Earth Science: Paleontology: Reading the Fossil Record
Using fossils and rock layers to understand Earth's past
Earth Science: Paleontology: Reading the Fossil Record
Using fossils and rock layers to understand Earth's past
Earth Science - Grade 6-8
- 1
A fossil shell is found in a rock layer on top of a mountain. What does this fossil suggest about the environment where the rock originally formed?
Think about where shelled organisms usually live today.
The fossil shell suggests that the rock originally formed in a marine or ocean environment. The area may have been underwater in the past and later uplifted to form part of a mountain. - 2
Explain the law of superposition and how it helps paleontologists read the fossil record.
The law of superposition states that in an undisturbed sequence of sedimentary rock layers, the oldest layers are at the bottom and the youngest layers are at the top. Paleontologists use this rule to determine the relative ages of fossils found in different layers. - 3
A student finds three fossils in different rock layers. Fossil A is in the lowest layer, Fossil B is in the middle layer, and Fossil C is in the top layer. Which fossil is probably the oldest, and why?
Use the order of the layers to compare relative ages.
Fossil A is probably the oldest because it is found in the lowest rock layer. In undisturbed sedimentary rocks, lower layers are usually older than layers above them. - 4
A fossil of a fern is found in an area that is now a dry desert. What can scientists infer about the past environment of that area?
Use the needs of living ferns to make an inference about the past.
Scientists can infer that the area may have been wetter in the past because ferns usually need moist conditions to grow. The fossil is evidence that the environment changed over time. - 5
What is an index fossil, and why is it useful for comparing the ages of rock layers in different places?
An index fossil is a fossil from an organism that was widespread but lived during a relatively short time period. It is useful because rock layers containing the same index fossil are likely to be similar in age. - 6
Two rock layers in different states both contain the same index fossil. What conclusion can geologists reasonably make about these two layers?
Index fossils are used like time markers in rock layers.
Geologists can reasonably conclude that the two rock layers formed during about the same time period. The matching index fossil helps correlate the layers across different locations. - 7
A fossilized footprint is found in sandstone. Is the footprint a body fossil or a trace fossil? Explain your answer.
The fossilized footprint is a trace fossil because it records evidence of an organism's activity rather than the preserved body of the organism. It shows that an animal walked through the area. - 8
List two examples of body fossils and two examples of trace fossils.
Body fossils are parts of organisms. Trace fossils are signs of what organisms did.
Two examples of body fossils are fossil bones and fossil shells. Two examples of trace fossils are footprints and burrows because they show activity rather than body parts. - 9
A rock layer contains fossil fish, fossil coral, and ripple marks. What type of environment did this layer most likely form in? Use evidence to support your answer.
This layer most likely formed in a shallow water environment such as a sea or ocean. Fossil fish and coral are marine life, and ripple marks can form when moving water shapes sediment. - 10
Why is the fossil record incomplete?
Consider what must happen for an organism to be preserved for millions of years.
The fossil record is incomplete because not all organisms become fossils. Many organisms decay, are eaten, or are destroyed before fossilization, and some fossils are later changed or destroyed by erosion, heat, pressure, or tectonic activity. - 11
In one rock sequence, a certain fossil species appears in older layers, becomes common in middle layers, and then disappears from younger layers. What might this pattern suggest?
This pattern might suggest that the species lived for a period of time, became more successful or widespread, and then went extinct or moved away from the area. The fossil record can show changes in populations over time. - 12
How can fossils provide evidence that Earth's environments and life forms have changed over time?
Compare fossils from different layers and locations.
Fossils show what organisms lived in the past and where they lived. When fossils in older layers differ from fossils in younger layers, or when fossils show past environments unlike today's environment, they provide evidence that life and Earth's environments have changed over time.