Saturday, July 24, 2010
We’ve been hearing rumbling of this paper for awhile now, but it became big news at SVP last year (in Bristol, England, a place I’ll never go back to): Jack Horner and John Scannella proposed a very radical taxonomic synonymy: Torosaurus, that famous long-frilled chasmosaurine, is actually just a very old Triceratops. This isn’t the first time Horner & Co. have applied ontogenetic lumping. Just last year, Horner & Goodwin suggested that Dracorex and Stygimoloch represent teenage and subadult stages, respectively, of Pachycephalosaurus. The paper has been met with mixed support, but this new effort has really split the community. This all seems to be an effort by Horner to show that the Maastrichtian period of the Cretaceous was a barren wasteland, taxonomically speaking, for the dinosaurs, and that diversity was in the crapper well before that giant meteor wiped the last few stragglers out.
And there’s nothing wrong with that. If you really can convincingly show that only one pachycephalosaur, one ceratopsid, one duckbill, and one tyrannosaur existed at the close of the Cretaceous, then yeah, the future looks grim for the Hell Creek dinosaur community. But that can be a tough pill to swallow. Any beefs I have with the Pachycephalosaurus paper have been addressed in an earlier post, so I won’t go too far into detail here. Mainly, I think the authors did a great job is showing that Dracorex is a teenager, Stygimoloch is a subadult, and Pachycephlosaurus is an old man. Those facts alone do not suggest synonymy. A larger sample size, more complete material, and better knowledge of the ontogenetic development of other pachycephalosaurs is necessary to make any kind of truly informed decision about the growth and development of a single poorly-represented genus.
Anyway, let’s talk about Triceratops and Torosaurus. The former is an extremely well-known, well-represented chasmosaurine ceratopsid from all over North America at the close of the Cretaceous. Torosaurus is known from the same deposits, but is extremely rare. Two species are tentatively known: T. latus and T. utahensis. The latter species was originally regarded as a species of Arrhinocertops by Gilmore, and, in fact, Horner & Scannella suggest that original classification may well bear out.
The authors based most of their claim on a paper from a few years ago by Horner & Goodwin which proposes an ontogenetic series for Triceratops. Key features of this growth series are the changing orientation of the brow horns and the eventual blunting and reabsorption of the epiparietals. The Horner & Goodwin paper details five distinct growth stages, though I do wonder about individual variation. Do all teenage Triceratops have slightly recurved brow horns? Would the brow horns really change orientation from basically straight as a baby to recurved as a teenager to forward-pointing as an adult? It just seems like a stretch. I’m all for horn growth, but I also know, from studies on centrosaurine ontogeny and bone beds, that ceratopsian exhibit a large degree of individual variation. Horner & Goodwin even acknowledge this when discussing Triceratops’ epiparietals, but we’ll get to that in a minute.
The point is that Torosaurus represents a sixth distinct growth stage, wherein the frill changes shape considerably—from wave-shaped in cross section to basically billboard-shaped and elongate, with squared-off upper corners instead of a nice round shape. Additionally, two large fenestrae appear as parietal bone is reabsorbed. Only as a full-grown adult does Triceratops come to resemble most of its chasmosaurine relatives. Apparently this all happens very quickly during the animal’s life, and there are no “in-between” specimens, although Horner & Scannella suggest that the oft-neglected Nedoceratops (Diceratops), with its incipient parietal fenestrae, may represent just such a “transitional” form.
The authors are quick to point out that the structure of the frill is the only skeletal feature that separates Triceratops from Torosaurus, but even this isn’t actually evidence for synonymy. If that’s your only justification for lumping two ceratopsids together, you may as well toss the entire Chasmosaurinae (except maybe Chasmosaurus) under one genus. It’s been repeatedly demonstrated that the parietal bone is the single most important bone in the ceratopsid body for providing taxonomic identification. Arrhinoceratops, Anchiceratops, Torosaurus, Mojoceratops, and Ajugaceratops all look pretty much the same beyond the parietal, although that last genus does have distinct brow horns. If anything, history has shown the safe bet—when it comes to horned dinosaurs—has been to split rather than lump.
