Scientists have assembled an open-and-shut case that a later extraterrestrial impact near the Yucatan peninsula was the cause of the mass extinction that snuffed out the dinosaurs 65 million years ago and paved the way for mammals to take center stage.
But the smoking gun -- or, in this case, the smoking crater -- is still lacking in the earlier Triassic-Jurassic crime.
"I'm convinced that something very dramatic happened," said paleontologist Hans-Dieter Sues of the Royal Ontario Museum in Toronto, a leading proponent of the impact theory.
While scientists try to solve what happened at the end of the Triassic, they already have determined how it began 250 million years ago. Evidence unveiled in February showed that a giant hunk of space rock slammed into the planet to end the Permian period, causing the biggest of the five major mass extinctions in the Earth's history and ushering in a time of remarkable evolutionary experimentation in the Triassic.
About 230 million years ago, the first dinosaurs arose from earlier reptiles. But there were many other types of animals competing with the dinosaurs. There were other forms of reptiles, including giant, semiaquatic crocodile-like phytosaurs such as Leptosuchus that were among the dominant carnivores of their time. There were the rauisuchians such as Postosuchus, a fearsome bipedal predator.
Reptilian herbivores included aetosaurs such as Desmatosuchus, an armored quadruped, and the somewhat cow-like therapsid Placerias.
All of these were knocked out at the end of the Triassic. The situation in the seas was even more devastating. Multitudes of marine creatures disappeared. Clams and corals were clobbered. And many species of land plants also were lost.
"The Triassic-Jurassic boundary wiped out the competitors to the dinosaurs. It's only after the boundary that you get a dinosaur-dominated ecosystem,'' said Columbia University paleontologist Paul Olsen, a leading expert on dinosaurs of this period. "It's really quite a dramatic change.''
The dinosaurs won the lottery, this time
"Whatever happened didn't affect the dinosaurs,'' Sues said. "If an impact was responsible for the Triassic-Jurassic extinction, as I believe, the strange thing is that at the end of the Cretaceous, when we have another mega-impact, the dinosaurs go out. It makes a point that there's a lot of randomness to this -- sort of like a giant lottery.''
Most of the dinosaurs roaming the Earth during the late Triassic were not the giants that emerged in the subsequent Jurassic and Cretaceous periods, with one exception. Plateosaurus, a long-necked, bipedal herbivore, reached lengths of about 20 feet.
More typical of the early dinosaurs was Coelophysis, a small bipedal hunter that was a precursor to much-later meat-eating giants such as Allosaurus and Tyrannosaurus.
Olsen noted that the forms of dinosaurs with which many people are familiar -- creatures like Brontosaurus (formally known as Apatosaurus), Allosaurus and Stegosaurus -- did not develop until after the competition had been eliminated.
A long-standing theory was that gradual changes in climate and sea levels spelled doom for many species over a period of millions of years. But experts said that theory is contradicted by new evidence showing that the extinction came suddenly.
University of Washington paleontologist Peter Ward said carbon isotope evidence found in rocks from the Queen Charlotte Islands off Canada's British Columbia coast demonstrated a swift collapse in marine plankton populations at the Triassic-Jurassic boundary.
"This thing was real fast,'' Ward said.
Other theories include massive volcanic activity that released gases into the atmosphere that caused intense global climate changes, and the explosion of a nearby star (a supernova) so intense that it blew away the Earth's protective ozone layer. But many experts feel the evidence points to a massive impact that triggered a global ecological crisis.
The smoking crater?
Scientists who favor the impact theory suspect that a humongous Canadian crater site in Quebec called the Manicouagan structure very well may be where the comet or asteroid landed.
But the most recent efforts to assign a date to the crater determined that the impact took place 214 million years ago -- 14 million years too early.
Sues said the antiquity of the rock at the impact site may have fooled the dating techniques used by scientists by providing a mix of different kinds of radioactive signatures.
"As the impact happened at Manicouagan, it happened into some of the world's oldest known rocks,'' Sues said.
"So as this rock gets molten, it may give you an artificially skewed date -- either too young or too old -- because there was so much radioactive material already in this more than two billion-year-old rock. The dates get all screwed up."
"We have to identify rocks that are the actual impact meld, the stuff that bubbled up seconds after the gigantic object hit. And it's very difficult to recognize that in the field."
Olsen noted that nothing unusual takes place in the fossil record at 214 million years ago -- not even a mild die-off. "So it's a bit mysterious that you would have this giant impact and have absolutely nothing going on," Olsen said.
Another vital clue is known as shocked quartz -- distinctive crystals of the rock quartz containing fractures caused by a huge amount of pressure exerted over a small period of time. The only places where shocked quartz has been found are at known extraterrestrial impact sites, nuclear test sites and in high-pressure laboratory experiments.
"Shocked quartz is evidence of an impact,'' said Smithsonian Institution paleontologist Michael Brett-Surman.
Scientists have searched for shocked quartz in rocks dating to the Triassic-Jurassic boundary. Unconfirmed traces of it have been found in Italy. Ward said experts are analyzing the rock from the Queen Charlotte Islands for shocked quartz.
Olsen said another piece of evidence supporting the impact theory is the high surge in fern pollen and spores in the fossil record at the Triassic-Jurassic boundary. Because ferns can repopulate quickly after a calamity, this suggests that they covered the land after an impact that may have decimated forests over a large area.
So, the detectives have amassed a lot of clues, but the culprit in this mass killing officially remains at large.
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