A newly unearthed dinosaur has been called the "Texas longhorn" of its family tree, and it's not hard to see why: Nasutoceratops titusi, a relative of the famous Triceratops, sported 3.5-foot-long horns, measured 15 feet long from nose to tail, and weighed 2.5 tons. But this fossil is significant for more than just its anatomy--the discovery of Nasutoceratops provides powerful evidence for a theory that may explain the astonishing diversity of dinosaurs in Western North America millions of years ago.
Nasutoceratops is a member of the ceratopsid family, a group of quadrupedal herbivores that lived during the Campanian stage of the Late Cretaceous period, between 84 and 70 million years ago. Like many ceratopsids Nasutoceratops bears horns and a neck frill. But “It's a bizarre animal in some respects," says paleontologist Scott Sampson of the Denver Museum of Nature and Science and leader of the team that discovered Nasutoceratops. In most ceratopsids, the major differences lie in the horns and frills. Trim them off and it's hard to tell the animals apart, he observes. But in addition to its oversized horns, Nasutoceratops differs from its cousins in having a massive nose. "It represents a previously unknown group of dinosaurs," Sampson asserts.
Sampson also argues that Nasutoceratops provides strong evidence of dinosaur provincialism, a phenomenon that caused vast speciation of dinosaurs across western North America. His team found the specimen in the Grand Staircase-Escalante National Monument in southern Utah. During most of the Late Cretaceous, high sea levels flooded much of North America, creating a landmass named "Laramidia": a narrow stretch of land around the young Rocky Mountains, stretching from Alaska to Mexico. Laramidia was less than one third the size of present-day North America. Dinosaurs flourished on this sliver of land. More than a dozen species of ceratopsids have been unearthed as far north as Alaska—but there's something confusing about these discoveries.
"Beginning in the 1960s, paleontologists were starting to notice that dinosaurs they were finding in the north were different than the ones in the south," Sampson says. "There was a question in people's minds about whether these animals were living at the same time." Yet advances in dating technology confirmed that several separate but related species of ceratopsids were living on the same continent at the same time. For example, Styracosaurus albertensis, another ceratopsid, roamed Laramidia at the same time as Nasutoceratops, just a few hundred miles to the north in what is now Alberta. But how could groups of giant dinosaurs live only a few hundred miles apart on the same piece of land without interbreeding? Speciation usually occurs when an ancestral species is divided into groups that evolve separately over time because of some impassable barrier.
"I can't imagine why something the size of a small rhino can't walk to Alberta and wouldn't have a species range that far north," says Mark Loewen of the University of Utah, a coauthor of the new study. "Something is keeping these animals from moving back and forth from north to south."
And that's where the dinosaur provincialism theory comes in. Having compared plant, fish, mammal and crocodile remains from northern and southern Laramidia, Sampson and Loewen believe that some unknown barriers were separating and combining groups of species over millions of years.
“What we think is happening is that Laramidia was a crucible of evolution,” Loewen says. “It was really easy to separate small populations of species, who through sexual selection drift genetically one way or another. Later, when the barrier is removed and the populations come back together, they don’t interbreed and compete for resources instead."
The barriers that caused speciation are speculative at best, Loewen says. An ancient river system used to sprawl over Laramidia and could have separated populations, but modern giant herbivores are skilled swimmers. It has also been suggested that slight differences in climate and plant life could have promoted provincialism.
Not only did the ceratopsids of Laramidia diversify like crazy, but Loewen suspects this group evolved quickly. Today in Africa there are a handful of mammals that grow to over 1000 kilograms. In Laramidia, more than 20 species of dinosaur grew over 1000 kilograms on a much smaller landmass. This proves the lush, tropical continent of Laramidia was able to support lots of rapidly evolving animals.
Whenever we’re seeing these things, they’ve changed,” Loewen says. “ The group is way more diverse than we would have thought even ten years ago.”
Sampson thinks Nasutoceratops is just the beginning of a wave of fossils that will support the dinosaur provincialism theory. “It suggests that there was in fact a whole different group of these horned dinosaurs evolving for potentially millions of years in the south, while there was a distinct community in the north,” he says. “That’s what we’re expecting to find as we continue excavating.”