September 22, 2010
At the height of the golden era of dinosaur science, it takes something special for a newly described dinosaur species to stand out. Dinosaurs with dual sickle claws, humps, or unexpected bristles more readily grab the attention of the public than more familiar-looking forms, but looks aren’t everything. A pair of horned dinosaurs described today in PLoS One are noteworthy for reasons that go beyond their strange appearances.
A few months ago, I was fortunate enough to visit the Utah Museum of Natural History’s paleontology field crew at their camp in southern Utah’s Grand Staircase-Escalante National Monument. The arid, rocky landscape was rich in dinosaur fossils, but 75 million years ago this same place would have looked very different. During that time in earth’s history, near the close of the Cretaceous, a shallow sea over the center of North America divided the landmass into two separate continents: Appalachia to the east and Laramidia to the west. The area which is today Grand Staircase-Escalante National Monument was in the middle of the thin western continent, and according to Scott Sampson, University of Utah paleontologist and lead author of the new paper, the place was “a wet, swampy setting akin to present-day northern Louisiana.” Crocodiles, turtles, and at least 16 unique species of dinosaurs thrived here, including the two horned dinosaurs announced today.
Named Utahceratops gettyi and Kosmoceratops richardsoni, respectively, the two dinosaurs belonged to a subdivision of the horned dinosaur family called chasmosaurines. Roughly speaking, this group of horned dinosaurs can often be identified by their large brow horns and their wide, squared-off frills, and both Utahceratops and Kosmoceratops fit the classic chasmosaurine type. Nevertheless, both were distinct from species previously recognized and were significantly different from each other. While Utahceratops had relatively short brow horns in front of a large frill that was slightly indented inwards along its top margin, the slightly smaller Kosmoceratops had longer brown horns and an array of spikes that spilled forwards over its frill like a chasmosaurine comb-over.
In contrast to more familiar chasmosaurines like Triceratops, though, the browhorns in both Utahceratops and Kosmoceratops were oriented out to the side rather than forward. Why this should be so is unclear. Co-author Andrew Farke of the Raymond M. Alf Museum of Paleontology says, “it’s hard to know for sure, but in modern horned animals horn orientation usually relates to horn function.” Perhaps Utahceratops and Kosmoceratops had similar styles of combat with members of their own species, Farke hypothesizes, or maybe the peculiarity was inherited in both from a common ancestor. Likewise, according to Sampson, the sideways orientation of these horns is also seen in another horned dinosaur from Coahuila, Mexico, and may be a common characteristic of chasmosaurines from this part of Laramidia. Among these dinosaurs, “the sideways-oriented horns offer another means to lock heads and engage in contests of dominance,” Sampson says; “they would also have made effective visual signals, particularly in Kosmoceratops.”
Utahceratops and Kosmoceratops were also very different from the horned dinosaurs that lived in the same area a few million years earlier. While both of the new dinosaurs came from the Kaiparowits Formation, horned dinosaurs of a different sort have been found in the national monument’s 80-million-year-old Wahweap Formation. These older dinosaurs, such as the many-horned Diabloceratops, belonged to another branch of the horned dinosaur family tree called the centrosaurines. “If you were to line up the skulls [of the Wahweap and Kaiparowits dinosaurs] side by side,” Farke says, “they would look strikingly different!” especially since the two newly described dinosaurs had relatively larger and more ornamented frills.
Given the degree of completeness of both dinosaurs (entire skulls for both, the majority of the skeleton of Utahceratops, and the skeleton except the tail, feet, and forelimbs in Kosmoceratops), their description is a major contribution to our understanding of ceratopsid anatomy and diversity. What makes them especially noteworthy, however, is that they confirm the existence of disparate pockets of dinosaur evolution along the western continent of Laramidia. These genera were not evenly spread from Mexico to Canada, but were distinct from the horned dinosaurs that lived at the same time in the northern part of the continent. This is not just an accident of sampling, the authors of the new study propose, but a signal of a real biogeographical phenomenon.
The picture that emerges from the distribution of the horned dinosaurs around 75 million years ago is that there were at least two separate centers of chasmosaurine evolution on the western continent. Where there were Chasmosaurus, the recently-described Mojoceratops and Vagaceratops (previously Chasmosaurus irvinensis, but renamed by the authors of this study) to the north, Utahceratops, Kosmoceratops and Pentaceratops lived in the far south. Based on the distribution of these dinosaurs in space and time, Sampson, Farke and their colleagues propose that about 77 million years ago there was a barrier that separated chasmosaurines in the north from those in the south. This barrier would have precipitated the divergent evolution of the northern and southern groups identified in the study, though the close relationship of Kosmoceratops and Vagaceratops found by the scientists suggests that these dinosaurs shared a close common ancestor that spread after this impediment was removed about 75.7 million years ago. As Sampson states by e-mail, the recognition of this patten raises some important questions about dinosaur evolution on Laramidia:
The implications of this finding huge, given that Laramidia was a landmass about one-third the size of present day North America, and much of this area was likely off limits to large dinosaurs because of large mountain ranges to the west. Today we have a handful of rhino-to-elephant sized mammals living on Africa. At present, it seems that there were at least 15-20 rhino-to-elephant sized animals living on Laramidia 76 million years ago, despite the fact that it was less than one-fifth the size of Africa. How did so many species of giants co-exist on such a small chunk of land? The answer may relate to available fodder (more biomass than at present) and/or to metabolic rates (slower in dinosaurs than in mammals). Either way, these finding suggest that the hothouse world of dinosaurs was, at least in some respects, very different than the one we know today.
Furthermore, if the pattern of evolution in these horned dinosaurs really was influenced by the existence of an ancient barrier, then the same signs of isolation should be seen among other dinosaurs, as well. Hadrosaurs and tyrannosaurs probably would have been isolated in the same way, and the discovery and description of additional dinosaurs from Grand Staircase-Escalante National Monument will be pivotal in further tests of this idea. Determining just what the barrier might have been, though, is another mystery. At present there is no sign of an actual physical barrier, though the authors of the new study suggest a previously-unidentified mountain range, temporary flooding by the nearby sea, a turbulent river system, or some kind of ecological barrier as possibilities.
The story of Laramidia will not be resolved by one paper alone, but will take years of study by many paleontologists. This is a good thing. Through something as simple as the description of two dinosaurs, paleontologists have placed the fauna of an entire lost continent in a new context, and, armed with a new set of questions, paleontologists can return to the strata of southern Utah in search of answers.
Sampson, S., Loewen, M., Farke, A., Roberts, E., Forster, C., Smith, J., & Titus, A. (2010). New Horned Dinosaurs from Utah Provide Evidence for Intracontinental Dinosaur Endemism PLoS ONE, 5 (9) DOI: 10.1371/journal.pone.0012292
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