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
thumb|left|Cast of S. altus skeleton (specimen AMNH 5339), found in 1914
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. In 1917 Osborn also renamed Ornithomimus tenuis Marsh 1890 into Struthiomimus tenuis.
thumb|left|Skeletal diagram of S. altus specimen AMNH 5339
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.
thumb|Cast of BHI 1266, which may be a Struthiomimus sedens specimen
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.) 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. A larger specimen of S. altus is estimated to weigh about . The specimens belonging to "S." sedens measured long and weighed . Struthiomimus had a build and skeletal structure typical of ornithomimids, differing from closely related genera like Ornithomimus and Gallimimus in proportions and anatomical details.
thumb|Size comparison between S. altus and "S." sedens
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 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. 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.
Classification
thumb|Original skull of RTMP 1990.026.0001 in A, C and E and restored in B, D and F
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. 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.
Recognizing the difference between ornithomimids and other theropods, Rinchen Barsbold placed ornithomimids within their own infraorder, Ornithomimosauria, in 1976. 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.
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 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,
Paleoecology
thumb|left|[[Albertosaurus hunting Saurolophus with Struthiomimus in the foreground]]
Fossil remains of S. altus are known from the Oldman Formation, dated to between 78 and 77 million years ago during the Campanian stage of the late Cretaceous period. S. altus has also been found in the Horseshoe Canyon Formation. 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).