| Ornithomimus | |
|---|---|
 | |
| MountedO. edmontonicus skeleton,Royal Ontario Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | †Ornithomimosauria |
| Family: | †Ornithomimidae |
| Genus: | †Ornithomimus Marsh, 1890 |
| Type species | |
| †Ornithomimus velox Marsh, 1890 | |
| Other species | |
Ornithomimus (/ˌɔːrnɪθəˈmaɪməs,-θoʊ-/;[2] "bird mimic") is agenus ofornithomimidtheropoddinosaurs from theCampanian andMaastrichtian ages of theLate Cretaceous inwestern North America.Ornithomimus was a swift, bipedal dinosaur which was covered infeathers and equipped with a small toothless beak that may indicate anomnivorous diet. It is usually classified into two species: thetype species,Ornithomimus velox, and a referred species,Ornithomimus edmontonicus.
O. velox was named in1890 byOthniel Charles Marsh on the basis of a foot and partial hand from theDenver Formation ofColorado. Other seventeen species have been named since then, though almost all of them have been subsequently assigned to new genera or shown not to be directly related toO. velox. The best material of species still considered part of the genus has been found inAlberta, representing the speciesO. edmontonicus, known from several skeletons from theHorseshoe Canyon Formation. Additional species and specimens from other formations are sometimes classified in the genus, such asOrnithomimus samueli (alternatively classified in the generaDromiceiomimus orStruthiomimus) from the earlierDinosaur Park Formation.
The history ofOrnithomimus classification and the classification of ornithomimids in general has been very complicated. Thetype species,Ornithomimus velox, was first named byO.C. Marsh in 1890 and is based onsyntypes YPM 542 and YPM 548 (a partial hindlimb and forelimb, respectively), found byGeorge Lyman Cannon in theDenver Formation ofColorado on June 30,1889. The generic name means "bird mimic", derived from Greek words ὄρνις (ornis), "bird", and μῖμος (mimos), "mimic", in reference to the bird-like foot. Thespecific name means "swift" inLatin.[3] Simultaneously, Marsh named two other species:Ornithomimus tenuis (based on specimen USNM 5814) andOrnithomimus grandis. Both consist of fragmentary fossils found byJohn Bell Hatcher inMontana, which is today understood astyrannosauroid material. At first, Marsh assumedOrnithomimus was anornithopod, but this changed when Hatcher and A. E. Sullins found the specimen USNM 4736, a partial ornithomimid skeleton, in 1891 from theLance Formation ofWyoming. Marsh named itOrnithomimus sedens in1892, though the specimen remained unnumbered and not figured untilCharles Whitney Gilmore's 1920 re-description.[4] Marsh also namedOrnithomimus minutus was also created based on the specimen YPM 1049 (ametatarsus) in 1892,[5] but it has since been recognized as belonging to analvarezsaurid.[6]
A sixth species,Ornithomimus altus, was named in1902 byLawrence Lambe and was based on specimen CMN 930 (hindlimbs found in1901 inAlberta),[7] but this was renamed to a separate genus in1916:Struthiomimus, byHenry Fairfield Osborn.[8] In1920, Gilmore namedOrnithomimus affinis forDryosaurus grandis (Lull1911),[4] based on indeterminate material. In1930,Loris Russell renamedStruthiomimus brevetertius (Parks1926) andStruthiomimus samueli (Parks1928) intoOrnithomimus brevitertius,Ornithomimus mirifica andOrnithomimus samueli, respectively.[9] The very same year,Oliver Perry Hay renamedAublysodon mirandus (Leidy1868) intoOrnithomimus mirandus,[10] which is today seen as anomen dubium. In1933,William Arthur Parks created the speciesOrnithomimus elegans,[11] which is today seen as eitherChirostenotes orElmisaurus. That same year, Gilmore namedOrnithomimus asiaticus for material found inInner Mongolia.[12]
Also in 1933,Charles Mortram Sternberg named the speciesOrnithomimus edmontonicus for a nearly complete skeleton from the Horseshoe Canyon Formation (specimen CMN 8632).[13]
At first, it had been common practice to name each newly discovered ornithomimid as a species ofOrnithomimus. In the sixties, this tendency was still very strong, as is shown by the fact thatOskar Kuhn renamedMegalosaurus lonzeensis (Dollo1903) fromBelgium intoOrnithomimus lonzeensis (which is understood today to be anabelisauroid claw)[14] andDale Russell in1967 renamedStruthiomimus currellii (Parks 1933) andStruthiomimus ingens (Parks 1933) intoOrnithomimus currellii andOrnithomimus ingens, respectively.[15] At the same time, it was usual that workers referred to the entire ornithomimid material as simply "Struthiomimus".[16] To solve this confusion by scientifically testing the separation betweenOrnithomimus andStruthiomimus, Dale Russell in1972 published amorphometric study. It showed that statistical differences in some proportions could be used to distinguish the two and he concluded thatStruthiomimus andOrnithomimus were valid genera. In the latter, Russell recognised two species: the type speciesOrnithomimus velox andOrnithomimus edmontonicus (even though he had trouble reliably distinguishing it fromO. velox). He consideredStruthiomimus currellii to be a younger synonym ofOrnithomimus edmontonicus. However, Russell also interpreted the data as indicating that many specimens could not be referred to eitherOrnithomimus orStruthiomimus. Therefore, he created two new genera. The first one wasArchaeornithomimus.Ornithomimus asiaticus andOrnithomimus affinis were reassigned to this new genus, becomingArchaeornithomimus asiaticus andArchaeornithomimus affinis. The second one wasDromiceiomimus, meaning "Emu mimic". This comes from the old generic name for the emu:Dromiceius. Russell assigned several formerOrnithomimus species named during the 20th century, includingO. brevitertius andO. ingens, to this new genus asDromiceiomimus brevitertius. He also renamedOrnithomimus samueli into a secondDromiceiomimus species:Dromiceiomimus samueli.[17]
