"In North America, duck-billed dinosaurs were like the antelope of the African veldt—common overall, but highly variable and fantastic in appearance," said Philip Currie, a paleontologist at the University of Alberta in Edmonton who was not involved in the study.
Species of Gryposaurus had previously been discovered in Montana and Alberta, Canada, but experts were divided over whether these species or their close relatives extended much farther south.
"The identification of the new specimen from Utah has ended this debate," Currie said.
"It is exquisitely preserved. The new study clearly shows that this dinosaur had diversified into multiple species over a huge geographic area."
The discovery also raises new questions for paleontologists.
Differences between species of Grypsaurus suggest that some sort of unknown barrier may have prevented populations from mixing, lead author Gates said.
The Utah skull and a partial skeleton found at a second site a few miles away do show for certain that the newfound Gryposaurus was a giant compared to its northern relatives.
The dinosaur was probably 30 feet (about 9 meters) long and stood at least 10 feet (3 meters) tall, researchers say.
"The bones were larger, and the lower jaw was much stronger than [in] other species," Gates said.
The snout and beak were also bigger.
Although the creature's exact diet is unknown, it likely used its strong jaws and teeth to chomp and chew tough, woody plants.
Such food was probably abundant in the area 75 million years ago. Unlike the rugged canyon country of Utah today, experts say the duckbill lived in a large, humid floodplain.
"There was no grass but probably lots of conifer trees and flowering plants," Gates said. "It would have been lush."
The new Gryposaurus is only the latest of several important dinosaur discoveries in the Grand Staircase-Escalante National Monument.
(Related news: "Giant Turkey-Like Dinosaur Found in Utah" [April 7, 2006].)
In 1996 President Bill Clinton established the monument in part to preserve its rich fossil beds.
Alan Titus, a paleontologist at the monument, worked on the Gryposaurus project.
He said the area contains "one of the most complete records of late Cretaceous terrestrial vertebrate and ecosystem evolution in a single place in North America [and] possibly the world."
The geological feature known as the Kaiparowits Formation, situated in and around the monument, has yielded a number of other new dinosaur species and an abundance of small vertebrate fossils, including fish, lizards and mammals, Titus said. (See a photo of the monument.)
"This is astounding," he said, "yet we guess it is just the tip of the iceberg.
Only an estimated 5 percent of the potential fossil-rich badlands have ever been surveyed.
Researchers have been surprised not just by the large number of fossils in the area, but also by the region's biological uniqueness.
Many of the Kaiparowitz fossils are species known only from southern Utah, Titus said.
"This [large number of unique fossil species] is the last thing any of us would have predicted," Titus said.
"It leads many of us to conclude that we need to rethink long-held views on dinosaur diversity and biogeography."
Lead author Gates agreed.
"There's a whole world hidden in the rocks in the Grand Staircase," he said.
"It's mesmerizing to look out and imagine all the animals waiting to be discovered."
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