Struthiomimus: Beast of the Week

All birds are dinosaurs, but not all dinosaurs were necessarily birds!  This week's beast is a perfect, yet confusing, example of that.  Let's check out Struthiomimus!

Struthiomimus was a theropod dinosaur that lived in what is now North America, specifically Alberta, Canada, and parts of the Western United States, during the Late Cretaceous Period.  There are currently three recognized species of Struthiomimus on the fossil record, which lived during the span of 75 to 66 million years ago.  The most well-known of the three species, Struthiomimus altus, was the oldest, who's fossils are around 75 million years old.  From beak to tail, Struthiomimus measured about fourteen feet long.  The name Struthiomimus, translates to "Ostrich Mimic" in reference to...well...just look at it!

Sitting Struthiomimus altus life reconstruction by Christopher DiPiazza.

I like to bring up convergent evolution a lot on my site.  In case you haven't read about it before on here, or anywhere else, convergent evolution is when two different species of organisms, in this case animals, look very similar because they live similarly, but do not share a close common ancestor.  They each evolved to look the same way independently.  For a good modern example check out a fish and a whale, or a flying squirrel and a sugar glider, or a monitor lizard and a tegu, or a hedgehog and a porcupine...the list goes on!  Well, Struthiomimus is possibly the best example of convergent evolution in the dinosaur world because it looks so much like a modern ostrich!

You might be thinking "Hey!  But an ostrich is a modern dinosaur!  They probably are directly related to one another!" but they're not!  Struthiomimus, although a theropod, and therefore very closely related to birds, did not go on to evolve into ostriches we see today nor any other kind of bird for that matter.  It went extinct at the end of the Cretaceous like the rest of the non-avian dinosaurs.  In fact, when Struthiomimus was alive, true birds had already evolved and were sharing their habitat with it.  It is amazing how the same design can appear twice, but separated by tens of millions of years!  Evolution is crazy!

Struthiomimus skeletal mount on display at the American Museum of Natural History.

Struthiomimus wasn't the only dinosaur that looked the way it did, however.  It belonged to an entire family, called ornithomimidae.  Ornithomimids typically had long, thin necks, small heads with large, round eyes and beaks, long arms, long fingers with three grasping claws, long tails, and of course, extremely long and powerful legs for running.  It is likely they were the best runners of their time, being able to sprint at high speeds to evade predators when they had to.  It is difficult to say exactly how fast Struthiomimus could have ran, but today's ostriches can sprint close to fifty miles per hour and sustain long distance speeds of around thirty miles per hour.  It is possible ornithomimids could have been similar.

A different Struthiomimus skeleton as found on display at the American Museum of Natural History.

Struthiomimus' diet is pretty mysterious.  Its head and mouth were proportionally very small so it was likely not attacking large animals.  Its actual beak was toothless and had somewhat sharp edges on the sides, which could have been an adaptation for clipping plants or cutting up small animals.  Struthiomimus' arms were long and but didn't have the ability to reach up that high past their normal resting position.  The fingers were all about the same length, which is unusual amongst dinosaurs, and wouldn't have been very dexterous, although many earlier reconstructions of ornithomimids show them holding and carrying items in an almost primate-like fashion.  It is possible Struthiomimus used its forelimbs as hooks, to bend plants closer to its mouth, which it could reach easier with the help of its long neck.

Close up of a Struthiomimus' hand at the Oxford University Museum of Natural History.

In life, Struthiomimus almost definitely had feathers.  We know this because specimens of a closely related dinosaur, called Ornithomimus, was discovered with amazingly preserved long feathers that were attached to its arms.  Struthiomimus likely was the same, making it look even more like an ostrich in life.

That's all for this week!  Feel free to comment below or on our facebook page!

References

Barrett, Paul M. (2005). "The diet of ostrich dinosaurs (Theropoda: Ornithomimosauria)". Palaeontology 48 (2), 347–358. doi:10.1111/j.1475-4983.2005.00448.x

Longrich, N. (2008). "A new, large ornithomimid from the Cretaceous Dinosaur Park Formation of Alberta, Canada: Implications for the study of dissociated dinosaur remains." Palaeontology, 51(4): 983-997.

Nicholls, Elizabeth L.; Russell, Anthony P. (1985). "Structure and function of the pectoral girdle and forelimb of Struthiomimus altus (Theropoda: Ornithomimidae)". Palaeontology 28: 643–677.

Russell D (1972). "Ostrich dinosaurs from the Late Cretaceous of Western Canada". Canadian Journal of Earth Sciences 9 (4): 375–402. doi:10.1139/e72-031.

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. doi:10.1126/science.1225376.