Scott Sampson agonized over the decision to place Einiosaurus and Achelousaurus in different genera, since their parietals are so similar, or take Einiosaurus out of Styracosaurus in the first place, which is where it had informally sat for so long. Just recently, another “species” of Styracosaurus has been renamed as Rubeosaurus ovatus. A critical part of each of these decisions has been that the structure of the parietal bone. In Einosaurus and Achelousaurus, Scott eventually came to conclusion that, based on the fact that the former had a big hooked nasal horn while the latter had a roughened pachyrhinosaurine boss was enough to separate them at the generic level, although he briefly toyed with the idea that they merely represented sexual dimorphs of each other! This is how much the parietal bone means in ceratopsid taxonomy!
So, stating heroically that the structure of the frill is the only thing separating Triceratops from Torosaurus, as if that means something, is superfluous and misleading because you could say the same about any two other chasmosaurines or any two centrosaurines. It’s a good thing those two subfamilies differ in more than just parietal anatomy or we’d all be screwed.
Horner & Scannella also point out that Triceratops skulls are ridiculously common, and in fact so many are known that a convincing growth series has been established for the genus. By contrast, Torosaurus is amazingly rare, but it is known from the same time and places that one finds Triceratops. They take this to mean that Torosaurus must represent a very aged Triceratops, but that living such a long and fruitful life must have been a miracle given the rarity of that genus vs. the dozens of Triceratops specimens. I have problems with this for two reasons. First, just because an animal is uncommon doesn’t mean it doesn’t actually exist (taxonomically speaking). Nobody is accusing Bagaceratops or Udanoceratops to be a juvenile or full adult of Protoceratops, even though they’re ridiculously rare by comparison. I’m actually surprised Horner hasn’t tried synonymizing the two species of Protoceratops. Anyway, it’s entirely possible that Torosaurus preferred different environments than Triceratops, which would make sense if they’re going to avoid direct competition, and that environment might not be prone to fossilization. I’ve heard it argued that ceratopsids have such a great fossil record because they liked near-shore environments that increase their chances of being fossilized. Surely they didn’t all live the exact same lifestyle, and it’s possible that Torosaurus habitually lived in a different area than Triceratops. It’s also not fair to say that “no juveniles of this genus exist, therefore this genus must not exist.” I can name dozens of dinosaurs that aren’t known from juvenile forms, yet their validity is not doubted!
The second reason is that I doubt Triceratops would put a ton of effort into suddenly expanding its frill when death occurred so close to that period. As the authors acknowledge, Torosaurus skulls are very rare, yet juvenile, subadult, and adult Triceratops skulls are unbelievably common. Why would “old adult” Triceratops (Torosaurus) be so poorly represented? Horner & Scannella suggest that the mortality rate was higher among non-Torosaurus-stage Triceratops, but I have a hard time believing that. Such a strange survival separation isn’t seen in other ceratopsids—why would it be any different for Triceratops? That is to say, other ceratopsids have pretty equal survival rates, no matter what their growth stage. Triceratops doesn’t?
While the authors focus on Torosaurus latus, one may wonder about how they rectify Torosaurus utahensis. They question how many specimens of T. utahensis are diagnosable to the genus level, and even if it is valid, they throw up their hands and say that it’s a southern species of Triceratops, a different genus, or Arrhinoceratops—which it was originally referred to. Time will tell, I suppose, although Hunt & Lehman (2008) have stated that T. latus and T. utahensis are nearly indistinguishable, and in fact can only be discriminated based on the structure of the squamosal/parietal suture. Is Triceratops also present in Utah? I’m not sure, myself, but it might say something about Toroceratops.
