Struthiomimus

''Struthiomimus'' (meaning "ostrich mimic", from the Greek στρούθειος/''stroutheios'' meaning "of the ostrich" and μῖμος/''mimos'' meaning "mimic" or "imitator") is a genus of ornithomimid dinosaurs from the late Cretaceous of North America. Ornithomimids were long-legged, bipedal, ostrich-like dinosaurs with toothless beaks. The type species, ''Struthiomimus altus'', is one of the more common small dinosaurs found in Dinosaur Provincial Park; its abundance suggests that these animals were herbivores or omnivores rather than pure carnivores.

History of discovery

In 1901, Lawrence Lambe found some incomplete remains, holotype CMN 930, and named them '' Ornithomimus altus'', placing them in the same genus as material earlier described by Othniel Charles Marsh in 1890. The specific name ''altus'' is from Latin, meaning "lofty" or "noble". However, in 1914, a nearly complete skeleton (AMNH 5339) was discovered by Barnum Brown at the Red Deer River site in Alberta, prompting ''O. altus'' to be described as the type genus of a new subgenus, ''Struthiomimus'', by Henry Fairfield Osborn in 1917. Dale Russell made ''Struthiomimus'' a full genus in 1972, at the same time referring several other specimens to it: AMNH 5375, AMNH 5385, AMNH 5421, CMN 8897, CMN 8902 and ROM 1790, all partial skeletons. The type species, ''S. altus'', is known from several skeletons and skulls, In 1916 Osborn also renamed ''Ornithomimus tenuis'' Marsh 1890 into ''Struthiomimus tenuis''. This is today considered a ''nomen dubium''. In 2016, ROM 1790 was made the holotype of a new genus and species, '' Rativates evadens''.

In subsequent years William Arthur Parks named four other species of ''Struthiomimus'': ''Struthiomimus brevetertius'' Parks 1926, ''Struthiomimus samueli'' Parks 1928, ''Struthiomimus currellii'' Parks 1933 and ''Struthiomimus ingens'' Parks 1933. These are today seen as either belonging to '' Dromiceiomimus'' or to ''Ornithomimus''.

In 1997 Donald Glut mentioned the name ''Struthiomimus lonzeensis''. This was probably a ''lapsus calami'', a mistake for ''Ornithomimus lonzeensis'' (Dollo 1903) Kuhn 1965. ''Struthiomimus altus'' comes from the Late Campanian (Judithian age) Oldman Formation.

A possible second species of ''Struthiomimus'' is known from the early Maastrichtian (Edmontonian age) Horseshoe Canyon Formation. Because dinosaur fauna show rapid turnover, it is likely that these younger ''Struthiomimus'' specimens represent a species distinct from ''S. altus'', though no new name has been given to them.

Additional ''Struthiomimus'' specimens from the lower Lance Formation and equivalents are larger (similar to ''Gallimimus'' in size) and tend to have straighter and more elongate hand claws, similar to those seen in ''Ornithomimus''. One relatively complete Lance Formation specimen, BHI 1266, was originally referred to ''Ornithomimus sedens'' (named by Marsh in 1892) and later classified as ''Struthiomimus sedens''. One 2015 paper by van der Reest ''et al.'' listed BHI 1266 as ''Ornithomimus sp.'', while another paper the same year considered the specimen ''Struthiomimus sp.'' pending a re-evaluation of both genera.

Description

The size of ''S. altus'' is estimated as about long and tall at the hips, with a weight of around . ''Struthiomimus'' had a build and skeletal structure typical of ornithomimids, differing from closely related genera like '' Ornithomimus'' and ''Gallimimus'' in proportions and anatomical details.

As with other ornithomimids, they had small slender heads on long necks (which made up about 40% of the length of the body in front of the hips). Their eyes were large and their jaws were toothless. Their vertebral columns consisted of ten neck vertebrae, thirteen back vertebrae, six hip vertebrae, and about thirty-five tail vertebrae. Their tails were relatively stiff and probably used for balance. They had long slender arms and hands, with immobile forearm bones and limited opposability between the first finger and the other two. As in other ornithomimids but unusually among theropods, the three fingers were roughly the same length, and the claws were only slightly curved; Henry Fairfield Osborn, describing a skeleton of ''S. altus'' in 1917, compared the arm to that of a sloth. These might have been adaptations to support wing feathers. It is likely it had feathers all over its body. ''Struthiomimus'' differed from close relatives only in subtle aspects of anatomy. The edge of the upper beak was concave in ''Struthiomimus'', unlike ''Ornithomimus'', which had straight beak edges. ''Struthiomimus'' had longer hands relative to the humerus than other ornithomimids, with particularly long claws. Their forelimbs were more robust than ''Ornithomimus''.

Classification

''Struthiomimus'' is a member of the family Ornithomimidae, a group which also includes '' Anserimimus'', '' Archaeornithomimus'', '' Dromiceiomimus'', ''Gallimimus'', ''Ornithomimus'', and '' Sinornithomimus''.

