The Mysterious Torosaurus

What is Torosaurus? The answer depends on who you ask. While it is certain that the dinosaur was one of the biggest and most impressive horned dinosaurs of the Late Cretaceous, paleontologists are now actively debating whether Torosaurus was a distinct genus of dinosaur or the fully mature growth stage of the more famous Triceratops. Anatomy, bone microstructure and the geological context of the two dinosaurs make up part of the ongoing discussion, but there is one key bit of evidence that has remained in the background: a little-known species from Utah.

When paleontologists John Scannella and Jack Horner proposed that Torosaurus was really a grown-up Triceratops in the Journal of Vertebrate Paleontology last year, the immediate public reaction was one of despair that scientists were taking away another beloved dinosaur. This was not actually the case—Triceratops was named first and so its name would have priority if the hypothesis of Scannella and Horner is confirmed—but, frustratingly, the myth that paleontologists are trying to toss Triceratops remains. The confusion over the dinosaur name game has obscured some of important details of the “Toroceratops” hypothesis. The debate has been discussed in the context of dinosaur genera, for instance. But within each genus, Triceratops and Torosaurus, are two species, and all four species are relevant to the fate of Torosaurus.

In last year’s paper, Scannella and Horner proposed that the species Torosaurus latus was synonymous with Triceratops. It wasn’t clear which individual Torosaurus latus specimens should be referred to which Triceratops species: Triceratops horridus or Triceratops prorsus. But the overlap of Torosaurus latus with both Triceratops species in time and space was used as a supporting argument for why Torosaurus should be synonymized with Triceratops.

The paleontologists also briefly mentioned a second, southern species of Torosaurus. Fragmentary fossils of the dinosaur Torosaurus utahensis have been found in Utah, New Mexico and Texas from sites where no Triceratops remains have ever been found. If this geographic separation is real, and Torosaurus utahensis really is a valid species of Torosaurus, then this little-known dinosaur will have an important role to play in the wider argument over whether paleontologists have named too many dinosaurs.

While Torosaurus latus and both species of Triceratops were found and described by paleontologist O. C. Marsh during the great “Bone Wars” of the late 19th century, Torosaurus utahensis was a more recent discovery. In 1946, a monograph on the “Reptilian Fauna of the North Horn Formation of Central Utah” was published by Charles W. Gilmore. (Though this was a posthumous publication likely completed by a colleague; Gilmore had passed away the previous year.) The formation that was the focus of Gilmore’s attention represents the latest Cretaceous—a finding supported by the more recent discovery of Tyrannosaurus rex within it—and is about the same age as the northern formations that have yielded Triceratops and Torosaurus latus. At the time that Gilmore was working, though, the dinosaurs of the North Horn Formation were little known, and among the enigmatic specimens were remains from up to 11 individuals of an unknown horned dinosaur. The bones were so fragmentary that it was difficult to tell whether they were something entirely new or should be assigned to an already existing dinosaur, so Gilmore tentatively presented the remains as a new species of an already known dinosaur genus: Arrhinoceratops? utahensis.

Three decades later, paleontologist Douglas Lawson reassigned Gilmore’s dinosaur to Torosaurus utahensis on the basis of several skull characteristics, with one of the main differences from the northern Torosaurus latus being a proportionally shorter squamosal bone (the broad bones that make up the borders of the dinosaur’s big frill). The trouble is that many specimens assigned to Torosaurus utahensis are so fragmentary that it is nearly impossible to confirm whether they belong to this dinosaur or to another. Many are only identifiable as chasmosaurines, the horned dinosaur subgroup to which Torosaurus, Triceratops and others belong. Since the most distinctive parts of Torosaurus specimens are their frills, specimens lacking these parts can be infuriatingly difficult to assign. (In a 2005 reassessment of the original material found by Gilmore and specimens assigned to Torosaurus utahensis, Robert Sullivan and colleagues concluded that the species was only definitively known from the type specimen from Utah’s North Horn Formation. The other proposed specimens could not be confirmed.) Perhaps the difficulties could be mitigated by the discovery of a complete or near-complete specimen of Torosaurus utahensis, but at the moment, there are a number of specimens which may or may not be referable to this dinosaur.

