WEBVTT - What is a fossil?

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<v Speaker 1>Welcome to Brainstaff Friends, stuff works dot Com where smart

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<v Speaker 1>happens him Marshall Brain with a cold and with today's question,

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<v Speaker 1>what is a fossil? And if you think about it,

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<v Speaker 1>there's something very odd about the fact that a bone,

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<v Speaker 1>or a plant or a tree trunk can turn into

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<v Speaker 1>a piece of rock. So what's actually going on there?

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<v Speaker 1>The term fossil describes a wide range of natural artifacts. Generally,

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<v Speaker 1>speaking of fossil is any evidence of plant or animal

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<v Speaker 1>life that's preserved in the material of the Earth's crust.

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<v Speaker 1>But when most people talk about fossils they mean a

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<v Speaker 1>specific subsection of this group fossils, in which the shape

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<v Speaker 1>of the animal or plant has been preserved while the

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<v Speaker 1>actual organic matter of its body has disappeared. These amazing remnants,

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<v Speaker 1>which date to pre as historic times, were formed very

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<v Speaker 1>slowly by dynamic geological processes. In most cases, the fossilization

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<v Speaker 1>process began when a plant or animal died and was

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<v Speaker 1>quickly covered up with sediments, usually at the bottom of

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<v Speaker 1>a body of water. The loose sediments protected the bodily

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<v Speaker 1>remains from the elements, but more importantly, they protected them

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<v Speaker 1>from bacteria and other forces that caused weathering. In decay.

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<v Speaker 1>This slowed the decaying process down so that some of

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<v Speaker 1>the remains. In most cases, only the hard materials like

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<v Speaker 1>bone or shell were preserved for thousands of years without

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<v Speaker 1>getting eroded away or eating away or crushed by passing

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<v Speaker 1>hoofs or whatever. During this time, sediment layers continued to

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<v Speaker 1>build up above that piece of bone. Eventually, these sediment

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<v Speaker 1>layers became hard solid rock. Sometime after this hard rock

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<v Speaker 1>layer formed, water percolated down through the rock and washed

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<v Speaker 1>the preserved remains away. Since the rock above was hard

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<v Speaker 1>and rigid, it didn't fall down into the empty space

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<v Speaker 1>where the remains used to be. This empty space formed

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<v Speaker 1>a natural mold of the animal, perfectly preserving the shape

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<v Speaker 1>of the original remains. In some cases, percolating water carried

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<v Speaker 1>minerals into the mold. These minerals hardened to make a

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<v Speaker 1>natural cast of the form, just as an artist might

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<v Speaker 1>make a sculpture cast. By filling a mold with plaster,

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<v Speaker 1>all the original organic material disappeared, but nature left a

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<v Speaker 1>precise mineral reproduction of that plant or animal. In cases

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<v Speaker 1>where minerals did not fill the mold, paleontologists may fill

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<v Speaker 1>it themselves, creating an artificial cast. This is just one

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<v Speaker 1>scenario for fossilization. There are all sorts of other ways

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<v Speaker 1>nature might form a fossil. A lot of prehistoric insects,

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<v Speaker 1>for example, have been fossilized in amber. This sort of

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<v Speaker 1>fossilization occurred when the insect was enveloped in the liquid

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<v Speaker 1>sap from a tree, Just like the sediments at the

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<v Speaker 1>bottom of a body of water. The sap material protected

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<v Speaker 1>the insect from decay and eventually hardened into a rock

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<v Speaker 1>like material. Animal fossils are also found in tar pits, bogs, quicksand,

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<v Speaker 1>and volcanic ash. Another interesting fossil type is petrified wood.

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<v Speaker 1>Petrified wood generally forms when trees fall into a river.

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<v Speaker 1>They become saturated and then buried in mud, ash, silts,

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<v Speaker 1>or other materials. Minerals such as the silica and volcanic ash,

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<v Speaker 1>seep into the tree trunk and fill tiny pores in

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<v Speaker 1>the wood cells. This changes the overall composition of the wood,

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<v Speaker 1>turning it into stone while preserving the original structure of

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<v Speaker 1>the wood. The variety of minerals in petrified wood creates

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<v Speaker 1>strikingly vivid colors. In addition to fossilized plant and animal

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<v Speaker 1>body remains. Paleontologists study fossilized animal footprints and trails, and

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<v Speaker 1>even fossilized animal dung called corporate lights. These fossils are

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<v Speaker 1>enlightening because they reveal something about how prehistoric animals moved

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<v Speaker 1>and what they ate. The fossil record, the total collection

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<v Speaker 1>of fossils in the world is extraordinarily important to our

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<v Speaker 1>understanding of the Earth's history. Fossils tell us which plants

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<v Speaker 1>and animals existed in prehistoric times and where they lived.

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<v Speaker 1>They also tell us something about when they lived. Based

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<v Speaker 1>on the position of fossils in the layers of the

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<v Speaker 1>Earth's crust, paleontologists can determine which animals predate other animals

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<v Speaker 1>and which animals lived at the same time. Using carbon dating,

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<v Speaker 1>paleontologists can sometimes estimate the age of fossils. This provides

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<v Speaker 1>the age of the rock layer where the fossil was found,

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<v Speaker 1>which helps scientists date all the other material at that level.

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<v Speaker 1>Without fossils, we would have a much more incomplete picture

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<v Speaker 1>of the Earth's early history and life's early history. For

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<v Speaker 1>more on this and thousands of other topics, visit how

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<v Speaker 1>stuff works dot com.