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Mass Extinctions

Purpose:

Define the Earth’s mass extinction events in terms of qualitative and quantitative data.

Background Information:

Mass extinctions are short periods of time with very high rates of extinction. Since these mass

death events represent such punctuated periods of time, they are utilized as boundaries or divisions

between Eras and Periods of geologic time. Mass extinction events may affect a group of

organisms (i.e. marine invertebrates) or may be more widespread (i.e. marine and continental

ecosystems). Multiple events have been referenced for the onset of a mass extinction event,

including climate change (warming or cooling) and subsequent glaciation events, habitat loss to

invasion of another species, an overabundance of oxygen, or meteorite impacts. Due to mass

extinctions events in Earth’s history, over 99% of species that have lived on Earth are now extinct.

We are able to analyze mass extinction events within sedimentary layers by considering the

interval zone of the fossil. Its first appearance represents it evolution while its disappearance

approximates its extinction.

Background extinctions are those that occur between mass extinction events. These events are

not punctuated like mass extinctions; rather, these events are ongoing and occur over a long period

of time. Like mass extinctions, background events may occur due to climate change and habitat

loss. However, introduction of diseases to a population can also cause a background extinction of

a species. Background extinction events are part of the evolutionary cycle of biodiversity.

Opposite to mass extinction events, adaptive radiation is a punctuated period of speciation.

During an adaptive radiation event, organisms diversify and build more evolved populations over

a short period of time. These events usually occur as a result of rapid multiplication of a species

that has been introduced into a favorable environment for adaptation and reproduction. Often times

in Earth’s history, a mass extinction event is followed by a period of adaptive radiation. With a

large portion of the biodiversity of the environment eliminated, habitats open to evolution of new

species.

Extinctions

Adaptive Radiation

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Activity 1: Generalized Data

The following graph represents the biological gains and losses of various organisms during the

Phanerozoic Eon. Locate areas of mass death and sudden speciation. For each mass death event,

place an “X” along the time scale where the greatest biodiversity loss is indicated. For each sudden

speciation event, place an “!” along the time scale at the beginning of a significant gain in

biodiversity.

The following graphs represent the biological losses of various organisms during the Phanerozoic

Eon. For each high extinction rate event, place an “X” over the tip of sudden decreases in

biodiversity.

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Activity 2: Invertebrate Data

All the following graphs represent the biological gains and losses of various marine invertebrate

organisms. On the following graphs, locate areas of mass death and sudden speciation. For each

mass death event, place an “X” over the area of the graph where the greatest biodiversity loss is

indicated. For each sudden speciation event, place an “!” at the beginning of the gain in

biodiversity. Both a drastic loss and sudden gain in biodiversity may be shown simultaneously.

Phylum Cnidaria, Class Anthozoa Phylum Brachiopoda

Phylum Mollusca, Class Bivalvia

Phylum Mollusca, Class Cephalopoda Phylum Mollusca, Subclass Ammonoidea

Phylum Echinodermata, Class Echinoidea Phylum Hemichordata, Class Graptolithina

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The following chart concentrates on trilobite families. Note there are muliple time periods in the

life of trilobites that represent major biological losses. Note each of these with an “X.”

Phylum Arthropoda, Subphylum Trilobitomorpha

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Activity 3: Vertebrate Data

All the following graphs represent the biological gains and losses of various vertebrate organisms.

On the following graphs, locate areas of mass death and sudden speciation. For each mass death

event, place an “X” over the area of the graph where the greatest biodiversity loss is indicated. For

each sudden speciation event, place an “!” at the beginning of the gain in biodiversity.

Phylum Chordata, Subphylum Vertebrata

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As with the previous chart on page 5, place an “X” over the area of the graph where the greatest

biodiversity loss is indicated and an “!” at the beginning of the gain in biodiversity.

Phylum Chordata, Subphylum Vertebrata, Class Reptilia, Superorder Dinosauria

Phylum Chordata, Subphylum Vertebrata, Classes Amphibia, Reptilia-Aves, and Mammalia

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Activity 4: Climate Data

The following graph depicts oxygen isotope data, specifically Oxygen-18. Note that during warm

periods we see a decrease in the isotope. However, during cool periods, we see an increase. For

each warm period, write an “H” over the maximum temperature. For each cool period, write a “C”

over the minimum temperature.

