Ornithomimus

Ornithomimus; Temporal range:Late Cretaceous(CampaniantoMaastrichtian),76.5–66Ma PreꞒ Ꞓ O S D C P T J K Pg N
	MountedO. edmontonicusskeleton,Royal Ontario Museum
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	†Ornithomimosauria
Family:	†Ornithomimidae
Genus:	†Ornithomimus; Marsh,1890
Type species
	†Ornithomimus velox; Marsh, 1890
Other species
	†O. edmontonicus; Sternberg,1933; †O. samueli?; (Parks, 1928);

Ornithomimus(/ˌɔːrnɪθəˈmaɪməs,-θoʊ-/;^[2]"bird mimic" ) is agenusofornithomimid theropod dinosaursfrom theCampanianandMaastrichtianages ofLate Cretaceous Western North America.Ornithomimuswas a swift, bipedal dinosaur which fossil evidence indicates was covered infeathersand equipped with a small toothless beak that may indicate anomnivorousdiet. It is usually classified into two species: thetype species,Ornithomimus velox,and a referred species,Ornithomimus edmontonicus.O. veloxwas named in1890byOthniel Charles Marshon the basis of a foot and partial hand from theDenver FormationofColorado.Another seventeen species have been named since then, though almost all of them have been subsequently assigned to new genera or shown to be not directly related toOrnithomimus 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 asOrnithomimus,such asOrnithomimus samueli(alternately classified in the generaDromiceiomimusorStruthiomimus) from the earlierDinosaur Park Formation.

History of discovery

First species named

The history ofOrnithomimusclassification and the classification of ornithomimids in general has been very complicated. Thetype species,Ornithomimus velox,was first named byO.C. Marshin 1890 and is based onsyntypesYPM 542 and YPM 548 (a partial hindlimb and forelimb, respectively), found byGeorge Lyman Cannonin theDenver FormationofColoradoon 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 namemeans "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 HatcherinMontana,which is today understood astyrannosauroidmaterial. At first, Marsh assumedOrnithomimuswas anornithopod,but this changed when Hatcher found specimen USNM 4736, a partial ornithomimid skeleton, inWyoming.Marsh named itOrnithomimus sedensin1892.On that occasion,Ornithomimus minutuswas also created based on specimen YPM 1049 (ametatarsus),^[4]but it has since been recognized as belonging to analvarezsaurid.^[5]

A sixth species,Ornithomimus altus,was named in1902byLawrence Lambeand was based on specimen CMN 930 (hindlimbs found in1901inAlberta),^[6]but this was renamed to a separate genus in1916:Struthiomimus,byHenry Fairfield Osborn.^[7]In1920,Charles Whitney GilmorenamedOrnithomimus affinisforDryosaurus grandis(Lull1911),^[8]based on indeterminate material. In1930,Loris RussellrenamedStruthiomimus brevetertius(Parks1926) andStruthiomimus samueli(Parks1928) intoOrnithomimus brevitertiusandOrnithomimus samueli,respectively.^[9]The very same year,Oliver Perry HayrenamedAublysodon mirandus(Leidy1868) intoOrnithomimus mirandus,^[10]which is today seen as anomen dubium.In1933,William Arthur Parkscreated the speciesOrnithomimus elegans,^[11]which is today seen as eitherChirostenotesorElmisaurus.That same year, Gilmore namedOrnithomimus asiaticusfor material found inInner Mongolia.^[12]

Also in 1933,Charles Mortram Sternbergnamed the speciesOrnithomimus edmontonicusfor a nearly complete skeleton from the Horseshoe Canyon Formation (specimen CMN 8632).^[13]

Reclassification by Dale Russell

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 KuhnrenamedMegalosaurus lonzeensis(Dollo1903) fromBelgiumintoOrnithomimus lonzeensis(which is understood today to be anabelisauroidclaw)^[14]andDale Russellin1967renamedStruthiomimus currellii(Parks 1933) andStruthiomimus ingens(Parks 1933) intoOrnithomimus currelliiandOrnithomimus 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 betweenOrnithomimusandStruthiomimus,Dale Russell in1972published amorphometricstudy. It showed that statistical differences in some proportions could be used to distinguish the two and he concluded thatStruthiomimusandOrnithomimuswere valid genera. In the latter, Russell recognised two species: the type speciesOrnithomimus veloxandOrnithomimus edmontonicus(even though he had trouble reliably distinguishing it fromO. velox). He consideredStruthiomimus currelliito be a younger synonym ofOrnithomimus edmontonicus.However, Russell also interpreted the data as indicating that many specimens could not be referred to eitherOrnithomimusorStruthiomimus.Therefore, he created two new genera. The first one wasArchaeornithomimus.Ornithomimus asiaticusandOrnithomimus affiniswere reassigned to this new genus, becomingArchaeornithomimus asiaticusandArchaeornithomimus affinis.The second one wasDromiceiomimus,meaning "Emumimic ". This comes from the old generic name for the emu:Dromiceius.Russell assigned several formerOrnithomimusspecies named during the 20th century, includingO. brevitertiusandO. ingens,to this new genus asDromiceiomimus brevitertius.He also renamedOrnithomimus samueliinto a secondDromiceiomimusspecies:Dromiceiomimus samueli.^[17]

