What I'm Studying: New Data on Struthiomimus altus and the Evolution of Ornithomimids in the Belly River Group

Struthiomimus altus as imagined by Irwin Christman in 1921, courtesy of  copyrightexpired.com.

My current research project is concerned with the anatomy and evolution of Struthiomimus, an ornithomimid ("ostrich dinosaur") from the Late Cretaceous of Alberta.  A previous study on ornithomimids led by Carleton student Thomas Cullen (Cullen et al. 2013) reopened some interesting questions about how different species of ornithomimids can be distinguished, and how extensive their stratigraphic ranges were.  My master's thesis supervisors, Michael Ryan and Claudia Schröder-Adams, both also supervised that study and supported further investigation into the evolutionary history of this group.

A published figure incorporating data from the ROM Struthiomimus.

Struthiomimus altus is one of the most famous ornithomimids known to the general public, the first one to be discovered in Canada (Lambe 1902), and the first one anywhere to be known from a nearly complete skeleton (Osborn 1917). However, the type material at the Canadian Museum of Nature is fragmentary, the descriptive literature on the more complete at the American Museum of Natural History is "preliminary," and some informative newer specimens are still nearly undescribed.  One such specimen is the Royal Ontario Museum Struthiomimus, discovered by Levi Sternberg in 1934, near what is now Dinosaur Provincial Park. Some features of the ROM Struthiomimus were noted by Russell (1972), and his reconstruction of a Struthiomimus skull is a composite of it and an AMNH specimen (above).  One of my goals for this project is to fully describe the ROM Struthiomimus and compare it to other specimens.  The ROM Struthiomimus deserves special attention because:
• It is one of only two specimens referred to S. altus from the Belly River Group that has a precisely documented quarry locality (Currie & Koppelhus 2005, supplementary CD).
• It is the only specimen referred to S. altus from the Belly River Group to preserve certain portions of the skull.
• It is one of only a few specimens referred to S. altus that combines the cranial, axial and appendicular elements of a single individual.
These conditions make the ROM Struthiomimus an important specimen for evaluating the diagnostic characters of S. altus, and whether it is the same species as geologically younger occurences of Struthiomimus.

When Osborn (1917) named the genus Struthiomimus, he justified its distinction from the closely related Ornithomimus partly on the basis of its lower stratigraphic occurrence, which fit the pattern he had observed in other dinosaur groups. However, subsequent discoveries enlarged the stratigraphic ranges of both taxa, and they are now known to have overlapped (Kobayashi et al. 2006, Cullen et al. 2013). The current stratigraphic range of Struthiomimus is unusually long for a dinosaur genus, and the longevity of species, or rate of species turnover within Struthiomimus, if any, is poorly understood.  The Oldman and Dinosaur Park Formations were not recognized as formally distinct until the 1990s (Eberth & Hamblin 1993), and it is therefore uncertain which formation(s) much of the S. altus material from the Belly River Group was collected from.   I'll also be examining fragmentary ornithomimid material from the geologically older Milk River, Foremost and Oldman Formations to see if I can detect any changes in the taxonomic content or diversity of Albertan ornithomimids preceding S. altus in the fossil record.


References

Cullen, T. M., Ryan, M. J., Schröder-Adams, C., Currie, P.J. and Kobayashi, Y. 2013.  An ornithomimid (Dinosauria) bonebed from the Late Cretaceous of Alberta, with implications for the behavior, classification, and stratigraphy of North American ornithomimids. PLoS One 8 (3, e58853):1-9. 

Currie, P. J. and Koppelhus, E. B. (eds.) 2005. Dinosaur Provincial Park: A Spectacular Ancient Ecosystem Revealed. Indiana University Press, Bloomington. 

Eberth, D. A. and Hamblin, A. P. 1993. Tectonic, stratigraphic, and sedimentologic significance of a regional discontinuity in the upper Judith River Group (Belly River wedge) of southern Alberta, Saskatchewan, and nothern Montana. Canadian Journal of Earth Sciences 30: 174-200.

Kobayashi, Y., Makovicky, P., and Currie, P. 2006. Ornithomimids (Theropoda: Dinosauria) from the Late Cretaceous of Alberta, Canada.  Journal of Vertebrate Paleontology 26 (3): 86A.

Lambe, L. M. 1902. New genera and species from the Belly River Series (mid-Cretaceous). Geological Survey of Canada, Separate Report 774, Contributions to Canadian Palaeontology 2: 25-81.

Osborn, H. F. 1917. Skeletal adaptions of Ornitholestes, Struthiomimus and Tyrannosaurus. Bulletin of the American Museum of Natural History 35: 733-771.

Russell, D. A. 1972. Ostrich dinosaurs from the Late Cretaceous of western Canada. Canadian Journal of Earth Sciences 9: 375-402.