Let’s talk about ontogeny. Horner & Scannella posit that Triceratops retains a paedomorphic condition of a solid frill well into adulthood, but what is the basis of determining that a solid frill is a juvenile trait? In centrosaurines, even the youngest animals have small parietal fenestrae. What about chasmosaurines? Are juvenile and subadult animals known chasmosaurines? Certainly, that Ajugaceratops bonebed can shed some light on this, but I haven’t read much on it. Apart from that concern, Horner & Scannella make a good case for the possible development of fenestrae in Triceratops over time. They discuss several specimens that have thin or sunken areas of the parietal where the bone is uniquely textured. I think this is good positive evidence that actually does make a correlation between Triceratops and Torosaurus. An alternate explanation is that they are actually describing juveniles or subadults of Torosaurus! Despite pointing out a lot of Triceratops skulls that have thinning areas of the parietal, the authors do not or cannot show a “transitional” frill, where the parietal fenestrae is present, but very small, and surrounded by thinning bone and a rim. If the bone was reabsorbed (as it must have been), it would have been reabsorbed from the inside out, so skulls should exist that have circular thinning sections with a small perforation at their center.
Next, the authors discuss squamosal elongation in Triceratops. A series of squamosal bones are shown: almost all of them are recognizably Triceratops: L-shaped. Figure I is interpreted as being from Triceratops, but it looks very similar to figures J and K, which are labeled Torosaurus. The authors are trying to show a purported growth series, but to my eye, the Triceratops squamosals are very obviously different from the Torosaurus squamosals. Worse, a line graph showing squamosal elongation is shown (Figure 4). Triceratops subadults and “young adults” cluster around the center, as expected, but the Torosaurus individuals fall way to the right of everything else. There are no Triceratops or Torosaurus specimens straddling the gap between the two genera. It’s very obvious that Torosaurus squamosals are very different than Triceratops. Plus, one Torosaurus squamosal (ANSP 15192) sits above the cluster of Triceratops squamosals. It’s recognizably Torosaurus but is similar in length to subadult and “young adult” Triceratops? To my mind, that means Torosaurus is fundamentally different than Triceratops at a similar age.
The authors also discuss epiparietal and episquamosal morphology. In the Horner & Goodwin Triceratops paper, the authors observe that Triceratops’ epiparietals and episquamosals become incorporated into the edge of the frill over time, and that older adults no longer have spiky frill edges, but more scalloped edges where the epiparietals and episquamosals are tab-like and rounded. In every known specimen of Torosaurus where epiparietals and episquamosals are preserved, they are also heavily absorbed. However, there are some key differences. Whereas the epiparietals and episquamosals of adult Triceratops are tab-like and rounded, those of Torosaurus are elongate and somewhat flattened, with distinct squared-off edges instead of rounded edges. Did the episquamosals and epiparietals elongate along with the frill bones? I doubt it. Their distinct morphology really does seem distinct.
Torosaurus specimen ANSP 15192 is particularly interesting because it appears to represent a subadult Torosaurus: its nasal horn morphology pertains to an early growth stage based on Horner & Goodwin’s Triceratops paper, and its frill is not as long as MOR 1122 or YPM 1831, yet it still has parietal fenestrae. Of course, Horner & Scannella recognize the analogy with Triceratops but ignore the nasal horn morphology: “No ‘Torosaurus’ specimen has posteriorly curving postorbital horn cores, which would be indicative of immaturity.” However, ANSP 15192’s postorbitals do conform very well to figure (d) of that paper—what you might call a young adult. And guess what? That’s where its squamosal sits on Figure 4 of Horner & Scannella’s paper. In that same growth stage (d), the epiparietals and episquamosals of Triceratops are starting to fuse and become rounded.
I’d talk about the bone histeology of the frill, as that’s one of the more interesting aspects of the paper, but I don’t know enough about bone histeology to do so intelligently.
Horner also seems married to the idea that the dinosaurs were dwindling in diversity at the end of the Cretaceous, and he’s been whining that paleontologists have been overstating the diversity of the Hell Creek Formation for years. That very well may be, but his solution is to synonymize everything that comes out of the ground. Look at what we used to have: Nanotyrannus, Tyrannosaurus, Dracorex, Stygimoloch, Pachycephalosaurus, Triceratops, and Torosaurus. Diceratops has always been questionable. Now we might just have Tyrannosaurus, Pachycephalosaurus, and Triceratops. I’m not against that idea, I’m just saying that extraordinary claims require extraordinary evidence. While the idea that Torosaurus represents the “silverback” stage in Triceratops’ lifecycle may hold merit for future study, I don’t think that Horner & Scannella proved it beyond a reasonable doubt.
EDIT: Can I stop pressing Control-I now?