How an index fossil dates a rock layer
1 min 42 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Fossils and evolution of life, with the past questions on it.
What Dr. Mars says (the words of the video)
How can a fossil tell the age of a rock layer?
Dr. Mars splits a piece of shale broken from the shale layer.
Shale forms from layers of mud.
Inside is a trilobite fossil, the remains of a sea animal.
Dr. Mars looks up trilobites in the Reference Tables.
Trilobites as a group lived from the Cambrian to the Permian.
A long range gives only an approximate age.
His trilobite is Eldredgeops, letter R on the table.
It lived only in the Devonian Period.
So his shale formed in the Devonian, four hundred nineteen to three hundred fifty-nine million years ago.
An index fossil lived only a short time.
It is called an index fossil because it points to one short time in Earth's history.
Eldredgeops is an index fossil.
An index fossil also lived over a wide area.
So shale far away with Eldredgeops is Devonian too.
Brachiopods as a group lived from the Cambrian until today.
Brachiopods are sea animals with two shells.
Check yourself! Which one best tells the age of a rock layer: A, B, or C?
Look at how long each one lived.
Answer: B! Eldredgeops lived the shortest time.
That is the trilobite Dr. Mars split from the shale.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.