Paleontologists believe the first dinosaur was a small-bodied, lightly built, fleet-footed predator.
Early on in dinosaur evolution, two major groups shifted to plant-based diets. But the fossil record of how dinosaurs went vegetarian, until now, was scant. Also, the plant-eaters' relationship to their meat-eating ancestors was unknown.
Thomas Holtz is a paleontologist at the University of Maryland in College Park. He said the Falcarius fossils are a clearer window into the shift from meat eating to plant eating than is available for equivalent transitions within other dinosaur groups. Prior to the Falcarius discovery, the likely intermediate forms between meat eaters and plant eaters "are either not yet discovered or are very fragmentary and so not recognized as such," Holtz said. That's what makes Falcarius so rare.
The Falcarius fossils show this transition in action among a group of dinosaurs, the birdlike meat-eating dinosaurs of the late Cretaceous period (about 140 million years ago). The finding allows paleontologists insight into long-nagging questions of evolution.
"If you're going to take a Velociraptor-type dinosaur and make a large-bodied, lumbering, bizarre-looking plant eater, what is it you need to do? What parts of the body need to change and how?" Zanno said.
Among the evidence for a dietary transition are teeth that look like tiny birch or elm leaves, a shape good for shredding plant material and shared with all other plant-eating dinosaurs, Kirkland said. In addition, the pelvis bone is expanded toward the side, a sign consistent with the larger gut size required to digest leafy greens.
"To digest plants is more difficult than to digest meat," Kirkland said. "Plant eaters have big digestive systems to process plant material."
Later therizinosaurs, Zanno said, had shorter tails that balanced their more upright posture for eating leaves off trees. They also had shorter legs that better supported their heavier frames and shoulder joints that allowed the arms to reach branches above the head.
But why go vegetarian? Kirkland said paleontologists can only speculate. But the dietary shift represented by Falcarius coincides with the appearance of the first flowering plants in the fossil record, she said.
Paleontologists working in China and Mongolia have been excavating therizinosaurs for about 50 years, accounting for a dozen species. Then in the late 1990s Kirkland, with Douglas Wolfe of Arizona's Mesa Southwest Museum, discovered the first recognized therizinosaur in North America, a species named Nothronychus.
Nothronychus, which was found in New Mexico, was dated to about 90 million years agosignificantly younger than the oldest therizinosaurs from Asia. Paleontologists reasoned that the group originated in Asia and used a land bridge between Alaska and Siberia to populate North America.
Falcarius, however, is 125 million years old, as old as Beipiaosaurus, the oldest known therizinosaur from Asia, which was preserved with feathers. According to Zanno, Falcarius is also more primitive than Beipiaosaurus. Also, some evidence suggests that 125 million years ago the land bridge with Alaska did not exist.
"We have to change our thinking," Zanno said. "It's possible [that therizinosaurs] may have evolved in North America and spread through Europe to Asia."
According to Kirkland, "anywhere in the whole Northern Hemisphere is up for grabs, and Europe is clearly the migratory route versus Alaska. People have a hard time remembering Alaska didn't exist" 125 million years ago.
Holtz agreed that this discovery suggests that early therizinosaurs migrated through Europe instead of over the land bridge between Alaska and Siberia. "I would suspect that Falcarius, or a dinosaur very similar to it, might one day be found in England," he said.
Kirkland and colleagues have excavated more than 1,700 Falcarius fossils at a previously overlooked, 2-acre (0.8-hectare) site at the base of the Cedar Mountain rock formation in east central Utah.
The site was discovered by a fossil hunter who was selling the bones illegally on the black market. After he discovered parts of Falcarius's distinctive neck, he realized he had something important and turned over the site to paleontologists for further research.
"We're very fortunate he came forward. We've all benefited from that," Kirkland said. "But he did spend five months in jail and paid a [U.S.] $15,000 fine. He may be the first person to ever go to jail for fossil theft on public land."
A second mass mortality site was discovered in the area by Celina and Marina Saurez, twins who are geology graduate students at Temple University in Philadelphia. The pair believes both sites are associated with a spring.
Kirkland said several explanations could explain the mass mortality at the springs, including drought, toxic gases, or botulinum (a bacterial toxin). The dinosaurs congregated in large numbers at the springs, at least periodically.
Regardless of how the dinosaurs died, Zanno, the University of Utah graduate student, said the sitewhich contains the bones of babies, juveniles, and adultswill likely be the best place for therizinosaur research for centuries to come.
"We'll be able to do really cool stuff, like determine how fast the animals grew, see at what age they reached maturity. Are there physical differences between babies and adults, differences between males and females? How much variation is there in the species," she said. "These are future research questions."
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