| Struthiomimus | |
|---|---|
 | |
| Cast of anS. altus skeleton,Rocky Mountain Dinosaur Resource Center | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Ornithomimosauria |
| Family: | †Ornithomimidae |
| Genus: | †Struthiomimus Osborn, 1917 |
| Species: | †S. altus |
| Binomial name | |
| †Struthiomimus altus (Lambe, 1902) | |
Struthiomimus (meaning "ostrich mimic", from theAncient Greek στρούθειος/stroutheios, meaning "of the ostrich", and μῖμος/mimos, meaning "mimic" or "imitator") is agenus ofornithomimiddinosaurs from theLate Cretaceous of what is now westernNorth America. They were long-legged,bipedal, ostrich-like dinosaurs with toothless beaks. Thetype species,Struthiomimus altus, is one of the more common, smaller dinosaurs found inDinosaur Provincial Park. Their overall abundance, in addition to their toothless beak, suggests that these animals were mainlyherbivorous or (more likely)omnivorous, rather than purelycarnivorous. 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.[1]
In 1901,Lawrence Lambe found some incomplete remains,holotype CMN 930, and named themOrnithomimus altus, placing them in the same genus as material earlier described byOthniel Charles Marsh in 1890. Thespecific namealtus is fromLatin, meaning "lofty" or "noble". However, in 1914, a nearly complete skeleton (AMNH 5339) was discovered byBarnum Brown at theRed Deer River site in Alberta, promptingO. altus to be described as the type genus of a newsubgenus,Struthiomimus, byHenry Fairfield Osborn in 1917.[2]Dale Russell madeStruthiomimus 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.[3] The type species,S. altus, is known from several skeletons and skulls,[4] In 1916 Osborn also renamedOrnithomimus tenuis Marsh 1890 intoStruthiomimus tenuis.[2] This is today considered anomen dubium. In 2016, ROM 1790 was made the holotype of a new genus and species,Rativates evadens.[5]
In subsequent yearsWilliam Arthur Parks named four other species ofStruthiomimus:Struthiomimus brevetertius Parks 1926,[6]Struthiomimus samueli Parks 1928,[7]Struthiomimus currellii Parks 1933 andStruthiomimus ingens Parks 1933.[8] These are today seen as either belonging toDromiceiomimus or toOrnithomimus.
In 1997Donald Glut mentioned the nameStruthiomimus lonzeensis.[9] This was probably alapsus calami, a mistake forOrnithomimus lonzeensis (Dollo 1903) Kuhn 1965.Struthiomimus altus comes from the LateCampanian (Judithian age)Oldman Formation.[10]
A possible second species ofStruthiomimus is known from the earlyMaastrichtian (Edmontonian age)Horseshoe Canyon Formation. Because dinosaur fauna show rapid turnover, it is likely that these youngerStruthiomimus specimens represent a species distinct fromS. altus, though no new name has been given to them.[10][11]
AdditionalStruthiomimus specimens from the lowerLance Formation and equivalents are larger (similar toGallimimus in size) and tend to have straighter and more elongate hand claws, similar to those seen inOrnithomimus. One relatively complete Lance Formation specimen, BHI 1266, was originally referred toOrnithomimus sedens (named by Marsh in 1892[12]) and later classified asStruthiomimus sedens.[13] One 2015 paper by van der Reestet al. listed BHI 1266 asOrnithomimus sp.,[14] while another paper the same year considered the specimenStruthiomimus sp. pending a re-evaluation of both genera.[10]
The size ofS. 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).[15] A larger specimen ofS. altus is estimated to weigh about 233.8 kilograms (515 lb).[16] The specimens belonging to "S."sedens measured 4.8 metres (16 ft) long and weighed 350 kilograms (770 lb).[17]Struthiomimus had a build and skeletal structure typical of ornithomimids, differing from closely related genera likeOrnithomimus andGallimimus in proportions and anatomical details.[18]
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. Theirvertebral columns consisted of ten neck vertebrae, thirteen back vertebrae, six hip vertebrae, and about thirty-five tail vertebrae.[19] Their tails were relatively stiff and probably used for balance.[2] They had long slender arms and hands, with immobile forearm bones and limited opposability between the first finger and the other two.[20] 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 ofS. altus in 1917, compared the arm to that of asloth.[2] These might have been adaptations to support wing feathers.[21] 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 inStruthiomimus, unlikeOrnithomimus, which had straight beak edges.[11]Struthiomimus had longer hands relative to the humerus than other ornithomimids, with particularly long claws.[4] Their forelimbs were more robust thanOrnithomimus.[11]
Struthiomimus is a member of the family Ornithomimidae, a group which also includesAnserimimus,Archaeornithomimus,Dromiceiomimus,Gallimimus,Ornithomimus, andSinornithomimus.
Just as the fossil remains ofStruthiomimus were incorrectly assigned toOrnithomimus, the larger group thatStruthiomimus belongs to, theOrnithomimosauria, also underwent many changes over the years. For example, O.C. Marsh initially includedStruthiomimus in theOrnithopoda, a large clade of dinosaurs not closely related to theropods.[22] Five years later, Marsh classifiedStruthiomimus in theCeratosauria.[23][24] In 1891, Baur placed the genus withinIguanodontia.[25] As late as 1993,Struthiomimus was referred toOviraptorosauria.[26] However, by the 1990s, there were numerous studies that placedStruthiomimus within Coelurosauria.[27][28][29][30]
Recognizing the difference between ornithomimids and other theropods,Rinchen Barsbold placed ornithomimids within their own infraorder, Ornithomimosauria, in 1976.[31] 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,[32] a classification scheme that was adopted by other authors at the beginning of the current century.
Thecladogram follows the 2011 analysis by Xuet al.:[33]
| Ornithomimidae |
| ||||||||||||||||||||||||||||||
In a 2001 study conducted by Bruce Rothschild and other paleontologists, fifty foot bones referred toStruthiomimus were examined for signs ofstress fracture, but none were found.[34]
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.[2] 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.
There has been much discussion about the feeding habits ofStruthiomimus. Because of its straight-edged beak,Struthiomimus may have been anomnivore. Some theories suggest that it may have been a shore-dweller and may have been afilter feeder.[19] Some paleontologists noted that it was more likely to be a carnivore because it is classified within the otherwise carnivorous theropod group.[3][35] 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,[18] 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.[20] However, these adaptations might have been used for wing feather support instead.[21]
The legs (hind limbs) ofStruthiomimus were long, powerful and seemingly well-suited to rapid running, much like an ostrich. The supposed speed ofStruthiomimus 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 andDromaeosaurus) and tyrannosaurs (e.g.Daspletosaurus andGorgosaurus), which lived at the same time. It is estimated to have been able to run at speeds between 50 and 80 km/h (31.1 and 49.7 mph).[36]
Fossil remains ofS. altus are known from theOldman Formation, dated to between 78 and 77 million years ago during theCampanian stage of thelate Cretaceous period.[10]S. altus has also been found in the Horseshoe Canyon Formation. A younger species (which has not yet been named), which apparently differed fromS. altus in having longer, more slender hands, is known from several specimens found in theHorseshoe Canyon Formation and lowerLance Formation, between 69 and 67.5 million years ago (earlyMaastrichtian).[10]
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