And now, another episode of "Dinosaur Survivor."
In this show, the question isn't which dinosaur to throw off the island. Instead, scientists ask whether any of the ancient reptiles survived the cataclysmic strike of a space rock in the Gulf of Mexico some 65 million years ago.
Representing the no team: Pretty much every dinosaur hunter in the world.
Representing the yes team: A retired federal geologist from New Mexico, James Fassett.
For 25 years, Fassett has been touting a fossilized femur he found as proof that a pocket of long-necked herbivores called sauropods survived for hundreds of thousands of years after all the other dinosaurs.
"I'm not totally a Lone Ranger," Fassett said of his theory. "But I guess I am still in the minority."
In the latest installment of this long-running series, Fassett and two colleagues report in the journal Geology that a new technique dates the femur to 700,000 years after the extinction event.
But few experts are buying it. One of Fassett's critics offered a sarcastic response. "Anything is possible," said Spencer Lucas, a paleontologist at the New Mexico Museum of Natural History and Science in Albuquerque. "There could also be a Bigfoot in my back yard."
With the new dating technique, Larry Heaman and Antonio Simonetti from the University of Alberta in Edmonton vaporized tiny bits of Fassett's fossil with a laser. They then measured the amount of uranium and lead in the resulting dust. Because uranium radioactively decays into lead over millions of years, the process acts as an atomic clock.
If proved, the laser technique could revolutionize fossil dating, said Paul Renne, director of the nonprofit Berkeley Geochronology Center in California. Currently, paleontologists date fossils indirectly, by determining the age of the rocks in which they're found or by hunting for specks of fossilized pollen nearby, which also offer strong age clues. In contrast, the laser blasting method attempts to date fossils directly.
However, Renne and several other fossil-dating experts said the technique is too new to be reliable. "Uranium-lead dating is tricky business," said Alan Koenig, a rock-dating expert with the U.S. Geological Survey in Denver.
A primary concern: It is impossible to know when, exactly, uranium leached into the bone. After the sauropod died in what is now northern New Mexico, the calcium in its bones was eventually replaced by harder, longer-lasting minerals, including uranium. That's the fossilization process. But paleontologists say there is no way to know how long this might take. "It could be 10 years; it could be a million," Renne said.
The laser method, then, provides some indication of when uranium entered the fossil. It does not pinpoint when the animal died. To further complicate matters, uranium may have leached into the fossil multiple times as, say, floods separated by millions of years washed over it.
Heaman said he tried to account for this by laser blasting only "pristine" areas of the fossil. For instance, he avoided sections near cracks, which might have allowed newer uranium to leach in.
The technique yielded an estimated age of 64.8 million years, give or take 900,000 years. That range straddles the so-called K/T boundary, the geological flash point that marks the end of the age of dinosaurs. Fassett said that other data he has collected and published prove that the fossil is younger than the K/T boundary. In particular, he said that fossilized pollen from the Ojo Alamo sandstone formation near the fossil could only have come from an era after the K/T boundary.
To prove it, about 10 years ago, Fassett took Lucas and another skeptical paleontologist, Robert Sullivan of the State Museum of Pennsylvania in Harrisburg, to the fossil site and pointed to where he sampled the pollen. The two paleontologists collected their own sample and tested it. But they obtained different results, concluding that the pollen was not, in fact, as young as Fassett had claimed. In 2009, Lucas and Sullivan published this and other evidence rebutting Fassett's theory.
Fassett's new paper fails to address the rebuttal. Lucas went so far as to call it "bad science." In the past 20 years, other prospects of survivor dinosaurs in Montana, South America and China have failed to hold up under scrutiny.
As for how the erstwhile survivors might have marched on, Fassett said, "All I can do is guess." One idea: Sauropods buried their eggs immediately before the asteroid impact; months later, the youngsters hatched into a devastated world to start a new herd. Or, perhaps some sauropods survived far from the blast - in Alaska, where sauropod fossils have been found - and later migrated south.
Whatever the case, expect more episodes of "Dinosaur Survivor."
Even Lucas called the search for survivors worthwhile. "The world is a big place, so why couldn't there be a refugium where a few dinosaurs limped on?" he said. "But you're going to need really strong evidence."