A fossil discovered in Montana has given new momentum to the hypothesis that dinosaurs were thriving right up until a devastating meteor hit Earth 65 million years ago, causing their extinction.

Scientists from Yale University have found what is believed to be the youngest dinosaur fossil ever found, thought to be from just before the mass extinction took place.

The discovery, described in a study published in the online edition of the journal Biology Letters, contradicts the theory that the dinosaurs slowly went extinct before the cosmic impact.

The fossil -- a 45-centimetre horn believed to be from a triceratops -- was found in Montana's Hell Creek formation. It was located just below the K-T boundary, the band of the Earth's crust that represents the time period in which the meteor struck.

One of the main problems with the meteor theory has been the lack of any non-avian dinosaur fossils buried within 10 feet of the boundary -- known as the 'three metre gap.'

The absence of fossils, some paleontologists say, indicates dinosaurs were already extinct when the cosmic impact occurred.

Yale paleontologist Tyler Lyson, lead author of the study, says the new discovery proves otherwise.

"To all of our surprise the boundary was no more than 13 centimetres above this horn, and the significance is this indicates that at least some dinosaurs were doing quite well in this locale at the time of the meteor impact," he told CTV.ca.

There is evidence that avian dinosaurs thrived up to and into the K-T boundary. In fact, they are believed to have survived the meteor and evolved into modern-day birds.

But until now there has been no sign of non-avian dinosaurs, such as tyrannosaurus rex, triceratops, torosaurus or duckbilled dinosaurs anywhere even close to the boundary.

The new finding suggests the three-metre gap doesn't exist, Lyson said. Instead, he's now referring to the "13-centimetre gap."

The dinosaur that the horn came from is believed to have lived between "tens of thousands of years to just a few thousand years before the impact," but Lyson said the team is unable to determine a specific age.

Reached at the dig site in an insolated part of Montana, he said the team's goal now is to find additional fossils, ideally from a "diversity" of species, also within close proximity to the K-T boundary.

"We've found several candidates right now but we have to do our homework, we have to do the full analysis and unfortunately that takes time," he said.