Triceratops reigns alone again

Triceratops may no longer have an identity crisis. As paleontologists lock horns on whether these dinosaurs were just baby versions of the larger Torosaurus, the latest clues suggest the two were indeed separate kinds of dinosaurs.

A new study reveals immature and adult examples of both Triceratops and Torosaurus. “I don’t see any clear fossil evidence that one dinosaur turned into the other,” says Nicholas Longrich. He and fellow Yale paleontologist Daniel Field make their case online February 29 in PLoS ONE.

Comparisons between the dinos, which both lived about 65 million years ago in western North America, start with their skulls. Like many horned dinosaurs, Torosaurus sported a sizable frill of bone perforated with two big holes. Triceratops, in comparison, wore an unusually short and solid crown.

Searching for adult Triceratops, Longrich and Field grouped 35 skulls based on how fused together the bones were. Several Triceratops skulls had completely fused, a sign of maturity in modern animals. Some Torosaurus skulls showed bones still joined by sutures, which are hints of youth.

But fused bones may not be a reliable way to gauge age, says paleontologist John Horner of Montana State University in Bozeman. “We recently collected 100 new Triceratops specimens they haven’t seen,” he says. “We see a lot of variety in bone fusion.”

In 2010 Horner blamed Triceratops’ unusual frill on youth. Patches of thinning bone on some skulls were steps toward full-fledged holes and a Torosaurus adulthood, he and Montana State colleague John Scannella argued.

Triceratops’ skull would have to change dramatically to achieve this transformation. The depressions form in different places than the holes. Triceratops frills also have fewer spikes than those of Torosaurus.

Finding a skull halfway between existing specimens with small holes would show that such remodeling is possible. A transitional specimen matching this description has been discovered, says Horner. But Longrich and others believe the strange skull in question — which has no horn and extra holes in bizarre places — belonged to a sick Triceratops or a species called Nedoceratops.

“It’s an old adult that’s fully mature,” says paleontologist Andrew Farke of the Raymond M. Alf Museum of Paleontology in Claremont, Calif.

With new fossils in hand, though, Horner isn’t giving up on lumping together the two dinos.

“He’s a confirmed lumper … with a history of defending his ideas to the nth degree,” says paleontologist Peter Dodson of the University of Pennsylvania in Philadelphia. “This is going to be discussed for years and years.”