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Triceratops' ancestor identified

Nicholas Longrich / Yale University

An illustration depicts a newly discovered dinosaur called Titanoceratops, which predates the better-known Triceratops.

Paleontologists report that a massive horned dinosaur was roaming the American Southwest 5 million years before the well-known Triceratops ... or was that a Torosaurus?

The newly named species, Titanoceratops, weighed nearly 15,000 pounds and had an 8-foot-long skull. It lived during the Cretaceous period, around 74 million years ago.

The finding, accepted for publication in the journal Cretaceous Research, suggests that the large horned dinosaurs evolved their large size earlier than previously thought, reports Yale University paleontologist Nicholas Longrich.

The paleontologist got an inkling about the existence of this dinosaur while searching through scientific papers that described a partial skeleton discovered New Mexico in 1941. The skeleton was identified as Pentaceratops, a common species to the area, and was reconstructed as one for display at the Oklahoma Museum of Natural History in 1995.

"When I looked at the skeleton more closely, I realized it was just too different from the other known Pentaceratops to be a member of the species," Longrich said in a news release.

Nicholas Longrich / Yale University

The skull on the left is the Titanoceratops skull, the missing parts of which were reconstructed to look like a Pentaceratops. The illustration on the right shows the missing parts of the frill (shaded).

Instead, he says the dinosaur likely weighed twice as much as an adult Pentaceratops. It was similar to Triceratops, but with a thinner frill, longer nose and slightly bigger horns.

He suspects that Titanoceratops is the ancestor of both Triceratops and Torosaurus. "This skeleton is exactly what you would expect their ancestor to look like," he said.

More work is needed, however, before the assignment to a new species is confirmed. As pointed out by Brian Switek on the Smithsonian's Dinosaur Tracking blog, members of the Dinosaur Mailing List are debating whether Pentaceratops and Titanoceratops are different growth stages of a single species.

"The animal Longrich has named Titanoceratops certainly did exist," Switek writes, "but as with any other species, the animal's name is a scientific hypothesis that will likely be discussed and debated in years to come."

The discussion parallels the debate over whether Triceratops and Torosaurus fossils actually represent the juvenile and adult forms of the same animal. Some experts suggest that the name "Triceratops" (or more likely "Torosaurus") should go extinct, alongside "Brontosaurus." Others, however, insist that Triceratops and Torosaurus were truly different breeds of horned dinosaurs.

So how do you settle the debate? The best way is to find more fossils, and especially fossilized frills. That's exactly what Longrich is hoping for in Titanoceratops' case.

More stories on horned dinosaurs:

John Roach is a contributing writer for msnbc.com. Connect with the Cosmic Log community by hitting the "like" button on the Cosmic Log Facebook page or following msnbc.com's science editor, Alan Boyle, on Twitter (@b0yle).