Scientists have long disagreed over what caused the mass extinction of dinosaurs on Earth. One hypothesis is that the prehistoric animals were killed after an asteroid hit Mexico, 65 million years ago. Others believe that the dinosaurs were already extinct, or gradually declining, at that time.
Now, a newly-found dinosaur fossil is the strongest evidence to date that dinosaurs were still roaming the earth when the meteorite hit.
Critics of the asteroid impact hypothesis have pointed to the lack of dinosaur fossils right below the Cretaceous-Tertiary boundary, the layer of the earth thought to be formed after the asteroid hit the planet. If the dinosaurs died during the massive impact, why hadn't paleontologists found any bones in the zone right below the Cretaceous-Tertiary boundary?
Now, they have. Researchers from Yale University discovered a triceratops horn a mere five inches below the Cretaceous-Tertiary boundary, in the Hell Creek formation in Montana.
"The fact that this specimen was so close to the boundary indicates that at least some dinosaurs were doing fine right up until the impact," said Yale graduate student Tyler Lyson, lead author of the study.
It's been a big year for dinosaur discoveries. In February, scientists reported finding evidence of a new species which they named Brontomerus mcintoshi -- in Greek, "thunder thighs" -- on account of its powerful legs.
And in April, scientists reported finding fossils from a "sharp-toothed beast" that likely lived in the Western U.S. more than 200 million years ago.
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