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'Ornithomimosaurs' (meaning 'bird mimic lizards') or members of the clade 'Ornithomimosauria' are theropod dinosaurs, like ''Gallimimus'', which bore a superficial resemblance to modern ostriches. They were fast, fleet-footed, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia (now Asia, Europe and North America). The group first appeared in the Early Cretaceous and persisted until the Late Cretaceous. They appear to be related to less-derived coelurosaurian theropods such as ''Compsognathus'' and tyrannosaurids. Primitive members of the group include ''Pelecanimimus'', ''Shenzhousaurus'', ''Harpymimus'' and probably the huge ''Deinocheirus'', the arms of which reached eight feet in length. More advanced species, members of the family ornithomimidae, include ''Gallimimus'', ''Archaeornithomimus'', ''Anserimimus'', ''Struthiomimus'', and ''Ornithomimus''. Some paleontologists, like Paul Sereno, consider the enigmatic alvarezsaurids to be close relatives of the ornithomimosaurs and place them together in the superfamily Ornithomimoidea (see classification below).

Contents

Description

Diet

Taxonomy

Classification

Phylogeny

Other evidence

References

External links

Description

The skull of ornithomimosaurs, which atop a relatively long and slender neck, was small, with large eyes. Some primitive species (such as ''Pelecanimimus'' and ''Harpymimus'') had teeth, but most had toothless beaks.

The fore limbs ('arms') were long and slender and bore powerful claws. The hind limbs were long and powerful, with a long foot and short, strong toes terminating in hooflike claws. Ornithomimosaurs were probably among the fastest of all dinosaurs. Like many other coelurosaurs, the ornithomimosaurian hide was probably feathered rather than scaly.

Diet

Ornithomimosaurs probably acquired most of their calories from plants. Many ornithomimosaurs, including primitive species, have been found with numerous gastroliths in their stomachs, characteristic of herbivores. Henry Fairfield Osborn suggested that the long, sloth-like 'arms' of ornithomimosaurs may have been used to pull down branches on which to feed, an idea supported by further study of their strange, hook-like hands.^[1] The sheer abundance of ornithomimids — they are the most common small dinosaurs in North America — is consistent with the idea that they were plant eaters, as herbivores usually outnumber carnivores in an ecosystem.
Thanks to well-preserved specimens of ''Ornithomimus'', some of the anatomy of the inside of ornithomimid beaks has been observed. A 2001 study showed that the inside walls of the beak possessed vertical ridges, similar to those found in modern ducks. This led some paleontologists to suggest ornithomimids may have been filter feeders, which strained water through their beaks to consume small aquatic organisms.^[2] However, others have argued against this interpretation, pointing out that similar structures are found in the beaks of a wide variety of non-filter feeding beaked animals, including turtles, and have no relation to filter feeding.^[3]

Taxonomy

Named by Marsh in 1890, the family Ornithomimidae was originally classified as a group of "megalosaurs" (a "wastebasket taxon" containing any medium to large sized theropod dinosaurs), but as more theropod diversity was uncovered, their true relationships to other theropods started to resolve, and they were moved to the Coelurosauria. Recognizing the distinctivness of ornithomimids compared to other dinosaurs, Rinchen Barsbold placed ornithomimids within thir own infraorder, Ornithomimosauria, in 1976. The contents of Ornithomimidae and Ornithomimosauria varied from author to author as cladistic definitions began to appear for the groups in the 1990s. Paul Sereno, for example, used Ornithomimidae to include all of Ornithomimosauria in 1998, and subsequently switched to a more exlusive definition that nested Ornithomimidae (advanced ornthimomimes) within the larger stem-group Ornithomimosauria, a classification scheme that is mirrored by most other literature in the early 2000s.
In the early 1990s, prominent paleontologists such as Thomas R. Holtz Jr. proposed a close relationship between theropods with an ''arctometatarsalian'' foot; that is, bipedal dinosaurs in which the upper foot bones were 'pinched' together, an adaptation for running. Holtz (1994) defined the clade 'Arctometatarsalia' as "the first theropod to develop the arctometatarsalian pes and all of its descendants." This group included the Troodontidae, Tyrannosauroidea, and Ornithomimosauria. Holtz (1996, 2000) later refined this definition to the branch-based "''Ornithomimus'' and all theropods sharing a more recent common ancestor with ''Ornithomimus'' than with birds." Subsequently, the idea that all arctometatarsalian dinosaurs formed a natural group was abandoned by most paleontologists, including Holtz, as studies began to demonstrate that tyrannosaurids and troodontids were more closely related to other groups of coelurosaurs than they were to ornithomimosaurs. Since the strict definition of Arctometatarsalia was based on ''Ornithomimus'', it became redundant with the name Ornithomimosauria, and the later name remained in use, while the name Arctometatarsalia was mostly abandoned.
Another clade, Ornithomimiformes, was defined by Sereno (2005) as (''Ornithomimus velox'' > ''Passer domesticus'') and is useful in phylogenetic taxonomy only if alvarezsaurids or some other group are actually closer relatives of ornithomimosaurs than maniraptorans. The Ornithomimosauria is an inclusive clade that contains primitive ornithomimosaurs and ornithomimids proper.

