Fossil Records

Fossil Records

STEMscopedia Reading ​

​By the end of the reading you should:

• Understand the significance of fossils as clues to the past and how they can provide insights into organisms that lived millions of years ago.

• Identify and describe different types of fossils, including inclusion fossils, mold fossils, cast fossils, and trace fossils.

• Appreciate the importance of the fossil record in documenting the diversity, extinction, and change of life-forms throughout the history of Earth.

​How can fossils tell us about organisms that lived millions of years ago?
Fossils are preserved parts or traces of animals or plants that lived in the past. Fossils are clues to the past. They tell us what organisms lived in a certain place, how they looked, and what the environment was like in the place where they lived. Fossils can also show how organisms have changed over time. If you find fossils of seashells high on a mountain, you could conclude that the mountain used to be underwater at some point in time.

Multiple Choice

What are fossils?

• Artificially created objects

• Fragments of metal

• Pieces of rock

• Remains of living organisms

Multiple Choice

What do fossils tell us?

• What organisms lived in a certain place, how they looked, and what the environment was like

• What color they used to be

• What year they were formed

• What kind of rock they were found in

​There are three different kinds of fossils.

​Inclusion fossils are formed when all or part of an organism is covered in a solid substance, such as ice or amber, that preserves the material trapped inside. This kind of fossil is different from all the rest in that the actual tissues of the dead organism are still present, and in rare cases, its DNA may even still be intact.

Multiple Choice

What is an inclusion fossil?

• A fossil formed when all or part of an organism is covered in a solid substance that preserves the material trapped inside

• A fossil formed by the actual dead organism

• A fossil formed by its activities while it was alive

• A fossil formed when part of an organism has been buried in sediment and slowly decayed

Mold fossils are formed when part of an organism has been buried in sediment and slowly decayed. The dead organism creates an impression in the soft sediment. The sediment then hardens into solid stone, preserving the impression in stone.

Multiple Choice

What is a mold fossil?

• A fossil formed by its activities while it was alive

• A fossil formed by the actual dead organism

• A fossil formed when part of an organism has been buried in sediment and slowly decayed

• A fossil formed when all or part of an organism is covered in a solid substance that preserves the material trapped inside

Cast fossils are the kind you probably picture in your mind when you think of fossils. For a cast fossil to form, a dead organism must first be buried in sediment. Then, as the organism slowly decays, it leaves behind open spaces where minerals crystallize and fill in the exact shape of the decaying organism or part. This process is called petrification and produces stones that mimic the exact shape of the tree stump, bone, shell, or whatever part decayed in that spot.

Multiple Choice

What is a cast fossil?

• A fossil formed by the actual dead organism

• A fossil formed when part of an organism has been buried in sediment and slowly decayed

• A fossil formed by its activities while it was alive

• A fossil formed by petrification that produces stones that mimic the exact shape of the decaying organism or part

Trace fossils are fossils that are formed not by the actual dead organism but by its activities while it was alive. Examples of trace fossils include animal tracks preserved in stone, petrified nests, burrows, etc.

Multiple Choice

What is a trace fossil?

• A fossil formed when part of an organism has been buried in sediment and slowly decayed

• A fossil formed by its activities while it was alive

• A fossil formed when all or part of an organism is covered in a solid substance that preserves the material trapped inside

• A fossil formed by the actual dead organism

​Remember, fossils are rarely the actual organisms themselves. Rather, their body parts have broken down and been replaced with mineral solutions that have hardened into rock.

​Fossils do not have dates printed on them. Yet there are ways to figure out the age of a fossil.

One way to determine the actual age of a fossil is to determine the age of the rock layer in which the fossil is found. This makes sense because the organism that produced the fossil was trapped in the rock when it formed.

Multiple Choice

What is one way to determine the age of a fossil?

• Examine the color of the fossil

• Count the number of fossils found in the area

• Measure the size of the fossil

• Determine the age of the rock layer in which the fossil is found

​Layers of rock form one on top of the other. The bottom layer was laid down first, so it is the oldest layer. The top layer was laid down last, so it is the youngest layer. The layers in between are in order from oldest to youngest from bottom to top. The law of superposition states that in any sequence of rock or sediment layers, the youngest sediments or rocks are at the top of the sequence and the oldest are at the bottom.

Multiple Choice

What is the law of superposition?

• In any sequence of rock or sediment layers, the oldest sediments or rocks are at the top of the sequence and the youngest are at the bottom

• In any sequence of rock or sediment layers, the sediments or rocks are arranged randomly

• In any sequence of rock or sediment layers, the sediments or rocks are all the same age

• In any sequence of rock or sediment layers, the youngest sediments or rocks are at the top of the sequence and the oldest are at the bottom

​To get an idea of how this works, imagine that you baked a layer of cake for each day on Monday, Tuesday, Wednesday, and Thursday, setting each layer on top of the other on the day it was baked. The Monday layer would be at the bottom, and the Thursday layer would be on top. Let’s say you put walnuts in Monday’s layer and raisins in Thursday’s layer. You give the cake to a friend who knows that the bottom layer is oldest and the top layer is youngest. What could your friend conclude about the age of the walnuts and raisins? 

