March 1, 2012
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.
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.
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