The 45cm-long fossilised browhorn belonged to a family of plant-eating dinosaurs that included the famous three-horned Triceratops.
It was found at a geological site known as the Hell Creek Formation in the bleak badlands of south-east Montana, US, where many other dinosaur fossils have been unearthed.
What made this one special was its location, just 13cm below the rock layer that marks the Cretaceous-Tertiary or "K-T" boundary – the point in the fossil record where the dinosaurs died.
This suggests dinosaurs were around right up to the time all traces of their existence vanished.
In other words, they disappeared suddenly, as the result of an abrupt global disaster rather than a slow extinction.
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A huge asteroid or comet smashing into the Earth off the coast of Mexico at the end of the Cretaceous Period 65 million years ago is widely believed to have killed off the dinosaurs.
But some sceptics point to an absence of dinosaur fossils for three million years leading up to the impact as evidence that the creatures may have already gone when the meteor struck.
The "three million gap" has helped drive controversy over what happened to the dinosaurs, some of which evolved into birds.
However, the horn fossil appears to close the gap, according to scientists writing in the Royal Society journal Biology Letters.
The team, led by Dr Tyler Lyson, from Yale University in New Haven, US, wrote: "The in situ specimen demonstrates that a gap devoid of non-avian (bird) dinosaur fossils does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the K-T boundary impact event."
The scientists pointed out that a 125cm section of rock strata laid down after the impact was completely devoid of fossils.
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