Late last year paleontologists Jack Horner and Mark Goodwin made waves by proposing that what had previously been thought to be two distinct genera of "bone-headed" dinosaurs—Stygimoloch and Dracorex—were really just growth stages of Pachycephalosaurus. Together the three body types illustrated how the skull of this peculiar dinosaur was reshaped as it grew—juveniles did not look just like smaller versions of the adults—but Pachycephalosaurus was not the only dinosaur to undergo such changes. In a new paper just published in the Journal of Vertebrate Paleontology, Horner and John Scannella suggest that one of the largest horned dinosaurs to have ever lived was simply the adult stage of one of the most famous dinosaur celebrities.
Among the most intriguing dinosaurs named by paleontologist O.C. Marsh during the "Great Bone Rush" of the late 19th century were the ceratopsians Torosaurus and Triceratops. They were the last of their kind—found in the same end-Cretaceous formations across the American West—and they seemed to differ only in some details of the skull. Where Triceratops had a somewhat curved, solid frill, Torosaurus had a flatter, expanded frill with two large openings in it. Beyond these features and a few other minor characteristics in the skull, it has been nearly impossible to tell them apart.
As suggested by Horner and Scannella, the close resemblance between these two dinosaur body types was not due to a close evolutionary relationship, but because they were different life stages in the same animal. After collecting and examining dozens of specimens, the paleontologists found a graded continuum of growth from the smallest juvenile Triceratops all the way up to what has been called Torosaurus. (The difficult-to-classify specimen representing the genus Nedoceratops may also fall within this range of skull shapes.) On the basis of gross anatomy alone, it is easily seen how the frill of Triceratops changed as it aged, with large windows in the frill opening up as the dinosaur became an adult. But some of the most compelling evidence for these changes comes from bone anatomy that can be seen only under a microscope.
When Horner and Scanella looked at the bone structure of Triceratops brow horns, they found that what had previously been thought to be fully mature individuals still had some growing to do. These Triceratops specimens lacked the amount of dense, mature bone which would have been expected for a fully grown animal, and, instead, this kind of mature bone was found in the horns of Torosaurus. Since all the specimens identified as Torosaurus represent adults, and what were thought to be fully adult Triceratops are only young adults, the simplest explanation is that both are growth stages of Triceratops (which was named first, and therefore has priority for the genus name).
From what Scannella and Horner were able to tell, Triceratops retained juvenile characteristics (such as a solid frill) for most of its life before a rapid change before reaching maturity. As shown by the rarity of mature "Torosaurus" skulls, however, young adult Triceratops became preserved in the fossil record much more often. Why this should be so is a mystery, but the new hypothesis proposed by Scannella and Horner resolves the question of why paleontologists have not found any juvenile Torosaurus skeletons. "Immature 'Torosaurus’ actually have been known for over a century," the authors conclude, "but have been called Triceratops."
Scannella, J., & Horner, J. (2010). Torosaurus Marsh, 1891, is Triceratops Marsh, 1889 (Ceratopsidae: Chasmosaurinae): synonymy through ontogeny Journal of Vertebrate Paleontology, 30 (4), 1157-1168 DOI: 10.1080/02724634.2010.483632