Mass Extinctions

Empirical/Quantitative Skills

Analyzing Trilobite Evolution Using Data From The Fossil Record

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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Critical Thinking/Communication

  1. When did the three biggest extinction events take place for the trilobites according to your plot?
  1. Use the plot of extinction intensity for all marine genera during the Phanerozoic Eon (page 4). 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?

3. A) Considering the three extinction events above, write at least one paragraph about each

event explaining a possible cause for the extinctions.

B) Include a second paragraph about their recovery or lack of recovery. Hint: Use the

Trilobite Geologic Time Scale (chart with periods and trilobite families page 4)) for recovery

data. Type your minimum 1-page response to question 3.


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