A favored Conclusion theory to explain the extinction of dinosaurs, together with many other species, has been the globally catastrophic collision of a large asteroid with the Earth. ██████████ ████████ ██ ██ ████████████████ ████████ ███████ ██ █ █████ ██ ████ █████ █████████ ██ █ ██████████ █████ ████ ████ █████████████████ ████ ███ ████████ ██████ █ ███ █████████ ██████ ████████ ████ ███ ████████ ██████ ███ ███████████ ███████ ██ ███ ███████ ████████ ████████ ████ ██████ ███ ███████████ ██ ███████ ██████ ████ ████ ███ ██████████ ██ ████ ███ ███████ ███ ██████ ██████ ██ █████ ████████ █████████ ████████ █████ ████ ████████ ██████ ███ ████ ████ ██████ ██ ███ ████████ ████████ ██████ ███ ███ ████████████
The stimulus presents two competing theories for the extinction of the dinosaurs. The first is the asteroid-impact theory: a large asteroid collided with the Earth and caused a global catastrophe. The evidence for this theory is an extraterrestrial chemical element found in a layer of dust around the world, at a geological level corresponding to the time of the supposed impact and the extinctions.
The second is the volcanic-activity theory: massive volcanic eruptions put enough dust into the atmosphere to cool the planet. The evidence is that the Deccan region of India has extensive volcanic flows from the same time period as the supposed asteroid impact and the extinctions. Proponents of this theory argue that any asteroid impact was irrelevant to the extinctions.
Which one of the following, ██ █████ ████ ████████ █████████ ████ ███ ███████████████ ██████ ██ ██ █████ ███████████ ██ ███ ██████
Large concentrations of ████████ █████ ████ ██████ ████ █████ ██ ███████ ████████ ████ ██ █████ ████ ███████ ██ █████████ ███████████ ██ █████ ██████ ██████ ████ ████ ██ █████ ██████
The fact that some dinosaur species congregated in large groups for nesting tells us something about dinosaur behavior, but it has no bearing on what caused their extinction. Whether dinosaurs nested in groups or individually, the asteroid impact would have affected them either way. This doesn't help us evaluate whether the asteroid theory is complete or incomplete.
Dinosaur remains indicate ████ ████ ███████ ██ ████████ █████ ████ ████████ ██ █████ ████ ████ ███████ ██ ████ ████ █████ ████ ████████ ██ ██████ ███ █████ ███████ ██ ███████ █████████████
This is a tempting answer. You might be thinking that if dinosaurs could migrate over wide ranges to escape local catastrophes, then a single asteroid impact shouldn't have been enough to kill them all, which would suggest the theory is incomplete. But this reasoning overlooks what the asteroid theory actually claims. The theory describes a globally catastrophic event. The catastrophe wasn't limited to the area near the impact site. The theory holds that the collision sent dust into the atmosphere worldwide, causing global effects. Migration doesn't help when there's nowhere on the planet to escape to. So the fact that dinosaurs could travel to escape local effects doesn't tell us much about whether a global catastrophe could have wiped them out.
Legends from many █████████ ████ ██ ███ █████ ██████ ████ ██████ ██████ ██ ████ ██ ██████ ██████████ █████ ██ ███ ███████ ████ ████ ███████ ███████ ███████ █████████ ████ ████████ ████ ███ ███████ ██ ██████████
Whether ancient peoples were familiar with dinosaur fossils has nothing to do with what caused dinosaurs to go extinct. This is irrelevant to evaluating the asteroid-impact theory.
In the Gobi ██████ ██ ██████ █████ ███ ████ █████ ███████ ███ ███ ███ ██ ██████████ ██████ ████████ █████████ ██ ████ ████ ████ █████ ██ █████████████ ██████████ ████ ███ ███████ █████ ███ ██ ███ ████ ███ █████████ █████ ██ ██ ██ ████
The fact that dinosaurs lived in the Gobi desert when it was as dry as it is today tells us something about the range of environments dinosaurs could tolerate, but it doesn't give us any information about whether the asteroid impact was the cause of their extinction. This doesn't help us assess the completeness or accuracy of the asteroid theory.
The fossil record ██ ███████ ████ █████ ███ █████ ██ ████████████████ ████ █████ █ ██████████ ████ ████ ██ ████████ ███████ ████ ██ ██ ███ ███ ████████ █████ █████ █████ ███ ████ █████ ████ █ ██████████ ██ ███████ ████ ██ ██ ██
This is strong evidence that the asteroid theory is at least incomplete. Remember, the extraterrestrial dust layer marks the time of the supposed asteroid impact. The fossil record below that layer (i.e., before the impact) shows that dinosaur species had already declined from 35 to 13. That's a significant reduction that happened before the asteroid ever arrived, which means something other than the asteroid must have been driving those earlier extinctions.
Furthermore, dinosaur teeth found above the dust layer show a further decline from 13 to 5 species. This means some dinosaurs survived the impact and died off gradually afterward, rather than being wiped out all at once by the collision. If the asteroid were the complete explanation, we'd expect to see the extinctions concentrated at the moment of impact, not a pattern of gradual decline both before and after. This evidence strongly suggests that the asteroid-impact theory, even if it captures part of the story, cannot be the whole explanation.
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