how fossilization creates fossils

hard parts

Usually its only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also.

quick burial

Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure 1.5). This fish was quickly buried in sediment to become a fossil. Quick burial is rare on land, so fossils of land animals and plants are less common than marine fossils. Land People buried by the extremely hot eruption of ash and gases at Mt. Vesuvius in 79 AD.

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unusual circumstances

Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles, California. Although the animals trapped in the La Brea Tar Pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar. (Figure 1.7). Artists concept of animals surrounding the La Brea Tar Pits. In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified by thousands of scientists. The fossil record is our best clue to the history of life on Earth, and an important indicator

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exceptional preservation

Some rock beds contain exceptional fossils or fossil assemblages. Two of the most famous examples of soft organism preservation are from the 505 million-year-old Burgess Shale in Canada (Figure 1.8). The 145 million-year-old Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure 1.8). (a) The Burgess shale contains soft-bodied fossils. (b) Anomalocaris, meaning abnormal shrimp is now extinct. The image is of a fossil. (c) The famous Archeopteryx fossil from the Solnhofen Limestone has distinct feathers and was one of the earliest birds. Click image to the left or use the URL below. URL:

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fossils were parts of living organisms

It wasnt always known that fossils were parts of living organisms. In 1666, a young doctor named Nicholas Steno dissected the head of an enormous great white shark that had been caught by fisherman near Florence, Italy. Steno was struck by the resemblance of the sharks teeth to fossils found in inland mountains and hills (Figure 1.1). Most people at the time did not believe that fossils were once part of living creatures. Authors in that day thought that the fossils of marine animals found in tall mountains, miles from any ocean could be explained in one of two ways: The shells were washed up during the Biblical flood. (This explanation could not account for the fact that fossils were not only found on mountains, but also within mountains, in rocks that had been quarried from deep below Earths surface.) The fossils formed within the rocks as a result of mysterious forces. But for Steno, the close resemblance between fossils and modern organisms was impossible to ignore. Instead of invoking supernatural forces, Steno concluded that fossils were once parts of living creatures. Fossil Shark Tooth (left) and Modern Shark Tooth (right).

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how fossils form

A fossil is any remains or traces of an ancient organism. Fossils include body fossils, left behind when the soft parts have decayed away, and trace fossils, such as burrows, tracks, or fossilized coprolites (feces). Collections of fossils are known as fossil assemblages. Click image to the left or use the URL below. URL:

fossilization is rare

Becoming a fossil isnt easy. Only a tiny percentage of the organisms that have ever lived become fossils. Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure 1.2). Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria. Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventually turning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil. Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figure 1.3). How about a soft bodied organism? Will a creature without hard shells or bones become a fossil? There is virtually no fossil record of soft bodied organisms such as jellyfish, worms, or slugs. Insects, which are by far the most common land animals, are only rarely found as fossils (Figure 1.4).

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conditions that create fossils

Despite these problems, there is a rich fossil record. How does an organism become fossilized? A rare insect fossil.

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instructional diagrams

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questions

collections of fossils are known as

a) fossil collections

-->  b) fossil assemblages

c) ecosystem collections

d) ecosystem assemblages

nicholas steno in 1666 said that fossils were once part of living creatures because fossils from far inland looked like the teeth of a freshly caught great white shark.

-->  a) true

b) false

marine fossils found in tall mountains were

-->  a) once part of living creatures.

b) washed up during the biblical flood.

c) formed within the rocks as the result of mysterious forces.

d) of unknown origin.

almost every single organism that has ever lived became a fossil.

a) true

-->  b) false

which will more likely result in a fossil?

a) a zebra is attacked by a pride of lions and its meat is all eaten.

b) an insect dies in a grassy area.

-->  c) hard shelled marine organisms die and are rapidly buried by sediment.

d) all of the above

a __ fossil is left behind when the soft parts have decayed away.

a) hard

b) bone

c) trace

-->  d) body

_ fossils are burrows, tracks, or fossilized coprolites (feces).

a) soft

b) bone

c) body

-->  d) trace

the la brea tar pits in los angeles is an example of an unusual circumstance in which a variety of fossils are preserved.

-->  a) true

b) false

soft parts of an organism are never preserved so life before the evolution of hard parts is a complete mystery.

a) true

-->  b) false

diagram questions

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