Fossils and Evolution
Spring 2012
Due: Tuesday, Feb. 21
Homework Assignment (20 points):
DIVERSITY ANALYSIS
Acknowledgment: This exercise was developed by Dr. Jon Payne at Stanford University. Dr. Payne graciously passed it along to your
instructor, who modified it for use in UNI’s Fossils and Evolution course.
OBJECTIVES
Upon completion of this exercise the student will be able to:
1. Use a spreadsheet to calculate and graphically display diversity, originations, extinctions and
per-capita extinctions by geological time interval.
2. Understand the basis for recognizing “mass extinctions” in the fossil record.
MATERIALS
Download the file “Diversity analysis.xls” from http://faculty.cns.uni.edu/~groves/ by right-clicking on
the link and then saving to your hard drive or flash drive. Open the file using MS Excel.
INTRODUCTION
Paleontologists use fossils to address a wide variety of questions. Some of these questions involve
individual specimens, some involve collections of specimens from the same locality, and some involve
global patterns. Analysis of global data has been an important component of paleontological research
over the past several decades. In particular, many paleontologists have been interested in how global
animal diversity has changed over geologic time. To address this question, several researchers compiled
databases containing the age of the first and last known occurrences of fossil groups at taxonomic levels
ranging from phylum rank to genus rank. Among these researchers, Jack Sepkoski compiled the most
extensive and widely-analyzed databases of family and genus stratigraphic ranges. His genus database
contains the first and last known occurrences of over 36,000 marine animal genera. This database can be
used to calculate the diversity of animals through the Phanerozoic at 5-10 million year intervals, as well
as to calculate the proportion of new animal genera first appearing during a given interval and the
proportion of existing animal genera that went extinct during that interval. In this exercise you will use
the ranges of the ~17,000 genera to examine the history of extinction in the marine fossil record.
ACTIVITY
The datafile contains six columns:
Period:This is the name of the geologic period.
Stage: This is the name of the geologic stage.
Date: This is the age (in millions of years) of the start of the stage.
X-bL: Number of genera that enter the stage from the previous stage but do not survive to the next stage.
X-Ft: Number of genera that originate in the stage and survive into the next stage.
X-bt: Number of genera that occur before and after the stage.
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Step 1:
Label the column to the right of the existing columns “Total Diversity”.
Question 1:
How can you calculate total diversity for each stage from the information provided?
Step 2:
Insert the appropriate function to calculate total diversity for each stage.
[Note: if you want cell C3 to be the sum of cells C1 and C2, then in cell C3 type: = C1+C2. Also note,
once you type the appropriate formula in the top cell, you can drag it down through the entire column —
you don’t need to retype it for each cell.]
Step 3:
Now create a graph of total diversity versus geological time.
 Select the cells in the “Date” column and, by holding down the control button, simultaneously
select the cells in the “Total Diversity” column.
 Next, click on the bar graph icon and select XY scatterplot. [Or, click on “chart” under the
“insert” menu.] Select the sub-type in the lower left, with data points connected by straight lines.
 After clicking “Next” twice, you will have the opportunity to label your graph. Label the vertical
and horizontal axes appropriately.
 After you click “Finish”, the graph will appear on your spreadsheet. It is customary to have
geological time move from left (older) to right (younger). Since you have ages on the horizontal
axis, you will want to reverse the direction of the horizontal axis scale. To do this, double click
on the x-axis and use the scale tab to reverse the x-axis scale.
Now you should have a graph of total genus diversity for the entire Phanerozoic. Print a copy of the
graph to turn in to the instructor.
Question 2: What features of the graph stand out to you?
As may be apparent from the data columns provided to you, total diversity for a given interval represents
a combination of genera that range through the interval (i.e., were alive before and after), genera that
originate in it, and genera that go extinct in it. Therefore, changes in diversity from one interval to the
next reflect the difference between the number of genera originating in the second interval and the
number of genera going extinct during the first interval. In other words: Change in diversity =
Originations – Extinctions. Any changes in diversity you observe could reflect changes in number of
extinctions, number of originations, or both. In the next section of the exercise, you will explore the
relative role of extinction and origination in diversity changes.
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Step 4:
Identify the column that represents the number of extinctions in each time interval. Make a graph
(scatterplot) with geological time on the x axis and the number of extinctions on the y axis. Then make
another scatterplot with geological time on the x axis and the number of originations on the y axis.
[Note: Do not plot points associated with date = 0. These are spurious points, because they represent
todays values without knowledge of the future.] Print a copy of both graphs to turn in to the
instructor.
Question 3: Has the number of genera going extinct and/or the number of genera originating
changed systematically through the Phanerozoic? Do any intervals stand out for having unusually
high or low numbers of extinctions or originations?
It is sometimes helpful to think about originations and extinctions as operating in a manner similar to
births and deaths in a population. When we think about it this way, it is clear that the number of
extinctions and the number of originations in any given time interval depend upon the available
diversity. When diversity is high, more genera are available to go extinct, and more are available to give
rise to new genera. Therefore, it is better to calculate the proportion (or fraction) of genera that went
extinct in a given interval rather than the raw number of genera that went extinct. We will do this below.
Step 5:
Create a new column that contains the proportion of extinctions, determined as the number of
extinctions divided by the number of genera that entered the stage from the previous stage. Title the
column appropriately. Next, create a scatterplot of extinction proportion versus geological time. Print a
copy of the graph to turn in to the instructor.
Question 4: In general, has proportion of extinctions changed from the Cambrian to the Recent? Are
there any outliers (i.e., points that are very different from other nearby intervals)?
Question 5: Find the boundaries between the Paleozoic, Mesozoic, and Cenozoic eras. What do you
notice about the extinction rates at these boundaries? Why do you think this is the case?
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