I* How Fossils Form

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Getting the Idea You may not know it, but all around us is evidence Key Words | of Earth's past. Rocks, fossils, tree rings, glacial fossil snow, and other geologic features can give us clues about Earth's paleontologist history. Fossils are the preserved remains of plants and animals body fossil petrifaction or the traces left by plants and animals, such as footprints. A trace fossil paleontologist is a scientist who studies fossils to learn about plants and animals that lived in former geologic periods.

Types of Fossils Fossils can provide information about the size and shape of extinct organisms. Fossils can even tell us about their growth patterns and diseases. By studying fossils, paleontologists can make scientific determinations about the climate or weather of the past. Different types of fossils give us clues to Earth's past environment and the organisms that lived on Earth.

In general, scientists refer to two main types of fossils: body fossils and trace fossils. Body fossils are the remains of an organism. They can be just part of an organism, or they may be the entire organism.

Body fossils can include actual pieces of an organism, such as bones, shells, teeth, or seeds. These parts are sometimes preserved through processes such as petrifaction. Petrifaction occurs when the organic material of an organism is replaced with minerals and "turned into stone." A piece of petrified wood looks like the original wood, but all of the wood molecules have been replaced with minerals. Lesson 43: How Fossils Form

The process of petrifaction explains how some fossils were formed. However, body fossils are more commonly found as casts or molds. This is because the organic (living) material that makes up a living thing usually decomposes quickly and is gone before the actual organism or parts of an organism can be fossilized.

Casts and molds are created when the organic material of the organism decays but the impression of that organism remains as a mold in surrounding rock. When these molds are filled with minerals or other sediments, they are called casts.

Ways Fossils Form Fossil Process Notes

Molds and casts Acids eat away the skeleton or shell and If the mold fills with leave an impression (mold) in the rock minerals, it becomes a cast

Tracks and trails Imprints are left in mud, which later hardens Dinosaur tracks are fairly common

Carbonization Oils leave the plant, and the remaining matter Most common method of becomes a layer of carbon; in other cases, fossilizing plants nitrogen, hydrogen, and oxygen are distilled from the plant

Petrifaction Minerals in plant cells crystallize; minerals Common with plants enter openings or cavities in shells or bones

Replacement Object buried in mud has its molecules Rare replaced by minerals

Recrystallization Mineral aragonite in shells turns into calcite Preserves the general shape of the animal

Soft tissue Mummification, frozen in ice Fossils are relatively young preservation and do not last long

Traps Whole organisms locked in tar pits, asphalt, amber, etc.

A trace fossil shows an organism's behavior but is not formed from the organism itself. Most trace fossils are from animals. Trace fossils Include animal tracks, trails, burrows, and nests. Trace fossils of plants include marks left by their roots. Colorado CSAP Coach, Science, Grade 8

Think of a paw print a dog leaves in the mud, or a footprint you leave. Suppose those prints are not washed away by rain or swept away in some other way. Millions of years later, someone might find your fossilized footprint. A paleontologist could figure out how big your feet were, how fast you were walking, and maybe even how much you weighed. A scientist could learn a lot about the dog, too.

The sketches below show how scientists can learn about animals from their tracks. Fossil tracks contain the same kind of information about extinct animals.

Deeper tracks suggest Longer stride suggests a taller animal a heavier aninnal

Larger tracks suggest One track over another suggests a larger animal which came first

Trace fossils include fossilized animal droppings. These fossils are clues to animals' diets. Traces of burrows and nests help paleontologists learn how extinct animals lived and raised their young.

DISCUSSION QUESTION Imagine you are a paleontologist and you discover the fossilized bones of an ancient deerlike creature. When you investigate, you discover that its leg bone is broken. You also notice that some of the bones seem to have marks or scratches on them. What might these clues tell you about how the animal died? Lesson 43: How Fossils Form

LESSON REVIEW

1. Which can body fossils tell us about an animal?

A. the size and shape of the animal

B. how fast the animal ran

C. what sounds the animal made _^

D. the color of the animal's fur

2. Scientists can learn all of the following from fossil tracks EXCEPT

A. how fast an animal could run.

B. how much an animal weighed. ; . ,

C. how large an animal was.

D. what an animal ate. >

3. Casts and molds are both types of

A. animal-only fossils.

B. body fossils. .

C. plant-only fossils. - • -

D. trace fossils.

4. What do paleontologists study?

A. modern-day animals and plants -

B. volcanoes and earthquakes

C. weather, climate, hurricanes, and tornadoes

D. organisms that lived in the past