Just as the fossil remains of ''Struthiomimus'' were incorrectly assigned to ''Ornithomimus'', the larger group that ''Struthiomimus'' belongs to, the Ornithomimosauria, also underwent many changes over the years. For example, O.C. Marsh initially included ''Struthiomimus'' in the Ornithopoda, a large clade of dinosaurs not closely related to theropods. Five years later, Marsh classified ''Struthiomimus'' in the Ceratosauria.O. C. Marsh. 1896. The dinosaurs of North America. United States Geological Survey, 16th Annual Report, 1894-95 55:133-244 In 1891, Baur placed the genus within Iguanodontia. As late as 1993, ''Struthiomimus'' was referred to Oviraptorosauria. However, by the 1990s, there were numerous studies that placed ''Struthiomimus'' within Coelurosauria.J. A. Gauthier and K. Padian. 1985. Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight. In M. K. Hecht, J. H. Ostrom, G. Viohl, and P. Wellnhofer (eds.), The Beginnings of Birds: Proceedings of the International Conference Archaeopteryx, Eichstätt 1984. Freunde des Jura-Museums Eichstätt, Eichstätt 185-197F. E. Novas. 1992. The evolution of carnivorous dinosaurs. In J. L. Sanz and A. D. Buscalioni (eds.), The Dinosaurs and Their Environment Biotic: Proceedings of the Second Year of Paleontology in Cuenca. Institute "Juan Valdez", Cuenca, Argentina 126-163

Recognizing the difference between ornithomimids and other theropods, Rinchen Barsbold placed ornithomimids within their own infraorder, Ornithomimosauria, in 1976.R. Barsbold. 1976. K evolyutsii i sistematike pozdnemezozoyskikh khishchnykh dinozavrov he evolution and systematics of late Mesozoic carnivorous dinosaurs In N. N. Kramarenko, B. Luvsandansan, Y. I. Voronin, R. Barsbold, A. K. Rozhdestvensky, B. A. Trofimov & V. Y. Reshetov (eds.), Paleontology and Biostratigraphy of Mongolia. The Joint Soviet-Mongolian Paleontological Expedition, 3:68-75 Transactions The constituency of Ornithomimidae and Ornithomimosauria varied with different authors. Paul Sereno, for example, used Ornithomimidae to include all ornithomimosaurians in 1998, but subsequently changed to a more exclusive definition (advanced ornithomimosaurs) within Ornithomimosauria, a classification scheme that was adopted by other authors at the beginning of the current century.

The cladogram follows the 2011 analysis by Xu ''et al.'':

Paleobiology

In a 2001 study conducted by Bruce Rothschild and other paleontologists, fifty foot bones referred to ''Struthiomimus'' were examined for signs of stress fracture, but none were found.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.

''Struthiomimus'' was one of the first theropods envisioned from the outset as having a horizontal posture. Osborn in 1916 let the animal intentionally be depicted with an elevated tail. This newer view created an image much more reminiscent of modern flightless birds, such as the ostrich to which this dinosaur's name refers, but would only much later be accepted for all theropods.

Diet

There has been much discussion about the feeding habits of ''Struthiomimus''. Because of its straight-edged beak, ''Struthiomimus'' may have been an omnivore. Some theories suggest that it may have been a shore-dweller and may have been a filter feeder. Some paleontologists noted that it was more likely to be a carnivore because it is classified within the otherwise carnivorous theropod group. This theory has never been discounted, but Osborn, who described and named the dinosaur, proposed that it probably ate buds and shoots from trees, shrubs and other plants, using its forelimbs to grasp branches and its long neck to enable it accurately to select particular items. This herbivorous diet is further supported by the unusual structure of its hands. The second and third fingers were of equal length, could not function independently, and were probably bound together by skin as a single unit. The structure of the shoulder girdle did not allow a high elevation of the arm nor was optimised for a low reach. The hand could not be fully flexed for a grasping motion or spread for raking. This indicates that the hand was used as a "hook" or "clamp", for bringing branches or fern fronds at shoulder height within reach. However, these adaptations might have been used for wing feather support instead.

Speed

The legs (hind limbs) of ''Struthiomimus'' were long, powerful and seemingly well-suited to rapid running, much like an ostrich. The supposed speed of ''Struthiomimus'' was, in fact, its main defense from predators (although it may also have been able to lash out with its hind claws when cornered), such as the dromaeosaurids (e.g. '' Saurornitholestes'' and '' Dromaeosaurus'') and tyrannosaurs (e.g. ''Daspletosaurus'' and '' Gorgosaurus''), which lived at the same time. It is estimated to have been able to run at speeds between . Paul, regarding his comparative speed estimates, notes that "... just how swift is swift? In hard, precise measure, this can be a real can of worms; for just how fast living animals run is not well known." (Paul, G.S. 1988. ''Predatory Dinosaurs of the World''. New York: Simon & Schuster.)

Paleoecology

Fossil remains of ''S. altus'' are only known definitively from the Oldman Formation, dated to between 78 and 77 million years ago during the Campanian stage of the late Cretaceous period. A younger species (which has not yet been named), which apparently differed from ''S. altus'' in having longer, more slender hands, is known from several specimens found in the Horseshoe Canyon Formation and lower Lance Formation, between 69 and 67.5 million years ago (early Maastrichtian).

See also

* Timeline of ornithomimosaur research

References

Further reading

*
*Cranfield, I. (2004). ''The Illustrated Directory of Dinosaurs and other Prehistoric Creatures'' (pp. 30–33). Greenwich Editions. .
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External links

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Fossils of Canada
Ornithomimids
Campanian genus first appearances
Maastrichtian life
Late Cretaceous dinosaurs of North America
Hell Creek fauna
Horseshoe Canyon fauna
Lance fauna
Oldman fauna
Paleontology in Alberta
Fossil taxa described in 1917
Taxa named by Henry Fairfield Osborn