Game of Bones: The true identities of America's last ceratopsians

While we can learn a lot from fossils, there is still much we cannot determine just by examining petrified bones, feathers, and other structures. For one, it is difficult to determine whether or not a specimen represents an already-known species or is an entirely new one. This phenomenon occurs more often than one would think, and many extinct genera have a score of junior synonyms which were once considered to be different species. These often turn out just to be juveniles of already-known species, or display some pathology which had rendered them to appear different from the rest of their kind.

Of course, all adult animals known from fossils had to start somewhere – obviously, we have fossils of various dinosaurs ranging in age from embryonic to adult-most forms. Our knowledge of the ontogeny of dinosaurs ranges from species to species – in the case of some, such as Allosaurus and Maiasaura, we have records of complete life histories. In other species, juveniles can often be reassigned as other species or genera simply due to a lack of information on the species as a whole.

The two three-horned titans in question: Triceratops (left) and Torosaurus (right). Illustration by Nicholas Longrich.

One such ontogenetic debate originated last year concerning one of America’s fossilized sweethearts – none other than the three-horned darling that is Triceratops. Fossilized remains of Triceratops are abundant in the American west and are known from young individuals to adults. However, Triceratops wasn’t the only three-horned beast roaming North America at the end of the Cretaceous. Torosaurus,  a closely-related chasmosaurine, has been found alongside Triceratops from Colorado to Montana. However, unlike the remarkably complete life history we have of Triceratops, Torosaurusfossils are largely adult specimens.

Triceratops growth series by Gregory S. Paul. The bottom-most skull, and those
at right, represent Torosaurus. This series represents the logic of Horner et. al (2010).

The absence of young Torosaurus in the fossil record led Jack Horner et. al (2010) to believe that Torosaurus is not its own species, rather it represents the most mature specimens of Triceratops. It makes sense at first: we know animals get larger and, in some cases, more impressive as they age, and Torosaurus’ massive head and elongated frill are much more spectacular than the shorter, square frill of Triceratops. However, size and exaggeration of features do not necessarily contribute to the maturity of any given specimen, and this determination is even harder to make when all we have to work with are fossils.

When news broke that the world may lose Triceratops to cladistic lumping, a panic spread: what would we do without everyone’s favorite three-horn? How would museums cope with the innumerable info-graphics which would need to be reprinted? And the children… How would our children grow up in a world in which Triceratops was no longer a valid genus?

Thankfully, there is no need to worry. First of all, Triceratops was named in 1889, whereas Torosaurus was named in 1891, giving our herbivorous hero precedent over the long-frilled foe. Second of all, a paper has finally been published which puts this whole debate to rest. Well-known specimens of both genera were compared and analyzed, put through a gauntlet of 24 visibly-testable features which diagnose the chasmosaurines as juveniles or more mature specimens.

Testing such features as the curvature of the postorbital horns, the degree of scalloping of the parietals (bumps around the edge of the frill) and squamosals (pointed cheek bones), and the fusion of major bones of the skull allowed Nicholas Longrich and Daniel Field (2013) to put an end to this long-winded argument.  A total of 36 specimens from both genera were analyzed for the test.

Comparative ages and diagnostic features of the ontogenetic stages of Triceratops and Torosaurus. From
Longrich & Field, 2013.

When Horner cited Torosaurus as being just a mature Triceratops, he cited the fact that no juvenile specimens of the former have been discovered, whereas many young Triceratops have been discovered. Longrich and Field discovered that, while the overwhelming majority of Torosaurus specimens are, in fact, adults, at least one specimen represents a slightly younger animal, which instantly puts a hole in Horner’s logic. If young Torosaurus did exist, displaying ontogenetic characteristics diagnostic of a young animal far different from adult Triceratops, then it is impossible for the latter to be a “stepping stone” to the former. So, in your FACE, Horner! Torosaurus juveniles DO exist! Which means…

"And you read this in my voice!"

But, in all seriousness, both genera are valid. While they are both extremely similar in body-shape and lifestyle, they represent two different animals. And I’m sure Tyrannosaurus thought they tasted the same anyway.

A similar story of mistaken identity occurred between these three sympatric pachycephalosaurs. From left
to right, the specimens range from young to old.

Horner has proposed this lumping before with other species; coincidentally (or not), they all hail from about the same time and place. Besides Triceratops/Torosaurus, he is a strong proponent of Nanotyrannus representing a juvenile Tyrannosaurus, which has also recently been tested and supported. More closely related to the horned creatures in question, he has also suggested that the pachycephalosaurids Dracorex, Stygimoloch, and Pachycephalosaurus also represent three ontogenetic stages in the life of the lattermost species. I’m a bit hesitant to accept that Stygimoloch represents an intermediate between the other two, partially because I grew up with ol’ Styg as one of my favorite dinosaurs. Plus, the name is just awesome: Stygimoloch translates to “devil of the River Styx.”

I think we can all rest a bit easier tonight knowing that Longrich and Field have put an end to the attempted assassination of Triceratops as a genus. I know I can, anyway.