A new fossil discovery in the United States has reinforced the commonly accepted but hotly debated theory that dinosaurs were wiped out when an asteroid struck the Earth some 65 million years ago.
Some paleontologists believe the prehistoric giants died out well before the space rock slammed into modern-day Yucatan peninsula, Mexico, leaving school children around the world baffled about what to believe.
The skeptics point to the absence of dinosaur fossils in a sediment layer deposited before the Cretaceous-Tertiary impact boundary in eastern Montana and western North Dakota, an area that is abundant with dinosaur impressions.
The doubters argue that the so-called the "three-meter (10-feet) gap" is evidence that the oversized vertebrates were long gone by the time the space rock arrived.
But the asteroid theory strikes back in a report published Wednesday in Biology Letters, a journal of Britain's Royal Society, according to AFP news agency.
Scientists led by Yale University's Tyler Lyson, a veteran fossil hunter in the Montana area, say they have found a smoking gun in the form of a dinosaur horn 13 centimeters (five inches) below the Cretaceous-Tertiary sediment line.
The 45-centimeter (20-inch) brow horn from a ceratops dinosaur was plucked from the dirt at Hell Creek in southeastern Montana, at a hill called Camel Butte.
"Discovery of this dinosaur locality demonstrates that a Cretaceous 'three-meter gap' does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the ... impact event," the study says.
The discovery supports the 30-year-old theory that a massive asteroid, possibly 160 kilometers (99 miles) wide, slammed into Mexico and triggered a cataclysm that changed the Earth's climate and ended the dinosaurs' 150-million-year reign as the world's dominant beasts.