The great splat of an asteroid that might have wiped out the dinosaurs apparently didn't get all of them. New fossil evidence suggests some dinosaurs survived for up to half a million years after the impact in remote parts of New Mexico and Colorado.
The whole idea that a space rock destroyed the dinosaurs has become controversial in recent years. Many scientists now suspect other factors were involved, from increased volcanic activity to a changing climate. Either way, some 70 percent of life on Earth perished, and an asteroid impact almost surely played a role.
Scientists recently analyzed dinosaur bones found in the Ojo Alamo Sandstone in the San Juan Basin. Based on detailed chemical investigations of the bones, and evidence for the age of the rocks in which they are found, the researchers think some dinosaurs outlived the crash that occurred 65 million years ago and stuck around for a while.
"This is a controversial conclusion, and many paleontologists will remain skeptical," said David Polly, one of the editors of the journal Palaeontologia Electronica, in which the research was published today.
Lead researcher Jim Fassett of the U. S. Geological Survey in Santa Fe, New Mexico went to great lengths to establish when the bones originated.
"The great difficulty with this hypothesis — that these are the remains of dinosaurs that survived — is ruling out the possibility that the bones date from before the extinction," he said. "After being killed and deposited in sands and muds, it is possible for bones to be exhumed by rivers and then incorporated into younger rocks."
To try to eliminate that scenario, Fassett investigated the rocks surrounding the bones and studied date indicators, such as their magnetic polarity. He said the evidence "independently indicate[s] that they do indeed post-date the extinction."
He also found that the dinosaur bones from the Ojo Alamo Sandstone have distinctly different concentrations of rare earth metal elements than the deeper, older rocks that date from the time of the impact. This suggests that it's unlikely the bones originated in that older rock and were somehow relocated to the more recent, higher level of sediment.
Another piece of evidence seems to support the claim, too. The fossil remains include a group of 34 hadrosaur bones lying together, which Fassett said are "doubtless from a single animal." If the bones had been exhumed from the older rock by a river, they would have likely been scattered in several locations, and wouldn't be clustered together as they are.
Even if the dinosaur bones do turn out to belong to disaster survivors, there probably were very few of them compared to their population before the crash.
"One thing is certain," Polly said. "If dinosaurs did survive, they were not as widespread as they were before the end of the Cretaceous and did not persist for long."