John Scannella is a doctoral student at Montana Sate University and the Museum of the Rockies, located in Bozeman, Montana. His research centers on understanding the evolutionary history of Triceratops, a genus of dinosaurs known for a characteristic bony head frill and three horns.
In July 2010 Scannella and John “Jack” Horner, curator of paleontology at the Museum of the Rockies and regents professor at Montana State University, published a paper in the Journal of Vertebrate Paleontology revealing that Triceratops and “Torosaurus,” rather than representing two separate genera of dinosaurs, actually are different versions of the same dinosaur. With this groundbreaking report in mind, Britannica science editors Kara Rogers and John Rafferty asked Scannella about the discovery and what it means for scientists’ understanding of dinosaur diversity.
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Britannica: As your recent paper in the Journal of Vertebrate Paleontology points out, Triceratops and Torosaurus have been considered distinct dinosaurs, classified in separate genera, for more than a century. What led you to believe that the two may instead be the same species?
Scannella: A few years ago, Jack Horner and Mark Goodwin published the first description of how the skull of Triceratops changed as it grew up from a baby to an adult. The changes these animals underwent were pretty extreme—the horns above the eyes change from curving backwards in small juvenile trikes to curving forwards in more mature individuals, and the epoccipitals (spikes bordering the frill at the back of the skull) start out very triangular and flatten with age.
Torosaurus latus is a horned dinosaur that is found in the same geological formations and same geographic area as Triceratops. It looks very similar to Triceratops, except it has a much larger bony frill at the back of its skull. The frill of Torosaurus has two holes in it, whereas in Triceratops the frill is solid. When I started studying Triceratops, I read a paper written by John Ostrom and Peter Wellnhofer in 1990 in which they briefly suggested that Torosaurus could conceivably be what a male Triceratops looked like. Given what was being learned about how dinosaur skulls changed shape as they grew up, I thought that the hypothesis of Torosaurus being a separate species was an idea worth exploring further. It turned out that Jack (who is my advisor at Montana State University and my co-author on this study) had been thinking along similar lines for some time and had even suggested in public lectures that Torosaurus and Triceratops might be the same animal.
Britannica: In what way are the two dinosaurs related, and what evidence did you uncover that supports this relationship?
Scannella: All the evidence we have suggests that Torosaurus was not a separate dinosaur, but, instead, was simply what a full-grown Triceratops looked like. We’ve compared numerous skulls of Triceratops and Torosaurus and we’ve found intermediate morphologies between what is typically found in Triceratops and what is typically found in Torosaurus. The frill of Triceratops developed thin areas in the same regions where Torosaurus specimens have holes. As Triceratops grew up, the frill became longer, wider, and thinner and eventually formed the characteristic holes found in Torosaurus. We’ve also examined the bones of many Triceratops and Torosaurus under a microscope, and the tissues of Torosaurus specimens are more mature than those in even the largest Triceratops, which is pretty strong evidence that Torosaurus was a mature Triceratops.
Britannica: How much larger do you think Torosaurus was relative to Triceratops and is there a possibility that, rather than different growth stages, the two may represent different sexes of the same species?
Scannella: Some “Torosaurus” skulls are huge. They’re the size of my car. However, they are not all gigantic. But even “Torosaurus” skulls that are smaller than large Triceratops skulls have the indications of ontogenetic (developmental) maturity, which is very interesting, as it tells us something about variation in dinosaurs. Could this be sexual variation? It’s possible. It is not impossible that “Torosaurus” is what male Triceratops looked like, but our histological evidence so far suggests that “Torosaurus” specimens were more mature than even the biggest Triceratops skulls.
Britannica: Many Triceratops specimens are known, but Torosaurus specimens are comparatively rare. How many specimens of the two did you compare, and where were they discovered?
Scannella: I have personally examined well over a hundred Triceratops, easily. That’s one of the great things about Triceratops: there are a lot of them to study. The Museum of the Rockies alone has collected over 70 specimens from the Hell Creek Formation in Montana over the last 11 summers. “Torosaurus latus” is much rarer—it is known from fewer than a dozen specimens, and I’ve examined virtually every one that is available for study in the United States.
Britannica: How could your research and consideration of ontogeny (the development of an organism) impact current understanding of dinosaur diversity?
Scannella: If “Torosaurus” is Triceratops, then that decreases our perceived dinosaur diversity: where there once were two species there is now one. We are now realizing that it is likely that many of the dinosaur species that have been named in the past might just be growth stages of other dinosaurs. I think that, as more dinosaurs are re-examined with a consideration of ontogenetic change, dinosaur diversity as we’ve perceived it will continue to decline. Recognizing ontogenetic change as a major source of variation between specimens will give us a clearer view of dinosaur paleoecology.
Photo credits: Courtesy of John Scannella; Courtesy, Library Services Department, American Museum of Natural History, New York City, photograph, E.M. Fulda.