"That's unequivocal evidence I think," Fassett said. River-washed bones would be widely scattered and also show signs of wear and tear—unlike the current fossils, some of which he describes as "pristine."
Working with colleagues at the USGS in Denver, Fassett also examined the concentrations of uranium and rare-earth metals in the fossil bones.
"I thought if we could determine the trace-element compositions of the bones, we might discover that the [older] Cretaceous bones had a different chemical fingerprint than the [younger Paleocene] bones do," he said, "and indeed that turned out to be the case."
No Reason Why Not
It's not known why some species, including crocodiles and birds, survived the K-T event while many others did not. The answer could be tied to what exactly caused the mass extinction.
The popular theory is that a killer asteroid struck the Yucatán Peninsula, although experts have argued for massive volcanism, disease, climate change, or some combination of factors.
Fassett, who supports the asteroid-strike theory, said he can't explain why dinosaurs may have survived longer in some areas but not others.
"One guess is that the survivors lived in the northernmost parts of North America, at the greatest distance from the impact site, and then migrated south," he said.
"But that doesn't explain why [dinosaurs that lived later] haven't been found elsewhere. We don't have an answer for that."
Despite his caution, the Smithsonian's Sues said that the idea of Paleocene dinosaurs can't yet be dismissed.
"There is no a priori reason that dinosaurs could not have survived in some places," he wrote in an email to National Geographic News.
"Indeed, other than in the [U.S.] western interior and in Europe, we have as yet no concrete evidence when dinosaurs vanished."
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