed of the stream.

Fossilization

With further erosion, more of the ichthyosaur tail becomes visible. The fossil skull can also be seen, again due to erosive forces wearing away the surface of the land. A paleontologist who discovered these would be prompted to search for the complete skeleton.

The forces of erosion wear away the land surface. The eroding action of a stream has created a sheer cliff face (right). As a result, the tail of the ichthyosaur is beginning to be exposed and bones drop into the bed of the stream.

With further erosion, more of the ichthyosaur tail becomes visible. The fossil skull can also be seen, again due to erosive forces wearing away the surface of the land. A paleontologist who discovered these would be prompted to search for the complete skeleton.

area. Usually, all that the paleontologist finds at first is a piece, or pieces, of bone, projecting from the ground or the side of a cliff.

The hard work then begins. The overlying rock must be taken away to reveal the fossil skeleton, often by cutting the fossil into pieces. The bones then have to be covered with moistened tissue paper and then with a layer of sacking (burlap) soaked in plaster-of-Paris. When this dries and hardens, it protects the fossil during its long, and often rough, ride back to the museum.

Paleontologists must not neglect their record-keeping. Each specimen must be given a number, painted onto it, and a precise note kept of where it was found. This is necessary because later analysis of such information may show, for example, that some animals occur in different rock strata, and thus belong to different communities (see p. 14).

Extracting and dating fossils

Once the fossils have arrived back in the laboratory of the museum or university, the plaster jacket is removed, and the fossil bone extracted from its rocky home. Sometimes, this can be done with a hammer and chisel or, on smaller specimens, with sharp, steel needles. But when possible, scientists use chemicals to dissolve away the mineral cement that binds together the particles of sediment. This process can reveal beautifully preserved, 3-D skulls, showing as much detail as those of recently-dead modern animals.

Whatever method is used, the fossil bone is usually badly cracked after its many millions of years in the ground, and must be strengthened by impregnating it with solutions of plastics.

There are both relative and absolute methods of establishing the age of fos sils. The relative method depends on 2 facts. First, because sediments are deposited successively, one on top of the other, paleontologists can often establish that one organism lived at a later time than another simply because it is found in rocks that lie higher in the rock sequence of a particular area.

The second fact is that no group of animals or plants lives forever. So within any such sequence, a new group may appear, persist through a number of strata, and then disappear without trace. Some groups evolved early in geological history, some later; some existed for a long time before they became extinct, while others had only a brief span before they were replaced by competitors. So, each unit of geological time is characterized by a unique assemblage of animals or plants (see p. 14).

A vast amount of information about many groups of fossil plants and animals has been obtained from different rock strata all over the world. But much work had to be undertaken by early paleontologists and geologists before even the major subdivisions of geological time — into the different eras, periods and epochs (see pp. 8—9) — were established and accepted.

The absolute method of dating or aging a fossil establishes its age in "millions of years." It relies upon the fact •that some rocks contain radioactive minerals, which were incorporated in the rocks when they were originally deposited. These minerals change, or "decay," into other substances at a rate that can be measured. For example, the rate of decay of the radioactive mineral uranium-235 into lead-207 is such that half of it has turned into lead-207 in 713 million years. So, by studying the relative amounts of uranium-235 and lead-207 in a rock, it is possible to calculate how long the mineral has been decaying, and when the rock was deposited.

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Responses

  • Pia Lind
    How do mold wear away the surface of rocks?
    8 years ago
  • SAARA
    What method is used for prehistoric sea animal fossil?
    2 years ago

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