Principle of Fossil Succession
William Smith (late l700’s) noted that rock layers in widely separated areas could be identified and correlated by their distinctive fossil content
Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.
Fossils succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content.
What is a fossil?
There are several things that can classify as a fossil. Here's a little outline to help you identify them:
Fossils must:
1. Come from something once living
2a. Is the original organism (like things incased in amber, or preserved in ice)
2b. OR have sediment replace the object being fossilized (seashells and tree bark are often replaced by minerals)
2c. OR it must be a trace of the object (like footprints or droppings)
Fossils must:
1. Come from something once living
2a. Is the original organism (like things incased in amber, or preserved in ice)
2b. OR have sediment replace the object being fossilized (seashells and tree bark are often replaced by minerals)
2c. OR it must be a trace of the object (like footprints or droppings)
How do you make a fossil?
There are 3 things that promote fossilization.
1. Hard body parts (such as skeletal bones or exoskeletons)
2. Rapid burial and/or lack of oxygen (oxygen will decay the organism)
3. Undisturbed for a long time (keeps oxygen out and allows the surrounding sediment to harden around it)
1. Hard body parts (such as skeletal bones or exoskeletons)
2. Rapid burial and/or lack of oxygen (oxygen will decay the organism)
3. Undisturbed for a long time (keeps oxygen out and allows the surrounding sediment to harden around it)
Index Fossil
Some fossils are better than others for telling the relative age of rock layers. These are called index fossils.They are any animal or plant that is characteristic of a particular span of geologic time or environment.
A good index fossil has 2 important traits:
1) It lived over a wide geological range. Or in other words, it lived in many different places. Different areas, continents, and environments. This way you can find it in lots of different places so you can compare rock layers and their ages. It makes it easier to pinpoint the era it lived in.
2) It existed for only a short period of time (like one or 2 eras). This way if we find that fossil we know exactly what era the rocks surrounding it come from.
The fossil in the picture is called Eurypterus. It is an aquatic animal that lived only in the Silurian era. Therefore, if you find this fossil, you know exactly what age your rock layer is.
A good index fossil has 2 important traits:
1) It lived over a wide geological range. Or in other words, it lived in many different places. Different areas, continents, and environments. This way you can find it in lots of different places so you can compare rock layers and their ages. It makes it easier to pinpoint the era it lived in.
2) It existed for only a short period of time (like one or 2 eras). This way if we find that fossil we know exactly what era the rocks surrounding it come from.
The fossil in the picture is called Eurypterus. It is an aquatic animal that lived only in the Silurian era. Therefore, if you find this fossil, you know exactly what age your rock layer is.