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After being introduced to some widely used similarity indices through a brief introductory lecture and reading assignment, students receive a handout that includes the genus-level presence/absence data for Jurassic ammonite faunas from the Boreal Craton and four accreted terranes that make up much of what is now western North America. The handout also includes several tables that present values calculated from those data for four different indices, and a map showing the current distribution of the terranes (all extracted from Hammer and Harper, 2006). The students then work collaboratively, in pairs or small groups, to answer a few rudimentary questions that require first-order interpretation of the data to get them comfortable with how the data are presented, and to demonstrate how different indices produce different rankings of relative similarity for the five faunas. For many of the students, it is their first encounter with quantitatively derived, but clearly ambiguous results â and the struggle begins ("but which one is right?") The exercise then tests them with more complex questions that require (without specific announcement) that they devise multiple explanations for low indices (dissimilar faunas) between Sonomia and the Boreal Craton, and critically evaluate those possible explanations. When they are led to the correct conclusion, that a simplistic interpretation of the dissimilarity as a product of geographic separation conflicts with the well established timing of Sonomia's arrival in the Triassic (based on other geologic evidence), it impresses upon them the importance of considering all the data available. In this case, the data falsify the hypothesis that Sonomia was still far away from North America, and require a different explanation for the faunal contrast, such as a latitudinal separation and/or physical separation by some barrier (perhaps highlands created by the Sonoman Orogeny). The second phase of the exercise involves analysis of a pronounced contrast reported by Loch (2007) in the taxonomic composition of Lower Ordovician trilobite faunas in what is now eastern and western North America, quantified by the Jaccard Index. With the geologic context leaving no doubt that both faunas inhabited the same paleocontinent, the stage is set for discussion of a strong (again, perhaps paleolatitudinal) contrast in environment/lithofacies that produced the contrast. The questions posed in this part of the exercise challenge the students to recognize the strong influence of relative sample sizes (a pitfall of that index), a different taxonomic level (trilobite species, as compared to ammonite genera), and perhaps other factors such as stratigraphic imprecision and inaccuracy in taxonomic assignment in producing values very different from those in the first case study.
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