Struthiomimus (meaning "ostrich mimic", from the Ancient 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 what is now western North America. They were long-legged, bipedal, ostrich-like dinosaurs with toothless beaks. The type species, Struthiomimus altus, is one of the more common, smaller dinosaurs found in Dinosaur Provincial Park. Their overall abundance, in addition to their toothless beak, suggests that these animals were mainly herbivorous or (more likely) omnivorous, rather than purely carnivorous. Similar to the modern ostriches, emus, and rheas (among other birds), these dinosaurs likely lived as opportunistic omnivores, supplementing a largely plant-based diet with a variety of small mammals, reptiles, amphibians, insects, invertebrates, and anything else they could fit into their mouth, as they foraged.
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 species of a new genus, Struthiomimus, by Henry Fairfield Osborn in 1917. Dale Russell (1972) referred several additional specimens to S. altus: 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 1917 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 4.3 metres (14 ft 1 in) long and 1.4 m (4 ft 7 in) tall at the hips, with a weight of around 150 kilograms (330 lb). A larger specimen of S. altus is estimated to weigh about 233.8 kilograms (515 lb). The specimens belonging to "S." sedens measured 4.8 metres (16 ft) long and weighed 350 kilograms (770 lb). 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
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