Audio Lecture

Narrator               : Listen to part of a lecture on Geology.

Fossils are the remains of orgnisms that have been preserved. Some of the most common fossils are shells, skeletons, leaves, and insects. They are occasionally preserved in ice, but most have been burried in mud or sand that collects at the bottom of bodies of water, especially lakes, swamps, and oceans. In order for fossils to form, the animals and plants must be buried quickly; otherwise, theorganism will disintegrate. If they are buried in loose sediment, the soft tissues will begin to decay. But the harder structures such as bones and shells will remain intact for much longer. After years of pressure from the layers of sedment above them, the lower layers of sediment turn into rock, encapsulating the organisms.

There are several different mineral processes that continue the fossilization of oranisms in the sedimentary rocks. A few plants and animals become fossilized after mineral-rich water soaks into the pores an openings in the hard tissues of the plant or animal. In these fossils, theoriginal body of the organism is strengthened by the infusion of mineral deposits, and every detail of the organism is preserved. But in most fossils, the minerals in the water dissolve the original organism, leaving a fossil mold. Minerals continue to be deposited in the mold at the same time, a process that results in the replacement of the same shape. In the casts of these molds, the internal features of the organism are not preserved, but the outer structure is accurate in every detail. Sometimes the fine shapes of even very fragile feathers and for are preserved by mineral replacement.

Although the fossil record is incomplete, the composite of fossil findings chronicles the form of life that existed at various periods in the past. In a sense, the fossil record is a history of life. The location of fossil in layers of undisturbed sedimentary rock shows not only which groups of organisms lived at approximately  the same time but also indicates the order in which they were buried, that is, their relative ages. Plants and animals on the lower layers are presumed to be older than those buried after then in the layers above.

1.  What are the two most common place where fossils may be found?

(Click on 2 answers)

(A)   Ice

(B)   Mud

(C)   Sand

(D)   Water

2.  The professor briefly explains a process. Summarize the process by putting the events in order.

(click on a sentence. Then click on the space where it belongs. Use each sentence only once)

(A)   Mold of the organism preserved the shape of the organism

(B)   Water soaks into the organism

(C)   Organisms are buried in mud or sand

(D)   Minerals in the water dissolve the original organism.

3.  What is lost in the process of replacement ?

(A)   The fine shapes of fragile structures

(B)   The internal features of the plant or animal

(C)   The mineralsin the deposit

(D)   The original fossil mold

4.  Why are the layers of sedimentary rock important to the fossils record ?

(A)   The ages of the fossils may be determined by their location in the layers of rock

(B)   The shapes of the fossils may be preserved in the layers of rock

(C)   The rock protects the fossils from the mineral water that dissolves them

(D)   Plants and animals that are formed at the same time are buried in different layers of rock.