What must have been a grim day some 130 million years ago has proven to be a boon for today's paleontologists with the discovery of the remains from hundreds, perhaps thousands, of specimens of a bizarre new species of dinosaur in Utah.
Scientists aren't sure what may have caused the deaths of these feathered, rotund, sickle-clawed creatures called Falcarius utahensis. But the resulting jumble of fossilized bones in a two-acre area of Utah's Cedar Mountain Formation offers a rich sample of a range of individuals from a unique group once thought to be mostly limited to Asia.
An analysis of one of the dinosaurs also suggests this herd may represent a snapshot of evolution in action as the animal switched from being mainly meat-eaters to plant-eaters.
"There is a giant hillside full of the fossils and we've only started to open a small portion of it," said Lindsay Zanno, a doctoral student at the University of Utah's Utah Museum of Natural History and a co-author of a study about the dinosaur in this week's issue of Nature. "It's likely to be one of the best dinosaur quarries in the world."
Members of a Weird Family
The paleontologists speculate that a water source contaminated by bacteria from a rotting corpse or a sudden release of subterranean gas may have snuffed out the herd. But confirming either of the theories may prove impossible now.
The fact that so many remains were found in one place, however, bolsters the theory that these animals, which had descended from carnivores, had switched their diets to mainly plants.
"Carnivores generally don't live in large packs because there isn't enough meat to go around," said Tom Holtz, a paleontologist at the University of Maryland in College Park who did not take part in the study. "Whereas if you're a plant eater living in a herd, it's to your advantage because there are lots of other eyes around looking out for you."
The newly found species belongs to a group of dinosaurs, called therizinosaurs, that paleontologists have described as "stranger than strange."
This dinosaur group has an unusual mix of characteristics, from the long, hollow neck bones of plant eaters like brontesaurae, to the wide hips of so-called bird-hipped dinosaurs to the comparatively large brain cases associated with advanced meat-eaters. Its name is derived from the word sickle, which scientists have used to describe its unwieldy clawed hands. Less-primitive specimens found in Asia have revealed the animal was also covered in shaggy feathers.
"They're like dinosaurs designed by committee," said Holtz. "They have bits and pieces that are similar to a whole range of species so they look like a weird amalgam of dinosaur groups."
This particular species featured 4-inch claws on its hands, measured 13 feet from snout to tail and stood just over 4 feet tall, according to the lead investigator and Utah's state paleontologist, James Kirkland.
The individuals uncovered so far in Utah are even more unique members of the therizinosaur family because they have the designs of a creature that ate both plants and meat. Kirkland explains this switch would have made sense since flowering plants had begun to flourish at the time.
The Utah versions had leaf-shaped teeth that would have worked well for chewing plants and a broad pelvis that could have accommodated the wider gut needed for digesting plants. Still, its thighs were slightly longer than the later, strictly plant-eating members of its family, suggesting it was also capable of chasing after the occasional small lizard for a bit of protein.
As for its spindly claws, Kirkland says "they probably used those like pitchforks for defense" rather than for ripping apart prey.
"Falacarius represents evolution caught in the act, a primitive form that shares much in common with its carnivorous kin, while possessing a variety of features demonstrating that it had embarked on the path toward more advanced plant-eating forms," said Scott Sampson, chief curator at the Utah Museum of Natural History and another co-author of the study.
Reformed Fossil Thief Made Discovery
This is only the second therizinosaur species to be found in North America. The other was a much later version -- only 90 million years old -- which Kirkland discovered in the late 1990s in New Mexico. Because it dated to the Late Cretaceous period, Kirkland and others had concluded that the older therizinosaurs in China had migrated over a land bridge from Asia through Alaska.
But 130 million years ago -- when the Utah creature was alive and well -- no such Alaska land bridge existed. This means the dinosaur could have been more widespread than scientists realized or that it migrated over Europe to North America, or it may have originated in what is now the American Southwest.
"I think the find is highly significant," said David Gillette, curator of paleontology at the Museum of Northern Arizona and a geologist at Northern Arizona University in Flagstaff. "The fact that we have Early Cretaceous specimens in North America throws open the question of where they originated."
These kinds of insights into the strange group of dinosaurs would never have come to light, if not for the latent conscience of a fossil thief.
A prospector from Moab, Utah, was the first to come across the find in the late 1990s while looking for ore and minerals in the desert. Even though he was on federal land, the prospector didn't immediately tell authorities about the cache, but excavated them and began selling material from the site.
Pretty soon, he realized the bones were highly unusual for the area. That's when he notified the state.
"He turned the material over to me and I said 'I won't go out of my way to go after you,' " Kirkland recalled. "But the stuff was coming into the market illegally and the Bureau of Land Management was already aware."
When asked to give a deposition about the prospector's actions, Kirkland agreed and the man was convicted, served five months in jail and paid a $15,000 fine.
"He knew this was possible and was willing to face the consequences. I was impressed he was willing to put his head on the chopping block," said Kirkland. "The guy is not evil, but he was no angel, either."
Kirkland adds that there are likely all kinds of dinosaur fossils that never make it into the scientific record because private bone hunters dig them up and then sell them for profit. But as this recent discovery shows, much remains to be found and revealed -- even in this country -- about what kinds of dinosaurs were among this planet's earliest residents.