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iOrnithomimosaurs
Fossil range: Cretaceous
Gallimimus 1 NHM3
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Superorder: Dinosauria
Order: Saurischia
Suborder: Theropoda
Infraorder: Ornithomimosauria
Barsbold, 1976
Families

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 Upper Cretaceous Period of Laurasia (now Asia, Europe and North America). The skull, sitting atop a long neck, was relatively small, with large eyes. Some primitive species had teeth but most had toothless beaks.

Struthiomimus sedens manus

Struthiomimus manus, showing claws (from OUMNH).

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 ornithomimid hide was probably feathered rather than scaly.

The group first appeared in the Lower Cretaceous and persisted until the Upper 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.

Ornithomimosaurs probably acquired most of their calories from plants but may have eaten small vertebrates and insects as well. Henry Fairfield Osborn suggested that the long, sloth-like 'arms' may have been used to pull down branches on which to feed. They may also have constituted a dangerous weapon. 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. The presence of gastroliths in the stomach of some ornithomimids fit this hypothesis.

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).

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, prominant 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 closr relatives of ornithomimosaurs than maniraptorans. The Ornithomimosauria is an inclusive clade that contains primitive ornithomimosaurs and ornithomimids proper.

Classification

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.

External links

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