SCIENCE

The Great Triceratops Debate Continues

Science Correspondent

January 31, 2011

What is Nedoceratops hatcheri? That depends on whom you ask.

For over 120 years the problematic skull of this horned dinosaur has been bounced around the literature under different names and attributions. While it was originally described as a distinct genus, Diceratops, some paleontologists later lumped it under Triceratops, at least until recent work raised the possibility that it really was a unique dinosaur. Then there was the problem of what to call it. The dinosaur's original name was occupied by a wasp, and two different publications proposed two different replacement names, with Nedoceratops just beating out the proposed name Diceratus.

Then came last year's controversial paper hypothesizing that the dinosaur Torosaurus was truly the adult stage of Triceratops. In this growth series, Museum of the Rockies paleontologists John Scannella and Jack Horner proposed, Nedoceratops represented a transitional stage between the young adult (Triceratops) and old adult (Torosaurus) stages, meaning that Nedoceratops should really be called Triceratops, too. But ceratopsian expert Andy Farke of the Raymond M. Alf Museum of Paleontology disagrees. In a recently-published PLoS One paper, Farke at long last gave Nedoceratops a detailed description and found that it stands apart from other horned dinosaurs.

Determining the identity of Nedoceratops is important for figuring out how many kinds of horned dinosaurs there were at the end of the Cretaceous, as well as testing ideas about the growth of Triceratops. The trouble was that very little had actually been written about this dinosaur. "obody had ever published a full scientific description of the skull," Farke explained via e-mail, "so it was an opportunity ripe for the taking."

What makes Nedoceratops unique—and has made it so frustrating to assign—is a mosaic of features on its skull. This dinosaur lacks a nasal horn, has brow horns that stick up almost vertically, and slot-like openings in its frill. The only known Nedoceratops skull also has uneven openings on its squamosal bones that make up the sides of its frill which have puzzled scientists for years.

"Nobody has been able to decide if these features are just the results of injury, abnormality, individual variation, or genuine differences between species," Farke says, but he makes a compelling case that the first three traits might be indications that Nedoceratops was unique. They do not seem to overlap with known specimens of Triceratops or Torosaurus. The openings in the squamosal bones are another matter. As interpreted by Farke:

The old thought was that these holes were the result of accidental "gorings" during horn-to-horn combat between rival dinosaurs. But, Darren Tanke and I recently noted that most aspects of the fenestrae argue against them being the result of injury. Instead, we think they were probably just the result of bone resorption in an area of the frill that was already thin to begin with. No horn thrusts required.

Additionally, the texture of the bone and the degree of fusion between the parts of the skull appear to indicate that this Nedoceratops skull represents an old individual. This makes it unlikely that it represents a transitional growth stage of Triceratops.

If Nedoceratops isn't really a pathological or young adult Triceratops, what does this mean for the still-debated "Toroceratops" hypothesis? Well, for one thing, the proposed Triceratops-Torosaurus continuum would lose its intermediate stage. More than that, though, Farke points out that the degree of changes required to turn a Triceratops skull into a Torosaurus skull are unknown in any other horned dinosaur, particularly the addition of bony knobs around the edge of the frill (epiossifications) and the opening of holes in the frill's parietal bones late in life. If these modifications actually occurred, Triceratops had an extremely unusual growth series. And, the icing on the cake, Farke mentions that a juvenile Torosaurus may have been hiding in plain sight for decades in a specimen called YPM 1831. Provided that further study confirms this identification, it would support the idea that Triceratops, Torosaurus and Nedoceratops truly were different dinosaurs.

The existence of three different horned dinosaurs in western North America at the same time would be important to investigations about the ecology and evolutionary history of the dinosaurs just before the mass extinction that wiped them out. Asked whether this indicates that dinosaurs were still going strong at the end of the Cretaceous or already dwindling, Farke replied:

I would suggest that dinosaurs were still going strong, but of course our view is very skewed towards western North America (where these horned dinosaurs lived). We know next to nothing about what was going on with dinosaurs elsewhere in the world at that time! Even within North America, many important dinosaur specimens from the end of the Mesozoic (including that of Nedoceratops) weren't collected with full geological data. Better field protocols are changing this (especially through ongoing work at Museum of the Rockies), but we have a long way to go yet.

Not everyone is going to agree with the new paper's conclusions, of course, but Farke is not exactly locking horns with his colleagues about this. There were no rumbles at the annual Society of Vertebrate Paleontology meeting last October, and as Farke documented on his own blog, the new Nedoceratops paper was actually improved through conversations with Scannella and Horner.

That is not to say that these scientists agree, though. When asked about whether Nedoceratops should be separated from Triceratops, Scannella replied:

The hypothesis that the single specimen of 'Nedoceratops' represents a distinct genus of horned dinosaur is based on noting how it differs from other specimens. If you're looking for differences between specimens, they're easy to find—but differences can't tell us anything about relationships; only similarities can do that. No evidence was presented which indicates 'Nedoceratops' was more mature than any other young adult Triceratops and its tiny parietal fenestra is what you would expect if it was in the process of developing large 'Torosaurus' fenestrae.

Furthermore, there may be additional evidence that Nedoceratops really does fall within the range of variation seen among Triceratops. Over the past decade the Museum of the Rockies has excavated multiple Triceratops specimens from the famous Hell Creek Formation, providing paleontologists with a way to determine just how much individuals varied from one other. According to Scannella, "There are numerous Triceratops specimens that overlap in anatomical traits with 'Nedoceratops,'" although these specimens have yet to be fully described.

I also asked Scannella about one other related point. When the public controversy over the Toroceratops hypothesis blew up last year, many critics on the Internet stated that Triceratops was larger than Torosaurus, and therefore the Torosaurus specimens could not be adult forms of Triceratops. I asked Scannella to respond to this point:

I am a lot taller than my Dad, but that doesn't make me older. When you have a huge sample size, like we now do for Triceratops, it is possible to see just how much variation is present. One of the things that varies is size. There are young Triceratops which are very large and there are more mature ones that are quite small. The sources of this variation may include things like ontogenetic variation, stratigraphic variation, sexual variation, and individual variation—so there is a lot to take into account.

The debate over the fate of Nedoceratops and Torosaurus is not over. Not by a long shot. No single paper is going to make all the difference here. Each academic article is another part of an ongoing discussion about how to identify dinosaur species and the implications those rearrangements might have. Being that Farke's paper is one of the first—but surely not one of the last—replies in this debate, I'll give him the last word:

Undoubtedly, many other paleontologists will have something to say about these issues. Some will agree, some will disagree, some will show parts of my paper are incorrect, and others will present more supporting data (at least I hope, on all counts). I suspect the next few years will feature much, much more discussion on these fascinating horned dinosaurs!

(Farke has also chronicled the process of writing the Nedoceratops paper in a three-part series at The Open Source Paleontologist: Part I, Part II, Part III.)

References:

Farke, A. (2011). Anatomy and Taxonomic Status of the Chasmosaurine Ceratopsid Nedoceratops hatcheri from the Upper Cretaceous Lance Formation of Wyoming, U.S.A PLoS ONE, 6 (1) DOI: 10.1371/journal.pone.0016196

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