Smithsonian.com

Triceratops (left) and Torosaurus (right). Art by Nicholas Longrich.

More than 120 years ago, the Yale paleontologist Othniel Charles Marsh described two of the most spectacular horned dinosaurs of all time. The first, named Triceratops in 1889, had three impressive horns jutting out of its face and a solid, curved frill. Two years later, Marsh named Torosaurus, another great, three-horned dinosaur, but with a longer frill perforated by two round holes. Although the two overlapped in space and time, they seemed distinct enough that paleontologists considered them to be separate dinosaur genera. That is, until Museum of the Rockies paleontologists John Scannella and Jack Horner suggested that these two dinosaurs were really one in the same.

Scannella and Horner presented their “Toroceratops” hypothesis at the 2009 Society of Vertebrate Paleontology meeting in Bristol, England, and the following summer their paper came out. Based on skull anatomy, bone microstructure and other lines of evidence, the paleontologists proposed that Marsh’s Torosaurus was really the skeletally mature form of Triceratops. As Triceratops grew, the dinosaur’s frill would have changed size and shape, and those trademark Torosaurus holes would have opened up. An enigmatic fossil named Nedoceratops seemed to show this intermediate anatomy and was cited by Scannella and Horner as a dinosaur caught in the act of changing. Poor reporting on the research sent the public into a tizzy—Triceratops fans wept, wailed and gnashed their teeth at the suggestion that paleontologists were taking away one of their favorite dinosaurs, but only those with an affinity for Torosaurus had anything to fear. Since Triceratops was named first, the name had priority and Torosaurus would therefore be sunk. (No one seemed to care a whit that poor, neglected Nedoceratops would suffer the same fate.)

But should we sink Torosaurus? In the two years since Scannella and Horner’s paper came out, paleontologists have gone back and forth about whether such a radical, late-life transformation in Triceratops was even possible. Early last year, ceratopsian expert Andrew Farke of the Raymond M. Alf Museum of Paleontology criticized the Triceratops transformation hypothesis and pointed out that Nedoceratops did not actually fit neatly into the sequence of changes Scannella and Horner had proposed. Naturally, the Museum of the Rockies paleontologists disagreed, and in a response published in December of 2011, Scannella and Horner reaffirmed the relevance of Nedoceratops to the extreme changes Triceratops might have undergone as it grew up.

Now another set of challengers has appeared. In a paper published last night in PLoS One, Yale University paleontologists Nicholas Longrich and Daniel Field concluded that Triceratops and Torosaurus truly were distinct dinosaurs, after all.

Most of what we know about Triceratops and Torosaurus has been extracted from skulls. Post-cranial skeletons are rare and, in the case of Torosaurus, incompletely known, and so the current argument is centered on how the skulls of these horned dinosaurs changed. In the new study, Longrich and Field coded twenty four different characteristics—relating to bone surface texture, fusion between skull bones, and other features—in a swath of Triceratops and Torosaurus skulls. The paleontologists then used this data to sort the different specimens into growth stages based on their cranial development. If Torosaurus truly represented the mature form of Triceratops, then all the Torosaurus should have come out as adults.

The skulls of Torosaurus YPM 1831 and Triceratops YPM 1822 compared. Image courtesy Nicholas Longrich.

Of the six Torosaurus examined, five fell into a range between young and old adults. But there was one particularly large individual that seemed to be significantly younger. When Andrew Farke issued his critique of the “Toroceratops” hypothesis last year, he noted that a skull designated YPM 1831 was a possible candidate for a young Torosaurus. The paper by Longrich and Field supported this idea—YPM 1831 grouped with the subadult dinosaurs. “It’s a little surprising considering how damn big the skull is—probably about nine feet long—but it’s not fully mature,” Longrich said. “It’s like a teenager,” he noted, “a physically big animal but not all that mature yet.” The development of ornaments on the skull, the fact that some bones are not fused, and a bone texture associated with rapidly growing bone are possible signs that this dinosaur was not yet an adult.

If YPM 1831 really was a subadult Torosaurus, then it is probable that Triceratops and Torosaurus were distinct dinosaurs. Indeed, if Torosaurus truly was the fully mature form of Triceratops, then we should not find any juvenile or subadult Torosaurus specimens. “[B]oth Torosaurus and Triceratops,” Longrich and Field concluded, “span a range of ontogenetic stages,” and the features which distinguished each dinosaur appear to have developed before full maturity.

