Provincial Dinosaurs

This week, a pair of authors from McGill University, Matthew Vavrek and Hans Larsson, published a paper in the prestigious Proceedings of the National Academy of Sciences in which they argued that dinosaurs living in North America during the last few million years of the Cretaceous Period were not divided up into distinct, geographically separated communities, or “provinces” (1). These authors plumbed a large storehouse of paleo data known as the Paleobiology Database in order to collect information about which kinds of North American dinosaurs lived in different locales during the Maastrichtian stage, spanning almost 6 million years (~71.3 - 65.5 million years ago). Among the varieties examined were Tyrannosaurus and Triceratops, as well as somewhat lesser known forms like the duck-billed Edmontosaurus, the long-necked Alamosaurus, the dome-headed Pachycephalosaurus, and the bony-armored Ankylosaurus. Vavrek and Larsson subjected these data to a rigorous statistical analysis, focusing in particular on the four most fossiliferous geologic units (i.e., those that have yielded more than 100 dinosaur specimens): the Horseshoe Canyon Formation of Alberta; the Hell Creek Formation of Montana and North Dakota; and the Lance Formation of Wyoming. They concluded that the evidence strongly supports the presence of a single dinosaur community inhabiting western North America during the Maastrichtian.

So what’s the big deal? Given that all of the animals noted above qualify as giants, shouldn’t we expect to find one community of dinosaurs living in Western North America during a single chunk of time? Well, yes, that would be the natural assumption, since bigger animals require more space to find sufficient food. The problem is that, beginning almost 50 years ago (2), paleontologists began noting that different varieties of Late Cretaceous dinosaurs tend to show up in the southern and northern parts of the Western Interior. For example, the giant sauropod Alamosaurus is known only from the south (e.g., New Mexico, Utah, Texas), whereas Triceratops tends to occur further north (e.g., Wyoming, Montana, Alberta, North Dakota). Thomas Lehman of Texas Tech University undertook an in depth study of the matter and concluded that dinosaurs were divided into northern and southern “provinces” during the Maastrichtian and (the preceding) Campanian stages of the Late Cretaceous (3,4,5).

Lehman expanded the scope to investigate the spectrum of vertebrate groups (animals with back bones), and found the same pattern; fishes, amphibians, lizards, turtles, crocodiles, and mammals all occurred in both north and south, but distinct representatives of these groups tended to be clumped latitudinally—that is, distinct genera and species were recovered in Alberta and Montana than in New Mexico and Texas. Even pollen fossils seemed to yield a parallel signal. Lacking any evidence of a physical barrier to north-south dispersal, Lehman hypothesized that some sort of climatic gradient must have caused the formation of unique plant communities, which in turn resulted in semi-isolated vertebrate communities.

Vavrek and Larsson’s study seems to fly in the face of any purported dinosaur provincialism. Their argument focuses attention on problems related to sampling and the vagaries of the fossil record. Based on probabilities alone, paleontologists are bound to recover the most common components of a fauna first. Sample sizes—that is, the number of specimens found—need to get relatively big before many of the rarer species are likely to show up. So any perceived differences between geologic formations and geographic regions may be due to biases in sampling rather than actual differences in the community composition. This is exactly what Vavrek and Larsson argue has occurred with our understanding of the Maastrichtian dinosaurs from North America. The paper’s closing statement sums it up: “These results suggest that dinosaurs were not as restricted in their ranges as once thought and that the fauna as a whole was largely homogenous.”

For the most part, I find these conclusions reasonable. Based on the four best-sampled geologic units of Maastrichtian age, I concur that there is no compelling evidence of geographically separated and biologically distinct dinosaur communities. I also applaud the application of rigorous statistical methods to the study of dinosaur paleobiology, something that has occurred all too infrequently in the past (once again, largely due to small samples).

Is that it then? Is the dinosaur provincialism hypothesis dead? By no means.

First off, Vavrek and Larsson focus their study only on dinosaur genera rather than species (for example, Triceratops versus Triceratops horridus). This strategy makes some sense, given that most Maastrichtian dinosaur genera contain only one species, and that many fossil specimens can only be identified to the genus level. Yet it must not be forgotten that genera are human categories with no biologically reality, and what we are really interested in here is communities of species—that is, populations of animals that are reproductively isolated from their near relatives elsewhere. In the much better sampled Campanian stage, several North American dinosaur genera (e.g., Parasaurolophus, Gryposaurus) include distinct, apparently latitudinally separated species. So it’s certainly conceivable that a similar pattern pertained to the Maastrichtian. Indeed some previous authors have raised the possibility of distinct species of Triceratops north and south within the Western Interior.