Classification

★ 'Infraorder ORNITHOMIMOSAURIA'

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★ ''Pelecanimimus'' (central Spain)

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★ ''Shenzhousaurus'' (Liaoning, Northeastern China)

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★ 'Family Deinocheiridae'

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★ ''Deinocheirus'' (Mongolia)

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★ 'Family Garudimiminae'

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★ ''Garudimimus'' (Mongolia)

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★ 'Family Harpymimidae'

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★ ''Harpymimus'' (Mongolia)

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★ 'Family Ornithomimidae'

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★ ''Anserimimus'' (Mongolia)

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★ ''Archaeornithomimus'' (China)

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★ ''Dromiceiomimus'' (Alberta, Canada)

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★ ''Gallimimus'' (Mongolia)

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★ ''Ornithomimus'' (Colorado and Alberta in North America)

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★ ''Sinornithomimus'' (Inner Mongolia, China)

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★ ''Struthiomimus'' (Montana and Alberta in North America)

Phylogeny

Cladogram after Barsbold & Osmólska (1990), Kobayashi & Lü (2003), and Ji ''et al.'' (2003).[1]
Ornithomimiformes (=Arctometatarsalia)
|-?Alvarezsauridae
`--'Ornithomimosauria'
|-?''Timimus''
|--''Pelecanimimus''
`--+--''Shenzhousaurus''
`--+--''Harpymimus''
`--Ornithomimoidea
|-?''Deinocheirus''
|--''Garudimimus''
`--Ornithomimidae
|--''Archaeornithomimus''
`--+--''Sinornithomimus''
`--Ornithomiminae
|--+--''Gallimimus''
| `--''Anserimimus''
`--Ornithomimini
|--''Struthiomimus''
`--+--''Dromiceiomimus''
`--''Ornithomimus''

Other evidence

Note that, ''Timimus'', early Cretaceous fossil remains (a femur) from Dinosaur Cove in Victoria in southeastern Australia are possibly ornithomimosaurian.

References

1. Nicholls, E. L., and Russell, A. P. (1985). "Structure and function of the pectoral girdle and forelimb of ''Struthiomimus altus'' (Theropoda: Ornithomimidae)." ''Palaeontology'', '28': 643-677.
2. Norell, M. A., Makovicky, P., and Currie, P. J. (2001). "The beaks of ostrich dinosaurs." ''Nature'', '412': 873-874.
3. Barrett, P. M. (2005). "The diet of ostrich dinosaurs (Theropoda: Ornithomimosauria)." ''Palaeontology'', '48': 347-358.

External links

★ A Guide to Ornithomimosauria with pictures

★ Khalaf-von Jaffa, Norman Ali Bassam Ali Taher (2006). Ornithomimid Dinosaur Tracks from Beit Zeit, West of Jerusalem, Palestine.Gazelle: The Palestinian Biological Bulletin. Number 56, August 2006. pp. 1-7.