Dinosaurs of the Lost Continent

In my last post, I addressed the hypothesis of dinosaur provincialism—that is, isolated communities of giant dinosaurs—on North America during the Late Cretaceous. Specifically, I discussed a recent paper by Vavrek and Larsson (1), who concluded on the basis of a rigorous statistical analysis that Maastrichtian dinosaurs (i.e., those living in the final 6 million years of the Mesozoic immediately prior to the K-T extinction) were not split into isolated provinces, as argued previously, but rather formed a single community. I cautiously agreed with their findings and noted a few caveats. For example, their study was restricted geographically—limited to the four most dinosaur-rich geologic units in the northern part of the Western Interior—leaving plenty of terrain for possible dinosaur provinces elsewhere.

Today I want to focus on another of my caveats: dinosaur provincialism in the preceding Campanian Stage (83.5 – 70.6 million years ago). If your goal is to understand large scale patterns and processes in the Mesozoic world of dinosaurs, the best place and time to look for answers is Campanian-aged rocks from the Western Interior on North America. Period. Nowhere else on Earth do we have continent-scale sampling of fossiliferous formations from the Mesozoic that have been intensively worked for more than a century. Literally dozens of different kinds of dinosaurs, many of them exceptionally preserved, have been recovered from as far north as Alaska and as far south as Mexico. And the great bulk of these occur in a 2 million year window of time, between about 77 and 75 million years ago.

The importance of this particular slice of Mesozoic time and space is heightened by the geographic context of the fossils. The Mesozoic was a hothouse world virtually devoid of polar ice caps. As a result, sea levels tended to be much higher than they are today, often flooding low-lying regions of continents with shallow seas. In North America, the Cretaceous Western Interior Seaway, extended from the Arctic Ocean in the north to the Gulf of Mexico in the south. For almost 30 million years (~95-67 million years ago), this so-called “epeiric” sea subdivided the continent into eastern and western landmasses, known as Appalachia and Laramidia, respectively. We know little of the dinosaurs of Appalachia, but geologic activity in the west has exposed an abundance of Campanian rocks along the eastern margin of the “lost continent” of Laramidia. Key geologic units that have yielded dinosaurs include the Dinosaur Park Formation of Alberta, the Two Medicine and Judith River formations of Montana, the Kaiparowits Formation of Utah, the Kirtland and Fruitland formations of New Mexico, and the Aguja Formation of Texas.

Campanian Laramidia was home to arguably the greatest known fluorescence of dinosaurs, and the emerging diversity patterns are stunning. On the one hand, wherever we look, the same groups of dinosaurs tend to show up. Bird-hipped herbivores include horned dinosaurs (ceratopsids), duck-billed dinosaurs (hadrosaurs), smaller ornithopod dinosaurs (hypsilophodonts), armored ankylosaurs (nodosaurids and ankylosaurids), and dome-headed dinosaurs (pachycephalosaurs). Lizard-hipped theropods are also very diverse, including ostrich-like ornithomimids, beaked oviraptorosaurs, and sickle-clawed dromaeosaurs and troodonts, with giant tyrannosaurs invariably filling the role of top predator. (Although poorly known, a great diversity of smaller-bodied theropods, such as feathered microraptorines and birds were almost certainly present.) On the other hand, the species-level representatives of these groups appear to be limited to small ranges. In particular, as first argued by Dale Russell (2), and later by Thomas Lehman (3), we find different genera and species in the north (Alberta and Montana) than we do in the south (Utah, New Mexico, and Texas). So, for example, Chasmosaurus is a horned dinosaur known only from the north, whereas Pentaceratops is limited to the south. Similarly, some species of Gryposaurus are known only from the north, whereas at least one species is restricted to the south.

Until recently, a relative dearth of identifiable dinosaurs from southern Laramidia made it difficult to test the provincialism hypothesis. This “southern gap” has now been partially remedied by a decade of fieldwork in the Kaiparowits Formation, abundantly exposed in Grand Staircase-Escalante National Monument, southern Utah. Working in close collaboration with the Bureau of Land Management, our interdisciplinary team—based out of the University of Utah, but including researchers from multiple institutions—has unearthed an entirely “new” assemblage of dinosaurs in the Kaiparowits,16 different dinosaur varieties so far, 11 of which can now be identified to the level of species (4). Several of these animals are represented by exceptionally preserved skulls and partial skeletons, often with skin impressions. Some of these species have been named and described, including the oviraptorosaur Hagryphus giganteus (5) and the hadrosaur Gryposaurus monumentensis (6). Other studies are nearing completion or well under way. I am happy to say that the discoveries keep on coming; the most recent field season yielded a plethora of amazing finds from a single quarry. First to be found was a hadrosaur about the size of T.rex; excavation of this specimen, which includes a nearly complete skull, yielded one skull and skeleton of a crocodilian and another of a turtle, as well as an ankylosaur with what appears to be an intact skull, and a possible pterosaur! All this from a single (albeit very large) site.

Remarkably, of the dozens of dinosaur species now identified from the Campanian of Laramidia, none can be confidently placed in both the north and the south—strong support for the notion of dinosaur provincialism. Sullivan and Lucas (7) argued previously that this provincialism is illusory, the result of animals arrayed in time rather than space. But recent advances in both the number and precision of radiometric dates (8) conclusively demonstrate temporal overlap of key formations (e.g., Dinosaur Park and Kaiparowits), as well as species belonging to particular groups (e.g., horned dinosaurs, hadrosaurs, and tyrannosaurs). A recent faunal review and statistical analysis by our working group (9) supports earlier claims from Lehman, showing that the late Campanian provincialism extends well beyond dinosaurs to encompass a variety of vertebrates.

