Assume a layer of rock (layer = bed) has three fossils whose geologic ranges are known. None of the fossils has a range short enough to tell us in which geologic period the bed was deposited.
However, if we plot the geologic range of each fossil we can see when they were contemporaries (i.e. in which geologic period(s) they all lived).
(All fossil illustrations after Illinois Geological Survey except Globigeria from Bureau of Economic Geology)
Here is an example.
Suppose we find these three fossils in a bed of rock:
First we go to the Fossil Reference Sheet (next page) and determine the fossil’s name and geologic range.
You will need to familiarize yourself with the Geologic Time Scale found in our class notes.
Next we make a chart and place X’s under each period of the fossil’s geologic range:
Name:                                      Є    O   S   D   M  IP   P   Tr   J   K   T   Q
Loxonema                                      X   X  X   X
Aulopora                                             X  X   X   X
Platystrophia                                 X   X
Notice the geologic ranges overlap in the Silurian Period. That is the age of the rock layer (bed).
Here is your assignment: Determine which period is the age of the 10 beds of rock.
When you have finished take the quiz to report your findings.