Fossils
1. Understanding Fossils
What are fossils?
Fossils are preserved remains, impressions, or traces of organisms that lived in the past.
They are usually found in sedimentary rocks.
Examples include dinosaur bones, shells, footprints, and leaf impressions.
How are fossils formed?
An organism dies and is buried by sediments such as mud or sand.
More sediment accumulates and covers the remains.
Over time, sediments become compacted and cemented into rock.
The remains or traces may be preserved as fossils.
Why are fossils important?
They provide evidence of organisms that lived millions of years ago.
They help scientists understand how life and environments have changed.
They help determine relative ages of rock layers.
They provide evidence of evolution and Earth’s history.
2. Types of Fossils
Type
Description and example
Body fossils
Preserved parts of organisms, such as bones, teeth, and shells.
Mold fossils
Hollow impressions left when an organism dissolves, such as a shell impression.
Cast fossils
Minerals or sediments fill a mold and harden, creating a replica.
Trace fossils
Evidence of activity, such as footprints, burrows, and tracks.
Petrified fossils
Minerals replace or fill remains, such as petrified wood.
Note: Categories may overlap; petrified wood is a type of body fossil.
3. Rock Layers and Relative Dating
Scientists study the position of rock layers and fossils within them.
Relative dating establishes which rocks or fossils are older or younger; it does not give exact ages in years.
Important principles:
Law of Superposition: In an undisturbed sedimentary sequence, the oldest layer is at the bottom and the youngest at the top.
Original Horizontality: Sediments are generally deposited in nearly horizontal layers; tilting or folding generally occurs later.
Cross-Cutting Relationships: A fault or igneous intrusion is younger than the rocks it cuts.
Inclusions: Rock fragments enclosed in another rock are older than the rock containing them.
Example: In three undisturbed layers labeled A, B, and C from top to bottom, A is youngest, B intermediate, and C oldest.
4. Fossils as Evidence
How do fossils help?
Fossils help identify and compare rock layers.
Layers containing the same distinctive index fossils may have formed during approximately the same period.
Scientists can correlate layers across different locations.
Fossils help establish the relative sequence of geological events.
What are index fossils?
Fossils of organisms that lived during a relatively short geological interval.
They were geographically widespread and are distinctive and recognizable.
They help correlate rock layers and estimate relative ages.
Examples: Certain ammonite, trilobite, and graptolite species. Not every member of these groups is an index fossil; the species must have a suitable geological range.
Fossil correlation example: A rock layer in Location A and one in Location B contain the same index fossil. Scientists can correlate these layers as approximately the same age.
Key idea: Index fossils help scientists compare rock layers from different places.