TEKS 5.7D Identify fossils as evidence of past living organisms and the nature of the environments at the time using models.

TEKS Lesson 5.7D: Fossils

What is a fossil?

Scientists who study certain kinds of rocks often get an idea of what life on Earth was like millions of years ago. They may uncover footprints that a dinosaur made as it walked through mud 100 million years ago. Or they may discover teeth that belonged to a
dog-sized horse that lived about 50 million years ago.

These footprints and teeth in stone are examples of fossils. Fossils are the preserved remains or traces of living things. Fossils give evidence about the nature of organisms that lived in the past. Fossils also tell about the organisms’ environments.

Many fossils can be found in rock. For example, trilobites were animals whose fossils
are found in ancient shale. Shale is a rock that can form over time in shallow water. Trilobites were hard-shelled animals that lived in the ocean about 500 million years ago.

1. Identify What are fossils? Where are they typically found? Identify what types of evidence fossils provide about the past.

_____________________________________________________________________

_____________________________________________________________________

How do fossils form?

Most fossils form when living things die and are buried by sediment. Pieces of rock, soil, or other material on Earth’s surface are called sediment. Over time, sediment presses together and hardens into sedimentary rock. This rock preserves the shapes of the once-living things. Sandstone, shale, limestone, and coal are examples of sedimentary rock. Scientists usually find fossils in these kinds of sedimentary rock.

Fossils in Rock Fossils form when the remains or traces of an organism are protected from decay. Most fossils form from animals or plants that once lived in or near quiet water. Quiet water can be found in places such as swamps, lakes, or shallow seas. Suppose an organism dies. Its soft parts often decay quickly or are eaten by animals. That is why typically only the hard parts of a plant or an animal leave fossils. These hard parts include bones, teeth, shells, seeds, or woody stems. It is rare for the soft parts of an organism to become a fossil.

The figure below shows an example of how a fish fossil can form in sedimentary rock. Erosion may wear away the rock. Then the fossil becomes visible.

 

 

        2. List Which parts of an animal or a plant are most likely to become fossilized?

_____________________________________________________________________

_____________________________________________________________________

Trace Fossils Evidence of the activities of ancient life forms comes from trace fossils. A fossilized footprint is one example of a trace fossil. The mud or sand that the animal stepped in was buried by layers of sediment. Slowly the sediment became solid rock. This rock preserved the footprint for millions of years.

• Fossil footprints can tell scientists about an animal’s size and behavior. Did the animal walk on two legs or four legs? Did it live alone or as part of a group?
• Other types of trace fossils also give clues about past living things and their behavior. A trail or burrow can give clues about the size and shape of an animal. It can also give clues about where the animal lived and how it obtained food.

Other Types of Fossils Other fossils form when the remains of organisms are preserved in substances such as tar, amber, or ice. For example, ancient animals have been preserved in amber. Amber is the hardened sap of evergreen trees. First, an insect is trapped on the sticky sap and dies. Then more sap covers it. The sap seals the insect from the air. This covering protects the insect’s body from decay.

3. Identify Fossils Identify the type of fossil that preserves evidence of an animal’s behavior.

1. carbon film
2. sediments
3. preserved remains
4. trace fossil

What evidence do fossils provide about past living organisms?

Scientists collect fossils from sedimentary rocks all over the world. They study the fossils to determine what past life forms were like. They want to learn what past animals looked like, what they ate, and what ate them. They also want to know how these animals cared for their young and how they moved around. For example, fossils of wastes from ancient animals provide evidence of what the animals ate. Dinosaur tracks can give evidence about how many legs a dinosaur walked on, how fast it moved, and when it lived. Dinosaur tracks may also give evidence about the social behavior of dinosaurs. For example, tracks can show whether dinosaurs moved in herds.

• Together, all the information that scientists have gathered about past life is called the fossil record. The fossil record provides evidence about the history of life.
• The fossil record reveals a surprising fact: Fossils occur in a particular order. Older rocks contain fossils of simpler organisms. Younger rocks contain fossils of more complex organisms. In other words, the fossil record shows that life on Earth changed over time. Simple, one-celled organisms gradually gave rise to complex plants and animals.
• The fossil record also shows that many types of organisms have become extinct. A type of organism is extinct if it no longer exists and will never again live on Earth.

 

4. Review What term best describes a type of animal that no longer exists?

1. preserved
2. extinct
3. complex
4. trace fossil

What evidence do fossils provide about past environments?

Scientists also want to know about the environment in the past. Scientists use fossils to build a picture of Earth’s past environments. The fossils found in an area tell whether the area was a shallow bay, an ocean bottom, or a freshwater swamp.

• Fossils also give evidence about the past climate of a region. For example, coal has been found in Antarctica. You may know that thick layers of ice and snow now cover Antarctica. But coal only forms from the remains of plants that grow in warm, swampy regions. The presence of coal shows that the climate of Antarctica was once much warmer than it is today.
• Scientists can use fossils to learn about changes in Earth’s surface. For example, fossils may give evidence that an area of dry plains once had many lakes and swamps.

5. Summarize What can scientists learn from fossils about the past environment in an area?

_____________________________________________________________________

_____________________________________________________________________

6. Identify Evidence Suppose you found fossils of ancient seashells and fish when you went for a walk in a dry, rocky area. How could you use these fossils as evidence to identify the nature of the environment of that area when the organisms were alive?

_____________________________________________________________________

_____________________________________________________________________

 

Lesson Check

  1.  List What are three things scientists can learn about an organism from its fossil?

__________________________________________________________________

__________________________________________________________________

  2.  Review Are all fossils made of rock? Explain.

__________________________________________________________________

__________________________________________________________________

  3.  Predict Which of the following could become a fossil?

1. A   mud
2. B   crystal
3. C   clam shell
4. D   volcanic rock

  4.  Identify Evidence Identify what the fossil record shows about past living organisms on Earth.

__________________________________________________________________

__________________________________________________________________

  5.  Infer What do deposits of coal tell about the past environment of Antarctica?

1. Antarctica’s climate was much colder.
2. Antarctica’s climate has stayed the same.
3. Antarctica’s climate was much warmer.
4. Antarctica must have always had a cool climate.

  6.  Identify Evidence A scientist finds a trace fossil of an animal’s underground burrow. The burrow contains a fossil jawbone. The jawbone belonged to a small mammal with teeth for eating seeds. What can you identify about the environment where this mammal lived?

__________________________________________________________________

__________________________________________________________________