Ornithomimus

Credit: Gabriel Ugueto.

*Ornithomimus* is an extinct genus of ornithomimid dinosaur that lived in North America during the Late Cretaceous.

## History

*Ornithomimus* has a complicated history. Othniel Charles Marsh (1890) named O. velox as the type, based on syntypes YPM 542 and YPM 548, found by George Lyman Cannon in the Denver Formation, Colorado on the 30th of June, 1889. It derives from the Greek ὄρνις (ornis), meaning "bird", and μῖμος (mimos), meaning "mimic", referencing the bird-like foot, with the epithet meaning "swift" in Latin. Marsh also named *O. tenuis* (based on USNM 5814) and *O. grandis*. Both are known from fragmentary remains found by John Bell Hatcher in Montana, which is now assigned to Tyrannosauroidea. Marsh first assumed *Ornithomimus* was an ornithopod, which changed when Hatcher found USNM 4736, a partial skeleton found by Hatcher from Wyoming, which Marsh (1892) named *O. sedens*. On the same occasion, *O. minutus* was erected from YPM 1049, a metatarsus now assigned to Alvarezsauridae, possibly *Trierarchuncus* but not assigned.

*O. altus*, a sixth species, was named by Lawrence Lambe in 1902, based on CMN 930, hindlimbs found in 1901 in Alberta, but was transferred to a new genus (*Struthiomimus*) in 1916 by Henry Fairfield Osborn. Charles Whitney Gilmore (1920) named *O. affinis* from *Dryosaurus grandis* based on indeterminate material. Loris Russell (1930) renamed *Struthiomimus brevetertius* and *Struthiomimus samueli* into *O. brevetertius* and *O. samueli*. The same year, Oliver Perry Hay renamed *Aublysodon mirandus* into *O. mirandus*, which is now a synonym and nomen dubium. William Arthur Parks (1933) erected *O. elegans*, which is either *Chirostenotes* or *Elmisaurus*. Gilmore, in the same year, named *O. asiaticus* from inner Mongolia. Charles Mortram Sternberg (1933) named *O. edmontonicus* from a near-complete skeleton (CMN 8632) from the Horseshoe Canyon Formation, Alberta.

It was first common to name every new ornithomimid as a species of *Ornithomimus*, and persisted up to the 1960s when Oskar Kuhn placed *Megalosaurus lonzeensis* into *Ornithomimus* and *Struthiomimus currelli* and *ingens* into *Ornithomimus*. At the same time, it was unusual for ornithomimids to be referred to *Struthiomimus*. To resolve the confusion, Dale Russel (1972) published a morphometric study on the statistical differences in proportions that could potentially differentiate them, concluding both were valid. He recognized two species: *O. velox* and *O. edmontonicus*, though he had trouble differentiating *O. velox*. *Struthiomimus currelli* was considered a younger synonym of *O. edmontonicus*. However, Russel also interpreted many specimens as not referable to either genera. He created two new genera: *Archaeornithomimus* and *Dromiceiomimus*.

Two tibia recovered from the Navesink Formation were named *Coelosaurus antiquus* by Joseph Leidy (1865). They were then attributed to *Ornithomimus* in 1979 by Donald Baird and John R. Horner as *O. antiquus*. Typically, *Ornithomimus* would have been considered a junior synonym of *Coelosaurus*, but found the name was already preoccupied by a dubious vertebra taxon, which was named by Richard Owen (1854), who was anonymous at the time. Baird referred several other specimens from New Jersey and Maryland. In 1997, Robert M. Sullivan used *O. velox* and *O. edmontonicus* as junior synonyms for *O. antiquus*, finding both indistinguishable (like Russel did), but both had the distinct features characteristic of *O. antiquus*. However, David Weishampel (2004) found *"Coelosaurus" antiquus* to be an indeterminate ornithomimosaur and a nomen dubium. A SVP 2012 abstract agreed with Weishampel, finding it differed from *Gallimimus* and *Ornithomimus* based on tibiae features. Gregory S. Paul (1988) placed all species in *Archaeornithomimus*, *Struthiomimus*, *Dromiceiomimus* and *Gallimimus* into *Ornithomimus*, but this has not been supported by others, including Paul's more recent work.

After Russel's study, researchers have managed to lump many species into *Ornithomimus*. Peter Makovicky, Yoshitsugu Kobayashi and Phil Currie (2004) studied Russel's proportional statistics and found no reason to separate *Dromiceiomimus*, making it a junior synonym of *O. edmontonicus*. However, they failed to include the type in the analysis. The same team later supported this in a 2006 lecture at the Society of Vertebrae Paleontology annual meeting, with their opinion shared by many later. They put *Dromiceiomimus samueli* as a junior synonym of *O. edmontonicus*, though Longrich suggested it may be a distinct, unnamed taxon from Dinosaur Park. Longrich named it *O. samueli* in a Dinosaur Park Formation faunal list.

*O. edmontonicus* from the Early Maastrichtian is valid, with *O. sedens* and *O. velox* from the Late Maastrichtian possibly being valid (Dale Russel (1972) could not determine which genus *O. velox* belonged, so he speculated it was an intermediary form of *Struthiomimus* and *Dromiceiomimus*, but placed it in *Ornithomimus* in 1985). *Struthiomimus sedens* and *"O." sedens* have not been compared.

