Could dinosaurs have survived asteroid?

The demise of the dinosaurs 65 million years ago has been unsuccessfully pegged to everything from poisoning and constipation to slipped disks and infertility. One of the most engaging ideas currently in vogue is that the dinosaurs were completely wiped out within a year or so after an asteroid slammed into the earth, spewing dust into the atmosphere that altered the climate and killed plants by blocking out the sun (SN: 6/2/79, p. 356). Now a paleontologist at the University of Norte Dame (Ind.) says he has evidence that challenges this theory too.

Keith Rigby and his colleagues recently discovered the bones of dinosaurs that they believe lived at least 40,000 to 200,000 years into the Paleocene epoch after the asteroid impact. The bones were found in streambed relics in the Hell Creek rock formation of east-central Montana.

The researchers do think there was an asteroid and they acknowledge its role in drastically reducing the number of dinosaurs; by their own reckoning the impact killed off at least 70 percent of these animals. But the asteroid did not deliver the final deathblow, they say. Their studies indicate that of the 13 dinosaur species living prior to the asteroid, 11 survived the impact. Moreover, the dinosaurs were already in decline before the asteroid hit. "The asteroid was the icing on a cake that had already been baked," says Rigby.

Rigby's group is not the first to suggest that dinosaurs lived beyond the crash. A few years ago, Jim Fassett, a stratigrapher at the U.S. Geological Survey in Reston, Va., reported what he believes are Paleocene dinosaur remains in New Mexico, which may have been left as much as 4 million years after the impact. Similar findings have been made in other places. And the impact theory has also been challenged on other grounds (SN: 8/31/85, p. 135). But none of these groups has been able to convince the scientific community of the validity of their claims. "This is the first time that there has been conclusive evidence that the dinosaurs did in fact survive the impact," says Rigby.

Rigby's findings have yet to be reviewed by other scientists because his group is just now submitting its complete results to SCIENCE. Fassett suspects that Rigby's most difficult problem will be to prove that the bones were not reworked--that is, not left during the Cretaceous period before the impact and then dislodged and reburied during the Paleocene. But Rigby says he has many pieces of evidence -- such as dinosaur teeth with sharp edges that would have been worn down by extensive reworking -- that support Paleocene origins.

Another uncertainty rests with the group's dating on bones, which was based on estimates of such inexact factors as sedimentary rates and changes in river flow patterns. Rigby notes that the dinosaurs could have lived even longer than his projections because the fossil record abruptly stops at a layer in which stream channels, the vehicle for collecting bones in high concentrations, disappear as the climate changes.

If Rigby's group is correct, then the question remains why the dinosaurs faced extinction while other species flourished. Rigby suggests that the dinosaurs' last days were spent along the banks of a Nile-like river with resources too limited to satisfy their greater needs. But the answer to "whodunit" must await the writing of the final chapter of this mystery.