A rare horned dinosaur known as Torosaurus may not be a distinct species, after all, according to a presentation given Friday at the annual meeting of the Society of Vertebrate Paleontology in Bristol, England.

Researchers have long recognized similarities between Torosaurus and Triceratops, the main distinctions being that Torosaurus is larger and has an expanded frill at the rear of the skull. But John Scannella, a doctoral student at Montana University, and his advisor, John R. Horner, have found that specimens attributed to the two species actually form a developmental continuum rather than falling into discrete groups. A Triceratops skeleton on display at the American Museum of Natural History in New York City, for example, exhibits a number of skull traits reminiscent of Torosaurus, including thin parietal bones and elongated squamosal bones. In addition, microscope examinations of thin slices of bone from Triceratops and Torosaurus specimens reveal that individuals attributed to Torosaurus are more mature than any of the ones assigned to Triceratops. Scannella and Horner therefore believe that the fossils that have been categorized as Torosaurus are just Triceratops individuals that reached mature adulthood before they died.

Scientists have wondered how two such similar groups could have shared the landscape—both ranged from Colorado to Saskatchewan at the end of the Cretaceous period. If Scanella and Horner are right, the answer is simply that the animals are one and the same species.

But the finding raises the question of why fossil hunters have recovered so few of the mature “Torosaurus” specimens—fewer than a dozen, compared to the many dozens of younger Triceratops. “If Torosaurus is Triceratops, then we’re finding a lot of animals that had a lot of growing up to do,” Scannella comments. Insights may come once researchers determine a more precise age at death for the individuals.

The finding provides more evidence that dinosaur diversity was declining before the animals became extinct, which, Scannella says, supports the idea that something other than an asteroid impact extinguished them.

Photo of Triceratops skeleton at the American Museum of Natural History by mrkathika via Flickr