Torosaurus utahensis is obviously a problematic dinosaur, but this doesn’t mean that it is irrelevant to arguments over the growth stages of Triceratops. Depending on whether the dinosaur is confirmed as valid or is synonymized with a different genus or species, the horned dinosaur may help resolve the great Toroceratops debate. A few clues were reported by ReBecca Hunt-Foster and Thomas Lehman in 2008. The paleontologists described a bonebed of horned dinosaurs found in the Javelina Formation of Texas. Thirty-seven identifiable skeleton elements were found from at least three individual animals, hypothesized to be one juvenile and two adults.

Based on Gilmore’s original specimen and the new elements found in Texas, Hunt-Foster and Lehman proposed that Torosaurus utahensis is distinguishable from Torosaurus latus in exhibiting a thickened bar of bone on the squamosal bone along the suture with the neighboring parietal bone (which makes up the middle part of the frill and is the bone containing the large holes that help distinguish Torosaurus from Triceratops) and a small bone called the epiparietal at the midline of the frill. Perhaps these features will be enough to distinguish the two Torosaurus species, or perhaps the more recently named species will be lumped into Torosaurus latus, but the existence of Torosaurus in a place where Triceratops is absent may help affirm the unique nature of Torosaurus.

The case that the dinosaurs we have called Torosaurus are simply fully mature Triceratops relies on the hypothesis that we are not going to find juvenile, sub-adult or young adult Torosaurus. If definitive juvenile specimens of Torosaurus are found then the large-frilled form cannot be considered the fully-grown stage of Triceratops. Paleontologist Andrew Farke recently pointed out one possible specimen of a subadult Torosaurus latus in the collections at Yale, and some of the bones described by Hunt-Foster and Lehman may belong to juvenile or sub-adult Torosaurus utahensis. The Yale skull requires further study, and the bones from Texas are too fragmentary to resolve the issue (near-complete skulls, or at least well-preserved frills, are needed), but they hint that young Torosaurus specimens may already rest in museum collections or might still await discovery in the field.

Perhaps, now that paleontologists are looking, Torosaurus may become known from its own growth series. Such a collection would allow paleontologists to compare how both Triceratops and Torosaurus grew up and visualize when the prominent adult traits of  each species became established. Then again, perhaps Torosaurus utahensis will turn out to be a different genus of dinosaur, and maybe Torosaurus latus will be sunk into Triceratops. There are a number of ways that the debate could be resolved. Further study is needed, and we could certainly use better specimens of Torosaurus utahensis. Until we really know what Gilmore’s enigmatic horned dinosaur truly is, those of us waiting to learn the fate of Torosaurus will be left in suspense.

References:

Farke, A. 2007. Cranial osteology and phylogenetic relationships of the Chasmosaurine Ceratopsid Torosaurus latus. In Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Bloomington: Indiana University Press. pp 235-257

Gilmore, C.W. 1946. Reptilian Fauna of the North Horn Formation of Central Utah. United States Geological Survey Professional Paper, 210-C, 53 p.

Hunt, R., & Lehman, T. (2008). Attributes of the Ceratopsian Dinosaur Torosaurus, and New Material from the Javelina Formation (Maastrichtian) of Texas Journal of Paleontology, 82 (6), 1127-1138 DOI: 10.1666/06-107.1

Sampson, S., & Loewen, M. (2005). Tyrannosaurus rex from the Upper Cretaceous (Maastrichtian) North Horn Formation of Utah: biogeographic and paleoecologic implications
Journal of Vertebrate Paleontology, 25 (2), 469-472 DOI: 10.1671/0272-4634(2005)0252.0.CO;2

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

SULLIVAN, R., BOERE, A., & LUCAS, S. (2005). REDESCRIPTION OF THE CERATOPSID DINOSAUR TOROSAURUS UTAHENSIS (GILMORE, 1946) AND A REVISION OF THE GENUS Journal of Paleontology, 79 (3), 564-582 DOI: 10.1666/0022-3360(2005)0792.0.CO;2

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.