RALEIGH — Scientists based in Raleigh and Chicago have discovered a new species of predatory dinosaur that was so dominant it apparently suppressed the rise of tyrannosaurs for millions of years.
Paleontologists Lindsay Zanno of N.C. State University and the N.C. Museum of Natural Sciences and Peter Makovicky of Chicagos Field Museum of Natural History found a partial skeleton of the giant carnivore, Siats meekerorum, in central Utah and worked with teams that excavated it over the course of two summer expeditions. They detailed the discovery Friday in the online journal Nature Communications.
Siats (pronounced see-atch) is believed to have been the largest predator in North America during its day, roughly 66 million to 100 million years ago. It is named after the Siat a man-eating monster from Ute tribal legend and for the Meeker family of Winnetka, Ill. The family has sponsored paleontologists working with the Field Museum, including some of Zannos early work.
The scientists estimate that Siats was more than 30 feet long and weighed more than 4 tons. That makes it the third-largest species of carnivorous dinosaur found on the continent. According to its bone structure, though, this individual was a juvenile, and its possible that adults could have rivaled the second-largest dinosaur, Acrocanthosaurus, which ruled just before Siats.
The tyrannosaurs of Siats time, meanwhile, were about the size of Labrador retrievers, said Zanno, and would have been no more nuisance to the giant Siats than jackals at a lions kill.
The large size difference, Makovicky said, suggests that the tyrannosaurs were being held in check, in an evolutionary sense, by the presence of Siats.
Then an unknown cause triggered the decline of Siats, making way for the arrival of the T. rex, which was more than twice as large.
Rather than two species of similar size essentially fighting it out for supremacy, scientists now believe that dominant species such as Siats had to die off to create an opening before another could rise to the top.
So it isnt that tyrannosaurs are better-suited to be top predators, Zanno said. They were just opportunists.
Filling in the blanks
The discovery of Siats fills an important gap in knowledge, she said. Paleontologists had been unsure not only what had ruled from atop the food chain during its apparent reign, but also what other species of dinosaurs thrived there and then.
Were really missing almost everything from that ecosystem, Zanno said.
As it turns out, that took a little of the surprise out of finding a new species, even one as large and important as Siats.
It was the summer of 2008, and Zanno was winding up her doctorate at the University of Utah when she joined a Field Museum-sponsored paleontological expedition. It headed for a parched, gray moonscape in the Cedar Mountain Formation of central Utah, where it was known from dating volcanic ash that the rock dated from the blank period.
We knew that pretty much anything we found there was going to be new to science, Zanno said.
Still, when she noticed some small pieces of bone sticking out of a hillside, the scientists got excited, in part because it was quickly clear that the bone belonged to a theropod dinosaur, a type that walked on its back legs and was almost certain to be a carnivore.
And plenty of suspense remained.
A few nubs of bone poking out of a rock and the fallen, eroded fragments underneath dont tell how much of the creature could be recovered. Plenty, as it turned out.
First, the paleontologists went at the rock with 60-pound jackhammers and pickaxes. Then, when they got down to the main level of the bones, they began using smaller tools such as chisels, rock hammers and even dental equipment.
We start out big, then get careful, Zanno said.
Pieces tell a lot
By the time they were done, they had dug up an area nearly the size of a small swimming pool and found parts of the creatures pelvis, backbone and a hind limb. That was enough to extrapolate much about it, including its size, the fact that it wasnt fully grown, and that it was indeed a new species.
It was the largest carnivorous dinosaur to be found in the United States in more than 60 years.
And more are on the way that will further fill in the blanks about Siats era.
Zanno has led her own expeditions to the area for the past two years and plans more. The Field Museum also has continued sending teams.
Already the researchers have more important finds, and could announce another species as soon as next year, Zanno said.