These findings have profound implications for our understanding of dinosaur ecology and evolution. The “island continent” of Laramidia was less than 20% the size of present day North America. Much of this landmass was covered with rising mountain ranges (primarily the Cordilleran Overthrust Belt, but perhaps the Laramide orogeny as well), sandwiching known Laramidian dinosaurs between a restless seaway to east and rising mountains to the west. So it’s remarkable to contemplate the notion of one diverse assemblage of dinosaurs—many with body masses in the rhino-to-elephant range—let alone multiple assemblages of such animals. To add insult to injury, some of these dinosaurs, especially among the hadrosaurs and ceratopsids, appear to have lived in large herds numbering at least in the hundreds of animals. How could so many giants make a living and persist over geologic time spans on such a diminutive landmass. Likely answers involve greater volumes of available plant food (primary productivity) and/or decreased dietary needs relative to modern-day warm-blooded mammals.

Given that most northern and southern dinosaur species within a given group appear closely similar, differing primarily in features associated with reproductive success (horns, frills, crests, etc.), they may well have played similar ecological roles. For example, the long-frilled horned dinosaurs (chasmosaurines) in the north and south, although distinct species, may well have consumed very similar kinds of plants. If so, the ecological niches filled by dinosaurs might have changed very little for millions of years during the Late Cretaceous. Behind this apparent ecological stasis, however, a variety of factors—perhaps including seaway migrations and other environmental changes—appear to have resulted in rapid evolutionary turnover of species (10). Like a long-running Broadway show, the players changed while the same story played out endlessly.

We have only begun to plumb the depths of knowledge relating to Laramidian dinosaurs. And we can count on many surprises to come. I will use this blog as an outlet to update readers on new discoveries as they are announced.

References
1) Vavrek, M. J. and Larsson, H.C. E. 2010. Low beta diversity of Maastrichtian dinosaurs of North America. Proceedings of the National Academy of Sciences,
2) Russell, D. A. 1967. A census of dinosaur specimens collected in western Canada, National Museum of Canada Natural History Papers, 36:1-13.
3) Lehman, T. M. 1997. Late Campanian dinosaur biogeography in the western interior of North America. Dinofest International Symposium Volume, pp. 223-24.
4) Sampson, S. D., Gates, T. A., Roberts, E. M., Getty, M. A., Zanno, L. E., Loewen, M. A., Smith, J. A., Lund, E. K., Sertich, J., and Titus, A. L. in press. Grand Staircase-Escalante National Monument: A new and critical window into the world of dinosaurs. Learning from the Land Symposium Symposium Proceedings.
5) Zanno, L. E. and Sampson, S. D. 2005. A new oviraptorosaur (Theropoda: Maniraptora) from the late Campanian of Utah and the status of the North American Oviraptorosauria. Journal of Vertebrate Paleontology, 25(4): 897-904.
6) Gates, T. A. and Sampson, S. D. 2007. A new species of Gryposaurus (Dinosauria: Hadrosauridae) from the Upper Campanian Kaiparowits Formation of Utah. Zoological Journal of the Linnean Society, 151:351-376.
7) Sullivan, R. M. & Lucas, S. G. 2006. The Kirtlandian land-vertebrate "age" – faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America. New Mexico Museum of Natural History Science Bulletin 35, 7-29.
8) Roberts, E.M., Deino, A.D., and Chan, M.A. 2005a. 40Ar/39Ar age of the Kaiparowits Formation, southern Utah, and correlation of coeval strata and faunas along the margin of the Western Interior Basin: Cretaceous Research, 26:307-318.
9) Gates, T.A., Sampson, S.D., Zanno, L.E., Roberts, E.M., Eaton, J.G., Nydam, R.L., Hutchison, J.H., Smith, J.A., Loewen, M.A., and Getty, M.A. in press. Biogeography of terrestrial and freshwater vertebrates from the Late Cretaceous (Campanian) Western Interior of North America: new information from the Kaiparowits Formation, south-central Utah. Palaeogeography, Palaeoclimatology, Palaeoecology.
10) Sampson, S. D. 2009. Dinosaur Odyssey: Fossil Threads in the Web of Life. University of California Press), 332 pp.
(Note: One of the book's chapters is dedicated to the story of Laramidian dinosaurs.)

Images (from top to bottom)
1) Late Cretaceous (Campanian) North America, showing the Cretaceous Western Interior Seaway subdividing North America into Laramidia (western landmass) and Appalachia (eastern landmass). Image credit: Ron Blakey http://jan.ucc.nau.edu/~rcb7/
2) The Kaiparowits Formation, Grand Staircase-Escalante National Monument, southern Utah. Image credit: Rebecca Hunt-Foster.
3) Reconstruction of the oviraptorosaur Hagryphus giganteus. Image credit: Michael Skrepnick.
4) The skull of Gryposaurus monumentensis, a new duck-billed dinosaur from the Kaiparowits Formation.
5) The skull of an unnamed horned dinosaur from the Kaiparowits Formation.