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AdequacyFossilRecord

AdequacyFossilRecord - 1 G331 The Nature and Adequacy of...

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1 G331: The Nature and Adequacy of the Fossil Record Approaches: x Rarefaction x Logarithmic Decay Model x How many species might have been alive in the past? What percentage are fossilized? x How many skeletonized species might have been alive in the past? What percentage are fossilized? x Example research study on Mississippian crinoids x Spindle Diagrams of Diversity Rar e fa c tion ± A technique that helps to determine when an adequate sample size has EHHQ UHDFKHG² D VRUW RI ³GLPLQLVKLQJ UHWXUQV´ DSSURDFK WR HYDOXDWL ng adequacy of sample size. When the rarefaction curve begins to flatten, additional sampling adds little new information. Examples: x How many WVU students would have to be randomly surveyed to find someone from 45 of the 50 states? There are 26,000 students. 0 10 20 30 40 50 0 100 200 300 400 500 600 700 800 900 1000 HomeState Students Sample of 1000 WVU Students x How many specimens should be randomly dredged from the continental shelf of Virginia to find most of the shelled invertebrate species? Only a small fraction of what is available.
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2 0 5 10 15 20 25 30 35 40 0 200 400 600 800 1000 1200 1400 Species Dredge Specimens Shelled Invertebrates, VA Shelf x How many specimens should be randomly counted from bulk rock samples to determine the approximate number of species in a geological formation? Only a small fraction of what is available. x We can use rarefaction to compare different groups while adjusting for sample size differences. For example, there are 181 genera of Mississippian crinoids in North America and 93 in the British Isles. Were there really twice as many genera living in North America, or does this reflect sampling differences? Sample size is based on the number of genera occurrences in time ³ bins ´ , essentially 11 biostratigraphic units within the Mississippian. North America is larger than the British Isles and has more Mississippian-age outcrops, and more
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3 rocks to sample. This might explain the difference between 181 and 93 genera. But when sample size is adjusted to be the same for both regions, we would expect to find 124 genera in North America. This is only 33% more rather than 95% more. Thus, the British Isles were more similar in the number of crinoid genera to North America than the raw counts would suggest. However, even with more sampling the number of British Isle genera would probably not equal that of North America. So there is a real difference between the two regions, but it is probably 33% greater, rather than 95%, for North America. Why? 0
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