Twotibiae from theNavesink Formation ofNew Jersey were namedCoelosaurus antiquus ("antique hollow lizard") byJoseph Leidy in1865. The tibiae were first attributed toOrnithomimus in1979 byDonald Baird andJohn R. Horner asOrnithomimus antiquus.[18] Normally, this would have madeOrnithomimus ajunior synonym ofCoelosaurus, but Baird and Horner discovered that the name "Coelosaurus" was preoccupied by adubious taxon, which was based on a single vertebra. It was originally namedCoelosaurus by an anonymous author now known to beRichard Owen in1854.[19] Baird referred several other specimens from New Jersey andMaryland toO. antiquus. Beginning in1997,Robert M. Sullivan regardedO. velox andO. edmontonicus as junior synonyms ofO. antiquus. Like Russell, he considered the former two species indistinguishable from each other and noted that they both shared distinctive features withO. antiquus.[20] However,David Weishampel (2004) considered "C."antiquus to be indeterminate among ornithomimosaurs, resulting in it being anomen dubium.[19] An SVP2012 abstract agreed with Weishampel by noting thatCoelosaurus differs fromGallimimus andOrnithomimus in the features of the tibiae.[21]
In1988,Gregory S. Paul classified the various species ofArchaeornithomimus,Struthiomimus,Dromiceiomimus, andGallimimus to the genusOrnithomimus.[22] This has found no acceptance among other workers and the name is not presently used by Paul himself.
Even after Russell's study, various researchers have found reasons to lump some or all of these species back intoOrnithomimus in various combinations. In 2004,Peter Makovicky,Yoshitsugu Kobayashi, andPhil Currie studied Russell's 1972 proportional statistics to re-analyze ornithomimid relationships in light of newly discovered specimens. They concluded that there was no justification to separateDromiceiomimus fromOrnithomimus, sinkingDromiceiomimus as a synonym ofO. edmontonicus.[23] However, they did not include the type speciesO. velox in this analysis. The same team further supported the synonymy betweenDromiceiomimus andO. edmontonicus in a2006 lecture at theSociety of Vertebrate Paleontology annual meeting.[24] Their opinion has been followed by most later authors.[25] Makovicky's team also consideredDromiceiomimus samueli to be a junior synonym ofO. edmontonicus, though Longrich later suggested it may belong to a distinct, unnamed species from the Dinosaur Park Formation which has yet to be described.[25] Longrich called the speciesOrnithomimus samueli in a faunal list for the Dinosaur Park Formation.[26]
Apart fromO. edmontonicus dating to the early Maastrichtian, two other species are presently considered to be possibly valid and are also from the late Maastrichtian.O. sedens was named by Marsh in 1892 from partial remains found in theLance Formation ofWyoming a year after the description ofO. velox. Dale Russell, in his 1972 revision of ornithomimids, could not determine which genus it actually belonged to, though he speculated that it may be intermediate betweenStruthiomimus andDromiceiomimus. In1985, he considered it to be a species ofOrnithomimus.[27] Although it has since been referred to mainly asStruthiomimus sedens (based on complete specimens fromMontana and some fragments from Alberta andSaskatchewan), these have yet to be described and compared to theO. sedens holotype.[25]
The other is the original type species:O. velox, at first known from very limited remains. Additional specimens referred toO. velox have been described from the Denver Formation and from theFerris Formation ofWyoming.[28] One specimen attributed toO. velox (MNA P1 1762A) from theKaiparowits Formation ofUtah was described in 1985.[27] Re-evaluation of this specimen by Lindsay Zanno and colleagues in2010, however, cast doubt on its assignment toO. velox and possibly even toOrnithomimus.[29] This conclusion was supported by a2015 re-description ofO. velox, which found that only the holotype specimen was confidently referable to that species. The authors of this study tentatively referred to the Kaiparowits specimen asOrnithomimus sp., along with all of the specimens from the Dinosaur Park Formation.[1]
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Like otherornithomimids, species ofOrnithomimus are characterized by feet with three weight-bearing toes, long slender arms, and long necks with birdlike, elongated, toothless, beaked skulls. They werebipedal and superficially resemblant toratites. They would have been swift runners thanks to their very long limbs and hollow bones. They also had large brains and large eyes. The brains of ornithomimids in general were large for non-avialan dinosaurs, but this may not necessarily be a sign of high intelligence. Some paleontologists think that the enlarged portions of the brain were dedicated tokinesthetic coordination.[30] The bones of the hands are remarkably sloth-like in appearance, which ledHenry Fairfield Osborn to suggest that they were used to hook branches during feeding.
