1. Ever ride the bus all the way to the end of the line? And have you noticed how the bus begins to empty out as you approach the final destination? And that sometimes, when you reach that final destination there’s still a gaggle (albeit a small gaggle) of people getting off? Use this metaphor to explain the Signor-Lipps Effect. As usual, your explanation should be good enough for anyone on campus to understand!
In effort to explain the Signor-Lipps Effect, the bus example on the lab handout provides a useful way to apply it. According to Signor-Lipps, mass extinction is immediate but it can look gradual in the fossil records. In regards to the bus trip, we can look at extinction as every person getting of the bus at each stop (species going extinct). On the final stop, the remaining people (species) exit the bus and no more people remain (catastrophic extinction). Though all the people remaining are gone, that does not mean people were not getting off the bus prior to the last stop. In reference to the fossil records, an animal may have gone extinct, but not in a catastrophic extinction (final bus stop). Species may have been going extinct throughout time (through several bus stops) before reaching a final extinction. Signor-Lipps present the challenge in being able to distinguish the difference in gradual extinction and catastrophic extinction in fossil records.
2. Dinosaurs went extinct at the Cretaceous-Tertiary boundary, but our earliest interpretations were that some groups went extinct well before then. Use the Signor-Lipps Effect to resolve this paradox.
Naturally species go extinct all the time. In regards to the Cretaceous-Tertiary period when animals went extinct, species were going extinct before that. In the Signor-Lipps explanation, diversity declined before the Cretaceous-Tertiary mass extinction but it can be difficult to compare the actual diversity from the apparent diversity. Gaps in fossil records suggests many things about the extinction process, and there are ways to test the hypotheses but it can often be biased (hence the title). However, environmental changes, climate changes, and natural selection are strong evidences that support species extinction before the Cretaceous-Tertiary mass extinction.
3. Write a detailed caption for the graph you made for the worksheet called “Figure 3”. Make sure you explain what each of the curves means, and why each curve has a different shape. You may want to refer to the captions in Signor and Lipps (1982) as a guide for the amount of detail I want.
The different lines in the graph represent diversity: apparent diversity, actual diversity. The blue curve represents apparent diversity which is what we see based on what is covered from the fossil record. The red and green curves represent the actual diversity in which extinction looks gradual. The green curve is the most constant looking line, but then all three curves show a severe drop-off when it reaches around 10 mya. Overall the graph is visually presenting the relationship of gradual diversity from catastrophic diversity before a mass extinction.
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4. Given the fact that we need to consider the S-L Effect, how can we distinguish between a catastrophic mass extinction and a gradual mass extinction in the fossil record?
To be able to distinguish gradual mass extinction from catastrophic mass extinction from the fossil record we would have to look at apparent diversity from actual diversity. It is difficult to be certain about the differences, however, because the curves can be altered. Some species, for instance, may not have a good record. Or a species may have been going extinct before the mass extinction, but the fossil range may not be consistent. This can make it difficult to put a date on when the fossilized species became extinct.
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