But Scannella disagrees. “Nothing in this paper falsifies the synonymy of ‘Torosaurus‘ and Triceratops,” he says. In particular, Scannella notes that the new study relies on comparative anatomical techniques, but does not employ studies of dinosaur bone microstructure which shows how individual skull bones were changing. Scannella explained:

Comparative morphology is useful in examining dinosaur ontogeny, however it shouldn’t be considered in a vacuum. There are other factors which provide a wealth of information on dinosaur growth. For example, by examining histology, the microstructure of the bones, we can actually see how the thick, solid frill of Triceratops expanded, became thinner, and developed the characteristic holes of the ‘Torosaurus‘ morph. You can look at a Triceratops squamosal under a microscope and see how it was transforming. We are also finding that the stratigraphic position of specimens is critical to understanding morphological trends.

Other subtle skull modifications are also in contention, such as how fusion between bones in the skull relates to maturity. Among other features, Longrich and Field looked at the fusion of skull bones to help determine which age bracket particular specimens fell within. “We think that what the fusions are telling you is that growth has slowed,” Longrich explained, “because you can no longer deposit new bone between those bones. This seems to be a fairly reliable indicator of maturity in relatively fast-growing animals like lizards, mammals, and birds.” In the case of Triceratops and Torosaurus, skull fusion seemed to occur in a particular sequence. “First the skull roof is fused, next the hornlets on the frill and cheeks fuse, then the beak and the nose fuse on. It’s a very regular pattern which suggests we can use this as a reliable way of getting at roughly where the animals fit in the developmental series,” Longrich said.

Yet Scannella and Horner have previously argued that the timing and degree of skull bone fusion aren’t as clear. Recently discovered specimens are contributing to the picture of how variable skull fusion might be. “The Museum of the Rockies has collected over a hundred new Triceratops from the Hell Creek Formation of Montana in the last decade,” Scannella said, and these specimens indicate that the details of skull fusion varies between individuals. “We have some huge, fairly mature Triceratops in which much of the skeleton is unfused; and there are also smaller, less mature specimens with many skeletal elements fused,” Scannella explained.

How the skulls of dinosaurs like Triceratops fused is not yet entirely clear, but, according to Andrew Farke, the degree of fusion between skull bones might be reliable for getting a general idea of how old an animal was. “There is little argument that the individual bones of the braincase tend to be unfused in young animals, and fused in old animals,” Farke pointed out, and further explained that “The same goes for the hornlets (epinasals and epijugals) on the face of ceratopsian dinosaurs,” he said, since “young animals tend to have unfused hornlets and old animals have fused hornlets.” Such features are what made the YPM 1831 Torosaurus stand out as a possible subadult to Farke’s eye.

Exactly which dinosaur YPM 1831 represents remains uncertain. The skull is the best candidate so far for a teenage Torosaurus, but this ambiguous specimen alone cannot end the debate. In fact, we have so much left to learn about Triceratops and Torosaurus—particularly about how their post-cranial skeletons changed as they aged—that a great deal of exploration and description remains to be done before this debate can be resolved. And this isn’t the only dinosaur name game in progress. The tiny tyrant “Raptorex” may have been a juvenile Tarbosaurus, the huge Anatotitan likely represents a mature Edmontosaurus, Titanoceratops was probably a big Pentaceratops, and the thick-skulled Dracorex and Stygimoloch might represent early growth stages of Pachycephalosaurus. Some of these changes sting—both Torosaurus and Anatotitan were childhood favorites of mine, and I’d hate to see them go—but, ultimately, these debates will help us better understand how dinosaurs grew up.

References:

Longrich, N., & Field, D. (2012). Torosaurus Is Not Triceratops: Ontogeny in Chasmosaurine Ceratopsids as a Case Study in Dinosaur Taxonomy PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0032623

The skull of Nedoceratops in oblique view, snout-view, and a line drawing (with grey areas denoting where the skull was reconstructed). From Farke, 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. “[N]obody had ever published a full scientific description of the [Nedoceratops] 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.

“[N]obody 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

Two Triceratops skulls. The one on the left represents the classic, young-adult form, and the one on the right represents the fully mature form (previously called Torosaurus). From the Journal of Vertebrate Paleontology paper.

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