By Zoë Corbyn of Nature magazine

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 paleontologists 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.

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 favor a gradual decline disagree.

The fossil--a 45-centimetre-long brow horn of what is either a Triceratops 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 paleontology student at Yale University in New Haven, Connecticut, and lead author of the paper published today in the Royal Society journal Biology Letters. "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.

Boundary blues

Kirk Johnson, a paleontologist at the Denver Museum of Nature and Science in Colorado, disagrees, arguing the fossil is "another nail in the coffin" for the gradual extinction theory. He is one of 41 authors of a review paper published last year that supports the sudden extinction model. "Everything we see is consistent with no diversity change," he says.

But Lyson and his colleagues saw only some of the typical markers of the K/Pg boundary in their rocks--a decrease in plant pollen and a spike in ferns, but not the spike in iridium or shocked quartz and glass spherules that are indicative of an asteroid strike. "A skeptic could argue they don't have the K/Pg boundary itself," Johnson says. The missing markers were correlated with the rock based on a nearby section.

Lyson says that more fossils from other dinosaur species will need to be found in the 3-metre gap before it is possible to say conclusively whether their extinction was gradual or catastrophic. However, he thinks that the find "does indicate" that at least some dinosaurs were "doing fine" right up to the point at which the asteroid struck. "The gap is artificial, and the more we sample the upper Cretaceous the more we are finding that," he says.

This article is reproduced with permission from the magazine Nature. The article was first published on July 13, 2011.