Ornithomimus differs from other ornithomimids, such asStruthiomimus, in having shorter torsos, long slender forearms, very slender, straight hand and foot claws, and hand bones and fingers of similar lengths.[31]
The twoOrnithomimus species today seen as possibly valid differ in size. In 2010,Gregory S. Paul estimated the length ofO. edmontonicus at 3.8 m (12 ft) and its weight at 170 kilograms (370 lb).[32] One of its specimens, CMN 12228, preserves afemur that is 46.8 centimetres (18.4 in) long.O. velox, thetype species ofOrnithomimus, is based on material of a smaller animal. Whereas theholotype ofO. edmontonicus, CMN 8632, preserves a secondmetacarpal eighty-four millimetres long, the same element withO. velox measures only fifty-three millimetres.[citation needed]
Ornithomimus, like many dinosaurs, was long thought to have been scaly. However, beginning in1995, several specimens ofOrnithomimus have been found preserving evidence offeathers. In 1995,2008, and2009, threeOrnithomimus edmontonicus specimens with evidence of feathers were found (two adults with carbonized traces on the lower arm, indicating the former presence ofpennaceous feather shafts, and a juvenile with impressions of feathers, which were up to five centimetres in length, in the form of hair-like filaments covering the rump, legs, and neck was also discovered). The fact that the feather imprints were found insandstone, previously thought to not be able to support such impressions, raised the possibility of finding similar structures with more careful preparation of future specimens. A study describing the fossils in2012 concluded thatO. edmontonicus was covered in plumaceous feathers at all growth stages and that only adults had pennaceous wing-like structures, suggesting that wings may have evolved for mating displays.[33] In2014, Christian Foth and others argued that the evidence was insufficient to conclude that the forelimb feathers ofOrnithomimus were necessarily pennaceous, citing the fact that the monofilamentous wing feathers incassowaries would likely leave similar traces.[34]
A fourth feathered specimen ofOrnithomimus, this time from the lower portion of the Dinosaur Park Formation, was described in October of2015 by Aaron van der Reest, Alex Wolfe, and Phil Currie. It was the firstOrnithomimus specimen to preserve the feathers of its tail. The feathers, though crushed and distorted, bore numerous similarities to those of anostrich, both in structure and distribution. Skin impressions were also preserved in the 2015 specimen, which indicated that, from mid-thigh to the feet, there was bare skin and that a flap of skin connecting the upper thigh to the torso. This latter structure is similar to that found in modern birds, including ostriches, but was positioned higher above the knee inOrnithomimus than in birds.[35]
In1890, Marsh assignedOrnithomimus to the cladeOrnithomimosauria, a classification that is still very common. Moderncladistic studies indicate it having a derived position in the ornithomimids. These, however, have only includedO. edmontonicus in their analyses. The relationships betweenO. edmontonicus,O. velox, andO. sedens have not been published.
The following cladogram is based on Xuet al.,2011:[36]
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The diet ofOrnithomimus is still highly debated. As theropods, ornithomimids might have been carnivorous, but their body shape would have been suited for a largely omnivorous lifestyle. Suggested food in its diet includes insects, crustaceans, fruit, leaves, branches, eggs, lizards, and small mammals.[30]
Ornithomimus had legs that seem clearly suited for rapid locomotion, with thetibia being about 20% longer than thefemur. The large eye sockets suggest a keen visual sense and also suggest the possibility that they werenocturnal.[37]
In a2001 study conducted by Bruce Rothschild and other paleontologists, 178 foot bones referred toOrnithomimus were examined for signs ofstress fracture, but none were found.[38]
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