The oldest horned dinosaur in North America was about the size of a dog, scientists have learned from a skull uncovered in 1997.
According to CBS News, authors of a study published in the journal PLOS One analyzed an Aquilops americanus fossil first unearthed 17 years ago in the Cloverly Formation in southern Montana. Horned dinosaurs, like the triceratops, were known to have flourished in North America.
"Aquilops lived nearly 20 million years before the next oldest horned dinosaur named from North America," study co-author Andrew Farke, a paleontologist with Raymond M. Alf Museum of Paleontology, said in a press release. "Even so, we were surprised that it was more closely related to Asian animals than those from North America."
A. americanus lived in the early Cretaceous period some 145.5 million to 65.5 million years ago.
"Scott [Madsen] initially thought it was generic, plant-eating dinosaur but it turned out it was one of these early horned dinosaurs. That is a really exciting find," Farke told CBS News of the fossil's original uncovering. "Up to this point, all that was known for horned dinosaurs were just isolated teeth and fragment bones.
"This new find is the first one that allows us to say exactly what kinds of horned dinosaurs lived in North America about 108 million years ago."
The researchers hypothesized that the dinosaurs could have been part of a migration from Asia, as there would have been a land bridge between Alaska and Russia.
"There was an influx of animals from Asia probably related to changes in ocean levels which would have opened up land bridges between some continents," Farke said. "There is about a 15 to 20 million year gap where we don't know what kind of horned dinosaurs were living in North America.
"Maybe that is when the ancestors of animals like Triceratops would have come to North America."