Second, the restricted quartet of geologic units (three formations in four places) used by Vavrek and Larsson covers a relatively limited latitudinal span, from southern Alberta in the north to Wyoming in the south (on the order of 10 degrees of latitude). So their study leaves wide open the possibility of distinct communities further south in Western Interior (and elsewhere on the continent). The south-only distribution of Alamosaurus is interesting here, perhaps reflecting a semi-isolated, upland, intermountain community (3,6).

Third and most important, a lack of dinosaur provincialism in the North American Maastrichtian does not negate the growing abundance of evidence documenting distinct dinosaur communities during the preceding Campanian Stage (~83.5 - 71.3 million years ago). The Campanian of the Western Interior of North America is much better sampled than the Maastrichtian, with more dinosaur species known from a 2 million year interval (about 76-74 million years ago) than for the entire 6 million years of the Maastrichtian. Moreover, in stark contrast to the Maastrichtian, most of the key fossil-bearing formations of the Campanian are now well dated, with multiple radiometric age estimated (i.e., absolute dates based on the decay of certain unstable radioactive isotopes found in volcanic ash) (7). So we can now begin to state with confidence which Campanian dinosaur species overlapped in time, but not in space.

Research conducted by our working group points to a very different picture for the Campanian than the Maastrichtian. In particular, a decade of work in the Kaiparowits Formation of Grand Staircase-Escalante National Monument (GSENM), southern Utah, has resulted in an entirely “new” dinosaur fauna (8,9), including the duck-billed Gryposaurus monumentensis (10) and the oviraptor theropod Hagryphus giganteus (11). Most of these new animals are still under study, with publications and announcements forthcoming. A comprehensive statistical analysis by Gates et al. (12), partially based on our GSENM results, finds robust support for Lehman’s hypothesis of vertebrate provincialism in the Western Interior during late Campanian. Remarkably, to date not a single dinosaur species can be confidently placed in both the south (e.g., Texas, New Mexico, Utah) and the north (e.g., Montana, Alberta).

Suffice it to say that the hypothesis of Late Cretaceous dinosaur provincialism in North America remains alive and well—and, in the opinion of our working group, robustly supported—at least for the Campanian. In my next post, I’ll address this topic again, describing some of our recent results in GSENM and probing deeper into the notion of provincial dinosaurs. Along the way, I’ll tell the story of a lost continent with grand implications for our understanding of the Mesozoic world of dinosaurs.

Notes and 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, published ahead of print doi:10.1073/pnas.0913645107.
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. 1987. Late Maastrichtian paleoenvironments and dinosaur biogeography in the western interior of North America, Palaeogeography, Palaeoclimatology, Palaeoecology, 60:189-217.
4) Lehman, T. M. 1997. Late Campanian dinosaur biogeography in the western interior of North America. Dinofest International Symposium Volume, pp. 223-24.
5) Lehman, T. M. 2001. Late Cretaceous dinosaur provinciality. In D. H. Tanke and K. Carpenter (Eds.), Mesozoic Vertebrate Life (pp.310-328) Indiana University Press.
6) Sampson, S. D. and Loewen, M. A. 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.
7) 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.
8) 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.
9) Sampson, S. D. and Loewen, M. A. 2010. Unraveling a radiation: a review of the diversity, stratigraphic distribution, biogeography, and evolution of horned dinosaurs. (Ornithischia:Ceratopsidae). Pp. 405-427 in M. J. Ryan, B. J. Chinnery-Allgeier, and D. A. Eberth (eds.), New Perspectives on Horned Dinosaurs. Indiana University Press.
10) 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.
11) 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.
12) 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.

Images
Top: Tyrannosaurus and Triceratops, by Michael Skrepnick
Upper Middle: Corythosaurus, by Michael Skrepnick
Lower Middle: Albertosaurus and Bambiraptor, by Michael Skrepnick
Bottom: Centrosaurus, Origin of a Mass Death Assemblage, by Michael Skrepnick
For more from Michael Skrepnick, go to: http://www.dinosaursinart.com/