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 Alberta, Canada and Wyoming, USA. Ornithomimids were long-legged, bipedal, ostrich-like dinosaurs with toothless beaks. The genus Struthiomimus currently contains three species.[1] The most well-known 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.[2]

Like many other dinosaurs discovered in the 19th century, the history of the various Struthiomimus species is convoluted. The first known fossils of were named Ornithomimus sedens by Othniel Charles Marsh in 1892, and a subsequent species was named O. altus by Lawrence Lambe in 1902. It wasn't until 1917 that Henry Fairfield Osborn named Struthiomimus from fossils discovered in 1914 from the Red Deer River site in Alberta.

Description
All species of Struthiomimus had builds and skeletal structure typical of an ornithomimid, differing from closely related genera like Ornithomimus and Gallimimus in proportions and anatomical details.[3] 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).[4] 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.[5] Their tails were relatively stiff and probably used for balance.[6] They had long slender arms and hands, with immobile forearm bones and limited opposability between the first finger and the other two.[7] 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.[6]

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.[1] Struthiomimus had longer hands relative to the humerus than other ornithomimids, with particularly long claws.[4] Their forelimbs were more robust than in the similar Ornithomimus.

Species
The type species, S. altus, is known from several skeletons and skulls,[4] and its size is estimated as about 4.3 metres (14 ft) long and 1.4 metres (4.6 ft) tall at the hips, with a weight of around 150 kilograms (330 lb).[8] Fossil remains of S. altus are only known definitively from the Dinosaur Park Formation, dated to about 75 million years ago during the Campanian stage of the late Cretaceous period.[1] 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, dated to about 70 million years ago (early Maastrichtian).[1] A third, even younger species, S. sedens, is known from the late Maastrichtian Lance Formation and Hell Creek Formation, dated to about 66 million years ago. S. sedens is also the largest Struthiomimus, similar to Gallimimus in size,[1] and measuring about 5 metres (16 ft) in length.

Discovery and history of study
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 was discovered by Barnum Brown at the Red Deer River site in Alberta, and officially described as the subgenus Struthiomimus by Henry Fairfield Osborn in 1917. Osborn complicated matters by assigning the better specimen, AMNH 5339, as the genoholotype.[6] 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 en ROM 1790, all partial skeletons.[10] In 1916 Osborn also renamed Ornithomimus tenuis Marsh 1890 into a Struthiomimus tenuis.[6] This is today considered a nomen dubium.

In subsequent years William Arthur Parks named four other species of Struthiomimus: Struthiomimus brevetertius Parks 1926,[11] Struthiomimus samueli Parks 1928,[12] Struthiomimus currellii Parks 1933 and Struthiomimus ingens Parks 1933.[13] These are today seen as either belonging to Dromiceiomimus or to Ornithomimus.