Name:
__________
Date: ___________________
INDEX FOSSILS
Imagine that you have been given a stack of old newspapers that had
the dates removed. As far as you can tell, the newspapers are in order by
date, yet you have no idea how old they are. You do have a reference book
that has the dates when various foods (including brand names foods) were
invented or stopped being made. While looking through the newspapers,
you find that there are advertisements for these foods. So if you were to
find an advertisement for hamburgers in your newspapers, you would know
that the paper was printed at a time both after hamburgers were invented
and after they became popular enough to advertise. A quick look in your food history reference book
tells you that the word hamburger, describing a beef patty, didn't become popular until around1895, so
you would know that your paper was printed no earlier than 1895. Since your newspapers are in order,
with the oldest on the bottom and the newest on the top, you also know that all of the papers above that
point where printed after 1895 as well.
But what if you wanted to know the date more closely? Well you would have to find a food that
was only around for a short period of time. In the 1980's, the Coca-Cola Company tried to make an
improvement on their most popular beverage, Coca-cola. The new coke as it was called was advertised
in every newspaper for a short period of time. No one liked drinking it, so the advertisements stopped,
and the company stopped making it. So, if you found while rummaging through the papers an
advertisement for the “New Coke” you would know that the paper was printed between 1985 and
1992.
The “New Coke” functions in the pile of newspapers like an index fossil in layers of rock. Index
fossils are fossils from animals that were widespread during a short length of geologic time, usually a
single geologic period, such as the Carboniferous or the Devonian. The animal (or plant) was very
abundant (there were lots of them), and they were found over a large range (usually all over the world)
during a short period of time, then became extinct. So, if you find a rock that contains an index fossil,
you then know that the rock formed during the time in which the animal that became the index fossil
lived. These fossils help us to date rocks relative to one another.
For example, if we know that a certain animal was only alive
during the Quaternary Period, such as the calico scallop (scientific
name: Pecten gibbus) any rock formation that contains a fossil of
this animal is no older than 1,800,000 (1.8 million) years ago. The
calico scallop functions as an index fossil for the Quaternary Period.
Fossils of calico scallops are only found in Quaternary rocks. Some
index fossils, such as graptolites, existed for such a short
(geologically speaking) period of time that geologists can date rocks
to a million year period. As you know from your previous lab, the Earth is 4,600 million (or 4.6 billion)
years old. So to be accurate within a million years is very, very good.
You will answer some questions that will help you to understand index fossils. If you open your
Reference Table booklet to pages 8 and 9, you will find a chart that lists events in the geologic past.
You will need to CAREFULLY read the chart in order to answer the questions.
First, open your Reference Tables to pages 8 and 9, and look at page 9. You will see a column
labeled “Time Distribution of Fossils.” It has types of animals listed vertically within gray bars. These
gray bars represent the length of time that these groups of animals existed. If you find the top end of the
bar labeled Dinosaurs (which ends because Dinosaurs became extinct and no more became fossils),
place your finger on it, and follow the line to the left onto page 8, you will see that there is a number,
65.5. This is the number of millions of years ago that Dinosaurs disappeared, as in 65 million years
ago.
Across the bottom of pages 8 and 9, you will see sketches of various index fossils, with a letter
in a circle above each fossil. If you go back to the column on page 9 labeled “Time Distribution of
Fossils” you will see that the circled letters are placed along the vertical gray bars that represent the
lengths of time various groups of animals have existed. The gray line that represents Brachiopods is
very long, and shows the amount of time that all different species (types) of brachiopods put together
lived. On this line, there are the letters “Z” and “Y.” These show the
periods of time where certain species of brachiopods lived. Z represents
the brachiopod Mucrospirifer, and Y represents the brachiopod Eospirifer.
Placing your finger on “Z” (on page 9's gray line) and moving over to the
left onto page 8, you will come to the word Devonian, which is the
geologic Period where this fossil existed. According to the Reference
Table, this Period occurred from 416 million years ago to 359 million years
ago. So if you found a rock containing a fossil of Mucrospirifer (Z), you
would know that the rock formed between 416 and 359 million years ago.
Using the two skills you just practiced, you can answer the following questions.
1. If you found a rock that contained the fossil of a vascular plant, you would know that the rock
containing the fossil formed no more than _______________ million years ago.
2. Ammonoids became extinct ____________ millions of years ago. Rocks containing ammonoid
fossils are at least this old.
3. Dicellograptus, labeled “K” (a Graptolite, a colonial animal) serves as an indicator fossil for which
Period?
4. The Crinoid, _________________, letter ____, is an index fossil for the Silurian Period, and the
Crinoid, ______________________, letter ____, is an index fossil for the Devonian Period.
5. The fossils of _________________ and ________________ serve as index fossils for the Quaternary
Period.
6. Define, in complete sentences, what an index fossil is.