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Lab 13: Mass Extinctions – How Catastrophic and How Gradual?

One of the fundamental problems in trying to understand biotic change across boundariesis trying to recover the true rate of extinctions within stratigraphic sections. In this lab we willlook at what is called the Signor-Lipps effect. The Signor-Lipps Effect explains how a fossilrecord that appears to be a gradual extinction can in fact represent a sudden extinction. Becauseof some species are rare and some are common, the probability of actually finding the rarespecies is much less than the common at any one level. Because the first and last appearance of aspecies defines its range, as we can know it, the rarer species seem to disappear from the fossilrecord long before the time of extinction, simply due to chance.

The debate about whether the K-T impact was actually responsible for the massextinctions, or whether it merely administered the coup de grace, to communities with alreadydeclining diversity, centers around this effect.

In this lab we will examine a published paper on the K-T boundary that purported toshow that marine fossils showed a gradual or stepped extinction pattern towards the K-Tboundary.

The Signor Lipps Effect

In 1982 Philip Signor and Jere Lipps wrote a paper (Signor and Lipps, 1982) for aconference on mass extinctions triggered by the Alvarez et al. (1980) paper setting forth theimpact model for the K-T boundary. In this paper they state:

Catastrophic hypotheses for mass extinctions are commonly criticized because many taxagradually disappear from the fossil record prior to the extinction. Presumably, ageologically instantaneous catastrophe would not cause a reduction in diversity or a seriesof minor extinctions before the actual mass extinction. Two types of sampling effects,however, could cause taxa to appear to decline before their actual biotic extinction. Thefirst of these is reduced sample size provided in the sedimentary record and the second,which we examine in greater detail, is artificial range truncation. The fossil record isdiscontinuous in time and the recorded ranges of species or of higher taxa can onlyextend to their last known occurrence in the fossil record. If the distribution of lastoccurrences is random with respect to actual biotic extinction then apparent extinctionswill begin well before a mass extinction and will gradually increase in frequency until themass extinction event, thus giving the appearance of a gradual extinction. . . . Therecorded ranges of fossils, especially of uncommon taxa or taxa in habitats notrepresented by a continuous record, may be inadequate to test either gradual orcatastrophic hypotheses.

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Signor and Lipps illustrate the concept with the following graph:

Figure 1. Alteration of diversity patterns by artificial range truncation. A, hypothetical diversitycurve, where diversity is suddenly reduced by a catastrophic extinction event; B, a cumulativeprobability curve, showing the likelihood of different amounts of artificial range truncation; C,apparent gradual decline in diversity after imposing the artificial range truncation suggested in Bon the hypothetical diversity pattern.

Range Through Method

In biostratigraphy, the temporal or stratigraphic range of a species is defined by the firstand last appearances in the stratigraphic sections. It is assumed that he species lived in between.

Additional finds of the same species between the first and last occurrences do not addadditional information about the range. Thus, in a graph of the distribution of species through a

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stratigraphic section, the ranges are generally shown as lines. This is called the “range through”method, because the species range through the interval between the first and last occurrence,

Case Study: The K-T Boundary, Zumaya, Spain

Ward et al. (1986) describe a fossiliferous marine section in Zumaya Spain. heymeticulously sampled it, trying to determine the pattern of extinction over the K-T boundary.The produced the graph on the following pages (Figure 3 and 4) showing the ranges of thevarious species they found.

By tabulating the number of ammonite species in each 1 meter increment, and plottingtha number against position below the boundary the obtained the following graph (Figure 2):

Figure 2: Ammonite species richness below the boundary at Zumaya (Ward et al., 1986).

From this graph they concluded that species richness was declining well before theboundary, which implied at the extinction was not catastrophic.

I. Try to duplicate their results by your own tabulation using figure 4 at 1 m intervals.Note that they truncated their results (at 180 m) before the actual bottom of theirsection (caused by fault), which was at 220 m below the boundary. Continue yourtabulation to be base of the section. What pattern do you get at the bottom of thesection and how does it compare with the pattern towards the top near the K-Tboundary? Note that with the range through method you count a species present forEVERY meter interval before its first and last occurrences.

II. Truncate their section by placing a line somewhere between the 20 and 60 m levelsand at somewhere between 140 and 180 m. Do the same kind of tabulation as you didin I (above) between these two new limits. Explain the pattern. Remember that in therange through method, you must count the first and last occurrence at the specimenclosest to your new limit (marked by a short horizontal line) at the real first and lastoccurrence.

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III. Compare the results of your tabulations with those of Ward et al. (1986) and comparethe latter to those of Marshal and Ward (1996) based on data collected during thesubsequent 10 years. Explain the changes in terms of the Signor Lipps Effect.

IV. What does this mean for the gradual or catastrophic end to the non-avian dinosaurs?

Figure 3: From Ward et al. (1986). The range chart is show on the following page enlarged(Figure 4).

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