Speaker 1: Welcome to Brainstaff Friends, stuff works dot Com where smart
Speaker 1: happens him Marshall Brain with a cold and with today's question,
Speaker 1: what is a fossil? And if you think about it,
Speaker 1: there's something very odd about the fact that a bone,
Speaker 1: or a plant or a tree trunk can turn into
Speaker 1: a piece of rock. So what's actually going on there?
Speaker 1: The term fossil describes a wide range of natural artifacts. Generally,
Speaker 1: speaking of fossil is any evidence of plant or animal
Speaker 1: life that's preserved in the material of the Earth's crust.
Speaker 1: But when most people talk about fossils they mean a
Speaker 1: specific subsection of this group fossils, in which the shape
Speaker 1: of the animal or plant has been preserved while the
Speaker 1: actual organic matter of its body has disappeared. These amazing remnants,
Speaker 1: which date to pre as historic times, were formed very
Speaker 1: slowly by dynamic geological processes. In most cases, the fossilization
Speaker 1: process began when a plant or animal died and was
Speaker 1: quickly covered up with sediments, usually at the bottom of
Speaker 1: a body of water. The loose sediments protected the bodily
Speaker 1: remains from the elements, but more importantly, they protected them
Speaker 1: from bacteria and other forces that caused weathering. In decay.
Speaker 1: This slowed the decaying process down so that some of
Speaker 1: the remains. In most cases, only the hard materials like
Speaker 1: bone or shell were preserved for thousands of years without
Speaker 1: getting eroded away or eating away or crushed by passing
Speaker 1: hoofs or whatever. During this time, sediment layers continued to
Speaker 1: build up above that piece of bone. Eventually, these sediment
Speaker 1: layers became hard solid rock. Sometime after this hard rock
Speaker 1: layer formed, water percolated down through the rock and washed
Speaker 1: the preserved remains away. Since the rock above was hard
Speaker 1: and rigid, it didn't fall down into the empty space
Speaker 1: where the remains used to be. This empty space formed
Speaker 1: a natural mold of the animal, perfectly preserving the shape
Speaker 1: of the original remains. In some cases, percolating water carried
Speaker 1: minerals into the mold. These minerals hardened to make a
Speaker 1: natural cast of the form, just as an artist might
Speaker 1: make a sculpture cast. By filling a mold with plaster,
Speaker 1: all the original organic material disappeared, but nature left a
Speaker 1: precise mineral reproduction of that plant or animal. In cases
Speaker 1: where minerals did not fill the mold, paleontologists may fill
Speaker 1: it themselves, creating an artificial cast. This is just one
Speaker 1: scenario for fossilization. There are all sorts of other ways
Speaker 1: nature might form a fossil. A lot of prehistoric insects,
Speaker 1: for example, have been fossilized in amber. This sort of
Speaker 1: fossilization occurred when the insect was enveloped in the liquid
Speaker 1: sap from a tree, Just like the sediments at the
Speaker 1: bottom of a body of water. The sap material protected
Speaker 1: the insect from decay and eventually hardened into a rock
Speaker 1: like material. Animal fossils are also found in tar pits, bogs, quicksand,
Speaker 1: and volcanic ash. Another interesting fossil type is petrified wood.
Speaker 1: Petrified wood generally forms when trees fall into a river.
Speaker 1: They become saturated and then buried in mud, ash, silts,
Speaker 1: or other materials. Minerals such as the silica and volcanic ash,
Speaker 1: seep into the tree trunk and fill tiny pores in
Speaker 1: the wood cells. This changes the overall composition of the wood,
Speaker 1: turning it into stone while preserving the original structure of
Speaker 1: the wood. The variety of minerals in petrified wood creates
Speaker 1: strikingly vivid colors. In addition to fossilized plant and animal
Speaker 1: body remains. Paleontologists study fossilized animal footprints and trails, and
Speaker 1: even fossilized animal dung called corporate lights. These fossils are
Speaker 1: enlightening because they reveal something about how prehistoric animals moved
Speaker 1: and what they ate. The fossil record, the total collection
Speaker 1: of fossils in the world is extraordinarily important to our
Speaker 1: understanding of the Earth's history. Fossils tell us which plants
Speaker 1: and animals existed in prehistoric times and where they lived.
Speaker 1: They also tell us something about when they lived. Based
Speaker 1: on the position of fossils in the layers of the
Speaker 1: Earth's crust, paleontologists can determine which animals predate other animals
Speaker 1: and which animals lived at the same time. Using carbon dating,
Speaker 1: paleontologists can sometimes estimate the age of fossils. This provides
Speaker 1: the age of the rock layer where the fossil was found,
Speaker 1: which helps scientists date all the other material at that level.
Speaker 1: Without fossils, we would have a much more incomplete picture
Speaker 1: of the Earth's early history and life's early history. For
Speaker 1: more on this and thousands of other topics, visit how
Speaker 1: stuff works dot com.