Lost World of Dinosaurs Survived Mass Extinction?

An isolated group of dinosaurs somehow survived the catastrophic event that wiped out most of their kind some 65.5 million years ago, a new study suggests.

Dinosaurs of this "lost world," in a remote region of the U.S. West, may have outlived their doomed relatives by as much as half a million years, according to James Fassett, an emeritus scientist with the U.S. Geological Survey in Santa Fe, New Mexico.

Fassett, who has argued for years that some dinosaurs survived the mass extinction, based his latest work on fossils from the San Juan Basin in what is now Colorado and New Mexico.

There, the bones of hadrosaurs, tyrannosaurs, anklyosaurs, and several other species were found together in a sandstone formation that dates to the Paleocene epoch—the time period after the so-called Cretaceous-Tertiary (K-T) extinction event, which is thought to have killed off the dinosaurs.

As with his past research, Fassett's latest find is likely to continue sparking controversy among paleontologists.

"Every few years someone claims to have [found] Paleocene 'surviving' dinosaurs," said Hans-Dieter Sues, associate director for research and collections at the Smithsonian Institution's National Museum of Natural History.

But so far, such fossils have eventually turned out to be older remains.

• "Unequivocal Evidence"

In his new study, appearing in the April 2009 issue of the journal Palaeontologia Electronica, Fassett argues that a single hadrosaur fossil helps prove that the San Juan dinos really are from the Paleocene.

After previous "survivor" finds, it was determined that the dinosaurs in question, initially entombed in sand or mud, had their bones exposed again later by natural forces such as river erosion.

The bones were then redeposited in younger rock layers, making them appear to belong to an earlier era.

But paleontologists found a concentration of 34 bones from a single hadrosaur in the San Juan Basin sandstone.

"That's unequivocal evidence I think," Fassett said. River-washed bones would be widely scattered and also show signs of wear and tear—unlike the current fossils, some of which he describes as "pristine."

Working with colleagues at the USGS in Denver, Fassett also examined the concentrations of uranium and rare-earth metals in the fossil bones.

"I thought if we could determine the trace-element compositions of the bones, we might discover that the [older] Cretaceous bones had a different chemical fingerprint than the [younger Paleocene] bones do," he said, "and indeed that turned out to be the case."

• No Reason Why Not

It's not known why some species, including crocodiles and birds, survived the K-T event while many others did not. The answer could be tied to what exactly caused the mass extinction.

The popular theory is that a killer asteroid struck the Yucatán Peninsula, although experts have argued for massive volcanism, disease, climate change, or some combination of factors.

(Related: "'Dinosaur Killer' Asteroid Only One Part of New Quadruple Whammy Theory.")

Fassett, who supports the asteroid-strike theory, said he can't explain why dinosaurs may have survived longer in some areas but not others.

"One guess is that the survivors lived in the northernmost parts of North America, at the greatest distance from the impact site, and then migrated south," he said.

"But that doesn't explain why [dinosaurs that lived later] haven't been found elsewhere. We don't have an answer for that."

Despite his caution, the Smithsonian's Sues said that the idea of Paleocene dinosaurs can't yet be dismissed.

"There is no a priori reason that dinosaurs could not have survived in some places," he wrote in an email to National Geographic News.

"Indeed, other than in the [U.S.] western interior and in Europe, we have as yet no concrete evidence when dinosaurs vanished."

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