​Another method of dating fossils is radioactive dating. Scientists can examine the radioactive particles in a sample of sediment from a fossil or sediment layer to determine its approximate age.

The collection of fossils and their placement in chronological order is known as the fossil record. It documents the existence, diversity, extinction, and change of many life-forms throughout the history of life on Earth.

Multiple Choice

What is radioactive dating?

• A method of dating fossils by examining the color of the fossil

• A method of dating fossils by examining the radioactive particles in a sample of sediment from a fossil or sediment layer

• A method of dating fossils by examining the size of the fossil

• A method of dating fossils by examining the location of the fossil

​Take a look at the illustrations on the right. They show fossils found in different layers of rock. What is the order of these fossils, from oldest to youngest?

Hotspot

Take a look at the illustrations on the right. They show fossils found in different layers of rock. What is the oldest fossil layer?

​What do you call scientists who study fossils?

They are called paleontologists. Paleontology is the scientific study of prehistoric life. Paleontologists use fossils to figure out three main things about fossils: the identity and origin of the fossil, the fossil's environment, and what the fossil can tell us about the history of Earth.

Multiple Choice

What is the study of prehistoric life called?

• Geology

• Paleontology

• Chemistry

• Biology

Multiple Choice

What do paleontologists use fossils to figure out?

• The type of rock the fossil is found in

• The identity, diversity, extinction, and change of many life-forms throughout the history of life on Earth

• The location of the fossil

• The age of the rock layer in which the fossil is found

​Seashells seem to be everywhere. Most of the time you will find them on beaches, but every now and then, you may find them far from the sea. For example, you may find a shell stuck in a rock high on a mountain. What could such a find tell you?

​How can fossils tell us what environments were like millions of years ago?

Scientists tell us that environments change over time. However, what evidence backs up this claim? In 1988, Gordon Hubbell and a team of fossil hunters dug a hole in a desert in Peru. This desert is one of the driest places on Earth; almost no rain falls there. After digging for a while, Hubbell discovered a jawbone and more than 200 sharp, triangular teeth. Hubbell is an expert in fossil sharks. He recognized that the teeth belonged to an ancestor of the great white shark.

​For about 20 years, the teeth were stored in Hubbell’s collection of fossils. Then, in the early 2000s, a team from the Florida Museum of Natural History in Gainesville figured out that the fossil teeth were 4 million years old! The conclusion? One of today’s driest places was underwater 4 million years ago. Fossils proved that the environment in this place had greatly changed.

​Here is another example. Today, the continent of Antarctica, which surrounds Earth’s South Pole, is the coldest place on our planet. Temperatures there can dive below -84ºC (-120ºF). No plants or animals permanently make their homes in the interior of Antarctica. Has Antarctica always been this cold and empty of living things? According to fossil evidence, the answer is no.

​Scientists exploring the Dry Valleys area in Antarctica found fossils of mosses and tiny animals. The fossils were unearthed in ancient lake beds. The scientists calculated that the fossils were about 14.1 million years old. At that time, shrubs grew from the ground around the lakes. Some insects lived there too. To support these living things, summer temperatures would need to have been much warmer than they are now. Again, fossils proved that an environment on Earth had changed greatly over time.

​Fossils are most often formed from the hard parts of organisms. The soft parts usually break down before they can be preserved in rock. However, there are some exceptions with inclusion fossils. For example, animals trapped under ice may be preserved whole. This is what happened to some woolly mammoths like the one shown in the image at the top right. Woolly mammoths are extinct animals that looked a bit like modern elephants. Some lived in the icy regions close to the North Pole. Some insects have been preserved whole too. This is especially true of insects trapped in tree sap. When the sap hardens, the insect is preserved inside. This happened to the mosquito in the image to the right.

Multiple Choice

What is an example of an organism that has been preserved as an inclusion fossil?

• An animal trapped under ice

• A leaf

• A rock

• A shark tooth

​How can we represent fossils and past environments using models?
Scientists often use models to describe objects or events in nature. A model is a representation of a real thing. A model may be as simple as a drawing or a shape made out of clay. It may be an animal reconstructed from just a few fossil bones. The fossils in the dinosaur skeleton shown below were used to make a model of a complete dinosaur.

​Another type of model is a diorama that shows both an organism and its environment. The environment may be in the present or in the distant past. The American Museum of Natural History in New York City shows two organisms related to human beings walking on a dusty plain in Africa. This model was based on actual fossil footprints preserved in rock. By studying the shapes of the footprints, scientists can draw conclusions about the body shapes and sizes of the organisms that made the prints.

Multiple Choice

What is an example of a model that shows an organism and its environment?

• A mosquito trapped in tree sap

• A diorama

• A shark tooth

• A woolly mammoth fossil