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."
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.