Ornithomimus: Beast of the Week

 This week we will be looking at a fast-running dinosaur that looked like modern birds. (but wasn't directly related to them) Make way for Ornithomimus!

Ornithomimus was a theropod dinosaur that lived in what is now mostly western North America, although some fragmentary fossils that appear to be from it have also been found on the east coast of the United States, including New Jersey.  It lived during the late Cretaceous period, between 76.5 and 66 million years ago, and therefore was one of the dinosaurs wiped out by the meteorite that ended the Mesozoic.  From beak to tail it would have measured about 12 feet (3.6 meters) long and was either an herbivore, or possibly an omnivore when alive.  Its genus name translates to "Bird Mimic" because of its birdlike (specifically ostrich) appearance.  

Ornithomimus edmontonicus life reconstruction in watercolors by Christopher DiPiazza.

Ornithomimus was a member of the ornithomimosaur family of theropods, characterized by their long, slender necks, long legs, long arms, and beaked faces.  Like Ornithomimus, many were toothless, but a few earlier members had small teeth.  Struthiomimus, Gallimimus, Harpymimus, and even the gigantic Deinocheirus, are all also members of this group.  Like many of its relatives, Ornithomimus had proportionally long arms with three fingers on each hand and long powerful legs, suggesting it was a fast runner.  A large number of Ornithomimus foot bones on the fossil record were found by paleontologists to be devoid of stress fractures, further supporting the idea they were adapted to running in life. Ornithomimus also had a slender beak with proportionally large eye sockets, suggesting it had strong eyesight in life.  

Ornithomimus edmontonicus skeleton on display at the Royal Tyrell Museum in Alberta, Canada.

Ornithomimus is currently known from two species.  The earliest of the two, called Ornithomimus edmontonicus, lived between 75.5 to 72 million years ago, in what is now Alberta, Canada.  It would have coexisted with other dinosaurs, like Albertosaurus, Anodontosaurus, Pachyrhinosaurus, Sphaerotholus, Hypacrosaurus, and its close relative, Struthiomimus.  The later species, Ornithomimus velox, was the slightly smaller species based on known material, and lived more in what is now the United States, from the very end of the late Cretaceous.  This species would have crossed paths with Tyrannosaurus, Triceratops, Pachycephalosaurus, Anzu, and Ankylosaurus to name a few, and like them, would have gone extinct when the asteroid that ended the Mesozoic hit the earth, 66 million years ago.  In the past Ornithomimus actually included a whopping seventeen more species that have since mostly been lumped into one of the two previously mentioned, or found to be different genera.

Fossilized remains of Ornithomimus feathers circled in yellow.  Specimen at the Royal Tyrell Museum.

For decades most experts suspected Ornithomimosaurs, like Ornithomimus, sported feathers in life based on its resemblance to modern birds and the presence of fossilized feathers in other kinds of dinosaurs.  More recently, however, this hypothesis was confirmed thanks to not one, but several separate specimens of Ornithomimus edmontonicus that sport fossilized remains of feathers.  Thanks to these exquisite fossils, we now know that young Ornithomimus had shaggy down-like feathers on their bodies, much like many modern birds.  We also know that adult Ornithomimus sported long feathers on its lower arms, like the wings of modern ostriches. It is worth noting that the Ornithomimosaurs were NOT direct ancestors of modern ostriches or any other birds (modern types of birds already existed during the time ornithomimosaurs were alive), but a beautiful example of convergent evolution, when two different kinds of animals independently evolve similar features and end up looking more closely related than they really are.

References

Makovicky, P.J., Kobayashi, Y., and Currie, P.J. (2004). "Ornithomimosauria." In Weishampel, D.B., Dodson, P., & Osmólska, H. (eds.), The Dinosauria (second edition). University of California Press, Berkeley: 137-150.

Rothschild, B., Tanke, D. H., and Ford, T. L., 2001, Theropod stress fractures and tendon avulsions as a clue to activity: In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, p. 331-336.

Zelenitsky, D. K.; Therrien, F.; Erickson, G. M.; Debuhr, C. L.; Kobayashi, Y.; Eberth, D. A.; Hadfield, F. (2012). "Feathered Non-Avian Dinosaurs from North America Provide Insight into Wing Origins". Science. 338 (6106): 510–514.