Palaeontologists have fired a new salvo in a 30-year-old war of words over the theory that the dinosaurs were wiped out by a space rock that smashed into Earth some 65 million years ago.
The hypothesis holds that an asteroid or comet whacked into the modern-day Yucatan peninsula in Mexico, igniting massive fires.
Billions of tonnes of wind-borne ash and dust filtered out light from the Sun, triggering a "nuclear winter" that cooled the planet and withered the vegetation on which the dinosaurs directly or indirectly depended.
After a reign that had lasted more than 150 million years, smaller creatures able to survive in tougher circumstances inherited the planet, notably mammals and the dinosaurs' avian offshoots, birds.
Not so, declare naysayers.
They do not contest the evidence for the Cretaceous-Tertiary impact but instead contend that dinosaurs had already petered out long before the cosmic smashup.
Their argument for this is a thick layer of sediment in the US states of eastern Montana and western North Dakota which is a treasure trove of dinosaur fossils.
A three-metre (10-feet) layer deposited before the Cretaceous-Tertiary boundary is devoid of dinosaur fossils, they say.
This means the dinosaurs must have gradually become extinct long before the impact, possibly through climate change or shifting sea levels, they suggest.
The issue became enshrined in palaeontological debate as "the three-metre gap."
But scientists reporting on Wednesday in Biology Letters, a journal of Britain's Royal Society say they now have found solid evidence to rebut the revisionists.
A team led by Tyler Lyson of Yale University has discovered a 45-centimetre (20-inch) brow horn from a ceratops dinosaur located just 13 centimetres (five inches) below the line where the Cretaceous-Tertiary sediment first begins.
They found it in the Hell Creek formation of southeastern Montana, at a hill known as Camel Butte.
"Discovery of this dinosaur locality demonstrates that a Cretaceous 'three-metre [10-feet] gap' does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the ... impact event," the study says.
Lyson admits, though, that it is intriguing that no fossils at all were found in a 125-centimetre (50-inch) -deep layer of sediment deposited after the big smash and calls for work to explain the mystery.

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