Paleontologists from the Utah Geological Survey and the Utah Museum of Natural History announced the discovery of a bizarre new species, Falcarius utahensis, in the May 5, 2005,. issue of Nature Magazine.  James Kirkland, Utah state paleontologist at the Utah Geological Survey and principal scientist for the new study.  He said, "This little beast is a missing link between small-bodied predatory dinosaurs and the highly specialized and bizarre plant-eating therizinosaurs."
     The new species was excavated from ancient gravely mudstones at the base of the Cedar Mountain rock formation, at a site named the Crystal Geyser Quarry after a nearby manmade geyser that spews cold water and carbon dioxide gas.  It was discovered in a mass graveyard containing hundreds, if not thousands, of individuals, including everything from hatchlings to adults.  The was discovered by a commercial fossil collector who later was convicted of fossil theft.  "We never would have found it, at least for 100 years or so, if he hadn't taken us to the site," Kirkland says.  "Once he figured out he had a new dinosaur, he realized scientists should be working the site. His conscience led him to get this stuff to me."
     Scientists have suggested a number of possible explanations for such mass deaths. Scott Sampson, chief curator at the Utah Museum of Natural History and an associate professor of geology and geophysics at the University of Utah.  Sampson says that they include drought, volcanism, fire and botulism poisoning from water tainted by carcasses.  Kirkland leans toward a theory developed by Celina and Marina Suarez, twins who are geology graduate students at Temple University in Philadelphia. Their research on carbonate-rich sediments in which the dinosaurs were buried suggests the area was near or in a spring, and that there were at least two mass die-offs.  That raises the possibility the dinosaurs were drawn repeatedly to the site by water or an attractive food source – perhaps plants growing around the spring – and then the spring occasionally would poison the animals with toxic gas or water, Kirkland says.
    With almost 1,700 bones excavated during the past three years, scientists have about 90 percent of Falcarius' bones, making it the most complete therizinosaurus specimen found to date.  Falcarius "is the most primitive known therizinosaur, demonstrating unequivocally that this large-bodied group of bizarre herbivorous group of dinosaurs came from Velociraptor-like ancestors," says study co-author Lindsay Zanno, a graduate student in geology and geophysics at the University of Utah and the Utah Museum of Natural History.  Sampson maintains that Falcarius did not descend directly from Velociraptor, but both had a common, yet-undiscovered ancestor, says study co-author and paleontologist
    Kirkland says Falcarius likely was covered with shaggy, hair-like "proto-feathers," which may or may not have had a shaft like those found in bird feathers. No feathers were found with the Falcarius fossils. Feathers rarely are preserved, but "a number of its close relatives found in China had feathers [preserved by unusual lake sediments], so the presumption is this animal too was feathered," Sampson says.
      The previously unknown species provides clues about how vicious meat-eaters related to Velociraptor ultimately evolved into plant-munching vegetarians.  The adult dinosaur walked on two legs and was about 13 feet long (4 meters) and stood 4.5 feet tall (1.4 meters). It had sharp, curved, 4-inch-long (10 centimeter) claws.
     Scientists do not yet know if the creature ate meat, plants or both.  Kirkland said, "Falcarius shows the beginning of features we associate with plant-eating dinosaurs, including a reduction in size of meat-cutting teeth to leaf-shredding teeth, the expansion of the gut to a size needed to ferment plants, and the early stages of changing the legs so they could carry a bulky body instead of running fast after prey."
     Falcarius, which dates to the Early Cretaceous Period about 125 million years ago, belongs to a group of dinosaurs known as therizinosaurs. Falcarius and Beipiaosaurus are about the same age and appear to represent an intermediate stage between deadly carnivores and later, plant-eating therizinosaurs. Falcarius is anatomically more primitive than the Chinese therizinosaurs.
     Falcarius had leaf-shaped teeth designed for shredding plants rather than the triangular, blade-like serrated teeth of its meat-eating relatives. Its pelvis was broader, indicating a larger gut to digest plant material, which is more difficult to process than meat.  Its lower legs were stubby, presumably because it no longer needed to run after prey. Compared with carnivorous relatives, Falcarius' neck was more elongated and its forelimbs were more flexible, perhaps for reaching plants to eat.  Sampson says: "Falcarius 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."
    Falcarius means sickle-maker, so named because later plant-eating therizinosaurs had 3-foot-long, sickle-like claws. The species name, utahensis, comes from the fact the new species was discovered in east-central Utah, south of the town of Green River.  Falcarius is the fourth new dinosaur species Kirkland has discovered in the Cedar Mountain Formation's Yellow Cat member (a unit of the formation) in 11 years. Others are meat-eaters Utahraptor and Nedcolbertia, and an armored dinosaur named Gastonia.
    "Therizinosaurs have been found for 50 years in China and Mongolia, but were not recognized as a distinct group until about 25 years ago," Sampson says. The only therizinosaur known previously from North America was Nothronychus, which Kirkland discovered in the late 1990s in New Mexico. It was 90 million years old, so scientists initially believed the older therizinosaurs in China had migrated over a land bridge from Asia through Alaska to the American Southwest.  But due to the constantly shifting plates of Earth's surface, Alaska didn't exist 125 million years ago – the age of both Falcarius and the oldest known Chinese therizinosaur, Beipiaosaurus.  So scientists now wonder if therizinosaurs originated in Asia and migrated through Europe to North America before the Atlantic Ocean basin opened up, or if they originated in North America and migrated through Europe to Asia. "Falcarius may have been home-grown," Kirkland says."  This discovery puts the most primitive therizinosaurs in North America," Zanno says. "This tells us that North America potentially could be the place of origin for this group of dinosaurs."