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Activity 5: Analyzing Mass Extinction Data

Use the data from the previous four activities to fill-in the chart below. The first column should

state the time period the mass extinction occurred. If the extinction event occurred at a boundary,

be sure to list the Era(s) and both Periods. For the Biodiversity column, choose whether the mass

extinction event was only associated with marine organisms, continental organisms, or both.

Consider the information you just analyzed and what you have learned about the evolution of life

via the fossil record in your Historical Geology course. In the third column, give examples of

organisms that were drastically affected by the mass extinction event. If the event involved both

marine and continental organisms, your examples should include at least one of each. And, in the

last column, use the data from Activity 4 to note any extreme heat/cold and glaciation events

correlated to the time period you stated for the mass extinction period.

Time

Era/Period

Biodiversity Involved

Marine/Terrestrial/Both

Organism

Examples

Climate

Extremes, Glaciation

1.

2.

3.

4.

5.

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Critical Thought (To Be Done Individually):

Consider the five major extinction events. The final one was significant in the development of

humans. What if that major extinction event had not occurred? How would the Earth’s biodiversity

be different? Which organisms would dominate Earth? What organisms that flourished after that

extinction event may not be here (or as abundant) due to a lack of that event? Would we be here?

Which fossils that we have studied in this course may not exist? Use the next page to write two-

three paragraphs describing a 4-Mass Extinction Event Earth.

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Activity 6: Analyzing Trilobite Evolution Using Data From The Fossil Record

(To Be Done Individually)

The following table provides data concerning Trilobite genera during the Phanerozoic Eon.

Specifically, the table lists the total number of genera that appeared during a particular time interval

and the number of genera that last appeared (went extinct) during that time interval. One way of

analyzing this type of data, in order to determine when mass extinction events occurred for a

particular group of organisms, is to calculate the “extinction intensity” for a particular time

interval. The “extinction intensity” is simply the percentage of total genera that went extinct during

that time interval. In other words, in the table below, the extinction intensity for each time period

may be calculated by dividing the third column (“number of genera that last appear in the interval”)

by the fourth column (“total number of genera in interval”) and multiplying by 100 to arrive at a

percentage. For simplicity, you should round up or down to the nearest whole percentage point.

First, complete the following table by calculating the extinction intensity and writing it in the fifth

column of the table. Next, plot the “Extinction Intensity (%)” versus the “Time Interval End

Date (mya)” on the following graph. Then use your plot to answer the questions that follow.

EVOLUTION OF TRILOBITE GENERA DURING THE PHANEROZOIC

Time Interval

Beginning Date

(mya)

Time Interval

End Date

(mya)

Number of

Genera That Last

Appear in

Interval

Total Number of

Genera in

Interval

Extinction

Intensity (%)

251 250 0 0 ---

260 251 6 6

271 260 9 20

299 271 5 30

318 299 9 66

359 318 22 105

385 359 17 65

398 385 114 209

416 398 77 469

423 416 37 156

444 423 29 223

458 444 185 479

470 458 76 448

488 470 71 462

495 488 94 489

507 495 178 749

520 507 54 279

532 520 22 399

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Activity 6 Questions:

1. When did the three biggest extinction events take place for the trilobites according to your

plot in the graph on page 11?

2. What was different about the last extinction event as compared to the earlier extinction

events listed in Questions 1?

3. What was the approximate background extinction rate for the trilobites during the

Phanerozoic? You may provide your answer as a range of extinction intensities. (It might

be helpful to re-read the section on background extinction rates on the first page of this lab

in order to answer this question.)

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4. Below is a plot of extinction intensity for all marine genera during the Phanerozoic Eon.

This plot indicates the major mass extinctions during the Phanerozoic. Which of the mass

extinction events shown below corresponds to the three major mass extinctions shown by

your plotted data for the trilobites?