*O. velox* is known from limited material, with additional specimens referred from the Denver and Ferris Formations. MNA P1 1762A, an *O. velox* from the Kaiparowits Formation, was described in 1985. Lindsay Zanno et al. (2010) have cast doubt on the assignment, even to the genus level. This was supported by a redescription of *O. velox* in 2015, finding only the holotype is referable. They tentatively assigned the specimen to O. sp., along with all the Dinosaur Park specimens.

## Description

*Ornithomimus* is characterized by feet with three weight-bearing toes, long slender arms, a long neck and a birdlike, elongated, beaked and toothless skull. Resembling an ostrich, they were bipedal. They were swift on their feet, with long limbs with hollowed bones and a large brain and eyes. *Ornithomimid* brains were large for non-avialan dinosaurs, but this is not a likely source for greater intelligence. Some think the enlarged portions were dedicated to kinesthetic coordination. The hand bones are sloth-like, where Henry Fairfield Osborn suggested they hooked branches during feeding. Differing from *Struthiomimus*, *Ornithomimus* has a shorter torso, long and slender forearms, a straight hand and foot claws that are very slender and metacarpals and fingers of similar length. Gregory S. Paul estimated *O. edmontonicus* was 3.8 meters (12 feet) long and 170 kilograms (370 pounds) heavy. CMN 12228 is a femur 46.8 centimeters (18.4 inches) long. *O. velox*, the type of the genus, is based on smaller individuals. The *O. edmontonicus* holotype is CMN 8632, a second metacarpal 84 millimeters long, with the same element in *O. velox* being 53 millimeters long.

*Ornithomimus* was first thought to be scaly, like many dinosaurs. However, at the start of 1995, several *Ornithomimus* individuals were found with evidence of feathers. In 1995, 2008 and 2009, three *O. edmontonicus* with feathers were found, including two adults with carbonized traces at the lower arm and the former presence of pennaceous feather shafts and a juvenile with feather impressions up to 5 centimeters long, with hair-like filament blanketing the rump, legs and neck. They were found in sandstone, which was previously thought incapable of such preservation, which raised the possibility of similar findings and found careful preparation is required. A 2012 study found *O. edmontonicus* was covered in plumaceous feathers at all ontogenetic stages, with only adults bearing pennaceous wing-like structures, which suggests they may have been part of mating displays. Christian Foth (2014) argued existing evidence was insufficient to determine if the forelimb feathers were pennaceous, citing the monofilamentous wings of Casuarius would leave a similar trace. Another *Ornithomimus* from the lower Dinosaur Park Formation, the fourth specimen, was described by Aaron van der Reest, Alex Wolfe and Phil Currie (2015), being the first *Ornithomimus* to preserve feathers on the tail. Though these feathers were crushed and distorted, they bore many similarities with the modern ostrich, in structure and distribution. Skin impressions in the 2015 specimen indicated bare skin spanned from the mid-thigh to the feet, where no scales were present and a flap of skin connected the upper thigh to the torso. This is similar to modern birds, including the ostrich, but is positioned higher above the knee in *Ornithomimus* than in birds.

## Classification

Marsh (1890) assigned *Ornithomimus* to Ornithomimosauria, which is still recognized as correct. Modern cladistic studies show a derived position in Ornithomimidae, but have only included *O. edmontonicus*. The relations between all species is unknown. Xu et al. (2011) conclude:

## Paleoecology

*Ornithomimus*' diet is debated. They may have been carnivorous, but their morphology is more suited towards a partly or somewhat herbivorous diet. Suggested foods include: insects, crustaceans, fruit, leaves, branches, eggs, lizards and small mammals. Ornithomimus is clearly adapted to running, with a tibia ~20% longer than the femur. The eyes suggest it had keen vision, and a possibility that they were nocturnal. Bruce Rothschild et al. (2001) find 178 foot bones with signs of stress fractures, but found none.

## Species

* *O. velox*
* *O. edmontonicus*
* *O. samueli?*

### Reassigned Species

* "O." mirandus
* *O. tenuis* (=*Daspletosaurus* or *Gorgosaurus*)
* *O. grandis* (=tyrannosauroidea)
* *"O." sedens*
* *O. minutus* (=alvarezsauridae, =*Trierarchuncus*?)
* *"O." altus*
* *"O." affinis*
* *"O." brevetertius*
* *"O." elegans* (=*Chirostenotes* or *Elmisaurus*)
* *"O." asiaticus*
* *"O." lonzeensis* (=abelisauroid)
* *"O." ingens*
* *"O." antiquus*
* *"O." bullatus*

## References

* https://en.wikipedia.org/wiki/Ornithomimus
* D. W. Fowler, J. P. Wilson, E. A. Freedman Fowler, C. R. Noto, D. Anduza and J. R. Horner. 2020. Trierarchuncus prairiensis gen. et sp. nov., the last alvarezsaurid: Hell Creek Formation (uppermost Maastrichtian), Montana. *Cretaceous Research* **116**:104560
* https://www.nature.com/articles/s41586-022-04446-1
* https://phys.org/news/2022-02-dinosaurs.html
* https://web.archive.org/web/20130929074932/http://archosaur.us/theropoddatabase/Tyrannosauroidea.html