Scientists have long debated how the dinosaurs met their end. Did a massive meteorite wipe them out quickly, or did the Age of the Dinosaurs end gradually? A new finding might settle the argument for good. Here, a brief guide:

What was this crucial discovery?
A single fossil — an 18-inch-long horn that most likely belonged to a plant-eating triceratops. It was found at a geological site, the Hell Creek Formation in the badlands of southeast Montana, where many other dinosaur fossils have been unearthed. Triceratops fossils are the most common in the area where this horn was found. 

What's so special about this fossil?
It was found just five inches below the rock layer marking the Cretaceous-Tertiary, or "K-T," boundary, the point where the dinosaur fossil record ends. Many paleontologists believe that's when, 65 million years ago, a catastrophic meteor strike smashed into Earth off the coast of Mexico and wiped out the dinosaurs. But scientists have been unable to find any fossils within 10 feet of the K-T boundary. That represents a time gap of 3 million years. Some scientists have cited this gap as evidence that the dinosaurs had been dying off gradually for ages before the meteor hit, due to catastrophic volcano eruptions, or other factors. Now it will be harder to support their case.

So does this settle it — dinosaurs died off all at once?
It makes things more clear, but this lone fossil won't be enough to end all debate. Finding "one dinosaur in the gap doesn't necessarily falsify the idea that dinosaurs were gradually declining in numbers," says researcher Tyler Lyson, a vertebrate paleontologist at Yale University, as quoted by LiveScience. "However, this find indicates that at least some dinosaurs were doing fine right up to the K-T boundary."

Sources: Christian Science Monitor, io9, LiveScienceTelegraph