Misassigned toOrnithomimus

Twotibiaefrom theNavesink FormationofNew Jerseywere namedCoelosaurus antiquus( "antique hollow lizard" ) byJoseph Leidyin1865.The tibiae were first attributed toOrnithomimusin1979byDonald BairdandJohn R. HornerasOrnithomimus antiquus.^[18]Normally, this would have madeOrnithomimusajunior synonymofCoelosaurus,but Baird and Horner discovered that the name "Coelosaurus" was preoccupied by adubioustaxon, which was based on a single vertebra. It was originally namedCoelosaurusby an anonymous author now known to beRichard Owenin1854.^[19]Baird referred several other specimens from New Jersey andMarylandtoO. antiquus.Beginning in1997,Robert M. SullivanregardedO. veloxandO. edmontonicusas 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."antiquusto be indeterminate among ornithomimosaurs, resulting in it being anomen dubium.^[19]An SVP2012abstract agreed with Weishampel by noting thatCoelosaurusdiffers fromGallimimusandOrnithomimusin the features of the tibiae.^[21]

In1988,Gregory S. Paulclassified the various species ofArchaeornithomimus,Struthiomimus,Dromiceiomimus,andGallimimusto the genusOrnithomimus.^[22]This has found no acceptance among other workers and the name is not presently used by Paul himself.

Present interpretations

Even after Russell's study, various researchers have found reasons to lump some or all of these species back intoOrnithomimusin various combinations. In 2004,Peter Makovicky,Yoshitsugu Kobayashi,andPhil Curriestudied Russell's 1972 proportional statistics to re-analyze ornithomimid relationships in light of newly discovered specimens. They concluded that there was no justification to separateDromiceiomimusfromOrnithomimus,sinkingDromiceiomimusas a synonym ofO. edmontonicus.^[23]However, they did not include the type speciesO. veloxin this analysis. The same team further supported the synonymy betweenDromiceiomimusandO. edmontonicusin a2006lecture at theSociety of Vertebrate Paleontologyannual meeting.^[24]Their opinion has been followed by most later authors.^[25]Makovicky's team also consideredDromiceiomimus samuelito 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 samueliin a faunal list for the Dinosaur Park Formation.^[26]

Apart fromO. edmontonicusdating to the early Maastrichtian, two other species are presently considered to be possibly valid and are also from the late Maastrichtian.O. sedenswas named by Marsh in 1892 from partial remains found in theLance FormationofWyominga 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 betweenStruthiomimusandDromiceiomimus.In1985,he considered it to be a species ofOrnithomimus.^[27]Although it has since been referred to mainly asStruthiomimus sedens(based on complete specimens fromMontanaand some fragments from Alberta andSaskatchewan), these have yet to be described and compared to theO. sedensholotype.^[25]

The other is the original type species:O. velox,at first known from very limited remains. Additional specimens referred toO. veloxhave been described from the Denver Formation and from theFerris FormationofWyoming.^[28]One specimen attributed toO. velox(MNA P1 1762A) from theKaiparowits FormationofUtahwas described in 1985.^[27]Re-evaluation of this specimen by Lindsay Zanno and colleagues in2010,however, cast doubt on its assignment toO. veloxand possibly even toOrnithomimus.^[29]This conclusion was supported by a2015re-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]

Description

Like otherornithomimids,species ofOrnithomimusare characterized by feet with three weight-bearing toes, long slender arms, and long necks with birdlike, elongated, toothless, beaked skulls. They werebipedaland 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 tokinestheticcoordination.^[30]The bones of the hands are remarkably sloth-like in appearance, which ledHenry Fairfield Osbornto suggest that they were used to hook branches during feeding.

Ornithomimusdiffers 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 twoOrnithomimusspecies today seen as possibly valid differ in size. In 2010,Gregory S. Paulestimated the length ofO. edmontonicusat 3.8 m (12 ft) and its weight at 170 kilograms (370 lb).^[32]One of its specimens, CMN 12228, preserves afemurthat is 46.8 centimetres (18.4 in) long.O. velox,thetype speciesofOrnithomimus,is based on material of a smaller animal. Whereas theholotypeofO. edmontonicus,CMN 8632, preserves a secondmetacarpaleighty-four millimetres long, the same element withO. veloxmeasures only fifty-three millimetres.

Feathers and skin

Ornithomimus,like many dinosaurs, was long thought to have been scaly. However, beginning in1995,several specimens ofOrnithomimushave been found preserving evidence offeathers.In 1995,2008,and2009,threeOrnithomimus edmontonicusspecimens with evidence of feathers were found (two adults with carbonized traces on the lower arm, indicating the former presence ofpennaceous feathershafts, 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 in2012concluded thatO. edmontonicuswas 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 ofOrnithomimuswere necessarily pennaceous, citing the fact that the monofilamentous wing feathers incassowarieswould 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 of2015by Aaron van der Reest, Alex Wolfe, and Phil Currie. It was the firstOrnithomimusspecimen 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 inOrnithomimusthan in birds.^[35]

Classification

In1890,Marsh assignedOrnithomimusto the cladeOrnithomimosauria,a classification that is still very common. Moderncladisticstudies indicate it having a derived position in the ornithomimids. These, however, have only includedO. edmontonicusin their analyses. The relationships betweenO. edmontonicus,O. velox,andO. sedenshave not been published.

The following cladogram is based on Xuet al.,2011:^[36]

Ornithomimidae

Archaeornithomimus asiaticus

unnamed

Sinornithomimus dongi

unnamed

	Anserimimus planinychus

	Gallimimus bullatus

unnamed

Qiupalong henanensis

unnamed

	Struthiomimus altus

	Ornithomimus edmontonicus

Paleobiology

The diet ofOrnithomimusis 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]

Ornithomimushad legs that seem clearly suited for rapid locomotion, with thetibiabeing about 20% longer than thefemur.The large eye sockets suggest a keen visual sense and also suggest the possibility that they werenocturnal.^[37]

In a2001study conducted by Bruce Rothschild and other paleontologists, 178 foot bones referred toOrnithomimuswere examined for signs ofstress fracture,but none were found.^[38]

References

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