Published online 13 July 2011 | Nature | doi:10.1038/news.2011.411
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The youngest dinosaur fragment yet?Scientists dispute fossil's significance to the extinction debate.
Zoë Corbyn
The discovery of what could be the youngest fossil of a dinosaur to date — from a period notorious for being free of their remains — has reignited a clash among palaeontologists about what caused the animals' extinction.
It is generally accepted that the enormous asteroid that slammed into Mexico's Yucatan Peninsula some 65 million years ago led to a mass extinction. But debate has raged about whether that included non-avian dinosaurs or whether they were already on the decline due to climate change, sea-level change or volcanic activity, with the impact merely providing the final blow.
Did the dinosaurs die off slowly from many factors, or abruptly in an asteroid strike? Mark Hallett
Those who found the fossil say it further strengthens the theory that the age of the dinosaurs ended abruptly following a sudden asteroid impact. But those who favour a gradual decline disagree.
The fossil — a 45-centimetre-long brow horn of what is either aTriceratops or a Torosaurus — was found in the Hell Creek Formation in southeastern Montana, just 13 cm below the Cretaceous–Paleogene(K/Pg) geological boundary — formerly known as the Cretaceous-Tertiary (K/T) boundary — a visible line of rock rich in the element iridium, which marks the geological timepoint at which the asteroid struck.
Mind the gap
The specimen's proximity to the K/Pg boundary not only makes it what is thought to be the youngest dinosaur fossil ever found, but also places it in the upper reaches of a roughly 3-metre zone of rock below the K/Pg boundary — the '3-metre gap'. This zone is known for its dearth of dinosaur fossils and has been used in the past to fuel the gradual decline theory.
"From this specimen we can say there were dinosaurs pretty much all the way up to the impact," says Tyler Lyson, a palaeontology student at Yale University in New Haven, Connecticut, and lead author of the paper published today in the Royal Society journalBiology Letters1. "It indicates that the impact was the likely cause of the extinction, and it was not gradual."
But David Archibald, a dinosaur expert at San Diego State University in California who argues for a "multiple causes" theory, says that the find changes nothing. "The basic error of the authors is the belief that finding one fragment of dinosaur suddenly makes this gap go away [when] it does not," he says.
He adds that the "paucity of fossil material" means it is impossible to use the gap to argue anything about whether the dinosaurs were declining or not before the asteroid impact. He also points out that the fossil might have been carried to the unusual location by natural processes — something that the paper deems unlikely.
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