| Ornithomimosaurs | |
|---|---|
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| Collection of seven ornithomimosaurs, clockwise from top left:Gallimimus,Anserimimus,Ornithomimus,Deinocheirus,Harpymimus,Struthiomimus and"Gallimimus mongoliensis" | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Maniraptoriformes |
| Clade: | †Ornithomimosauria Barsbold, 1976 |
| Subgroups[9] | |
| Synonyms | |
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Ornithomimosauria ("bird-mimic lizards") aretheropoddinosaurs which bore a superficial resemblance to the modern-dayostrich. They were fast, omnivorous or herbivorous dinosaurs from theCretaceousPeriod ofLaurasia (nowAsia,Europe andNorth America), as well as possibly Africa.[10][5] The group first appeared in theEarly Cretaceous and persisted until theLate Cretaceous. Primitive members of the group includeNqwebasaurus,Pelecanimimus,Shenzhousaurus,Hexing andDeinocheirus, the arms of which reached 2.4 m (8 feet) in length. More advanced species, members of the familyOrnithomimidae, includeGallimimus,Struthiomimus, andOrnithomimus. Some paleontologists, likePaul Sereno, consider the enigmaticalvarezsaurids to be close relatives of the ornithomimosaurs and place them together in the superfamily Ornithomimoidea (see classification below).
The skulls of ornithomimosaurs were small, with large eyes, above relatively long and slender necks. The most basal members of the taxon (such asPelecanimimus andHarpymimus) had a jaw with small teeth, while the later and more derived species had a toothlessbeak.[11] 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 othercoelurosaurs, the ornithomimosaurian hide was feathered rather than scaly.
Unambiguous evidence of feathers is known fromOrnithomimus edmontonicus, of which there are multiple specimens preserving feather traces.[12]Deinocheirus andPelecanimimus have been speculated to be feathered as well, the former due to the presence of apygostyle,[13] and the later due to possible impressions (otherwise taken to be collagen fibers). There is a debate on whether ornithomimids possessed thepennaceous feathers seen inPennaraptora.[14] Otherwise, a veryostrich-likeplumage and feather range is known in one specimen ofOrnithomimus.[15]
Named byO.C. Marsh in 1890, the familyOrnithomimidae 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 theCoelurosauria. Recognizing the distinctiveness of ornithomimids compared to other dinosaurs,Rinchen Barsbold placed ornithomimids within their owninfraorder, Ornithomimosauria, in 1976. The contents of Ornithomimidae and Ornithomimosauria varied from author to author ascladistic definitions began to appear for the groups in the 1990s.
In the early 1990s, prominent paleontologists such asThomas R. Holtz Jr. proposed a close relationship between theropods with anarctometatarsalian foot; that is, bipedal dinosaurs in which the upper foot bones were 'pinched' together, an adaptation for running. Holtz (1994) defined theclade Arctometatarsalia as "the first theropod to develop thearctometatarsalian pes and all of its descendants." This group included theTroodontidae,Tyrannosauroidea, and Ornithomimosauria. Holtz (1996, 2000) later refined this definition to the branch-based "Ornithomimus and all theropods sharing a more recent common ancestor withOrnithomimus 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 onOrnithomimus, it became redundant with the name Ornithomimosauria under broad definitions of that clade, and the name Arctometatarsalia was mostly abandoned.
ThepaleontologistPaul Sereno, in 2005, proposed the clade "Ornithomimiformes", defining them as all species closer toOrnithomimus edmontonicus than toPasser domesticus. Because he had redefined Ornithomimosauria in a much narrower sense, a new term was made necessary within his preferred terminology to denote the clade containing the sistergroups Ornithomimosauria and Alvarezsauridae — previously the latter had been contained within the former. However, this concept only appeared on Sereno's Web site and has not yet been officially published as a valid name.[16] "Ornithomimiformes" was identical in content to Holtz's Arctometatarsalia, as it has a very similar definition. While "Ornithomimiformes" is the newer group, Sereno rejected the idea that Arctometatarsalia should take precedence, because the meaning of the former name has been changed very radically by Holtz.[16]
Ornithomimosauria has variously been used for the branch-based group of all dinosaurs closer toOrnithomimus than to birds, and in more restrictive senses. The more exclusive sense began to grow in popularity when the possibility arose thatalvarezsaurids might fall under Ornithomimosauria if an inclusive definition were adopted. Another clade, Ornithomimiformes, was defined by Sereno (2005) as (Ornithomimus velox >Passer domesticus) and replaces the more inclusive use of Ornithomimosauria when alvarezsaurids or some other group are found to be closer relatives of ornithomimosaurs thanmaniraptorans, with Ornithomimosauria redefined to include dinosaurs closer toOrnithomimus than to alvarezsaurids.Gregory S. Paul has proposed that Ornithomimosauria might be a group of primitive, flightless birds, more advanced thanDeinonychosauria andOviraptorosauria.[17]
The cladogram below follows an analysis by Yuong-Nam Lee, Rinchen Barsbold, Philip J. Currie, Yoshitsugu Kobayashi, Hang-Jae Lee, Pascal Godefroit, François Escuillié & Tsogtbaatar Chinzorig. The analysis was published in 2014, and includes many ornithomimosaurian taxa.[9]
The cladogram below follows the phylogenetic analysis by Scott Hartman and colleagues in 2019, which has included a vast majority of species and uncertain specimens, resulting in a novel phylogenetic arrangement.[18]
| Ornithomimosauria |
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Below is a cladogram by Serrano-Brañaset al., 2020, showing an analysis more in line with previous assumptions about ornithomimosaur classification.[19]
| Ornithomimosauria | |
Ornithomimosaurs probably acquired most of their calories from plants. Many ornithomimosaurs, including primitive species, have been found with numerousgastroliths 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.[20] 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. However, they may have been omnivores that ate both plants and small animal prey.
Comparisons between thescleral rings of two ornithomimosaur genera (Garudimimus andOrnithomimus) and modern birds and reptiles indicate that they may have beencathemeral, active throughout the day at short intervals.[21]
Ornithomimosaurs are fairly well known for their gregarious life-styles. Some of the first findings of ornithomimosaurbonebeds were reported from theIren Dabasu Formation in 1993 byCharles W. Gilmore. The bonebed consisted of numerous individuals ofArchaeornithomimus ranging from young to adult remains.[22] Multiple specimens ofSinornithomimus were collected from a single monospecific bonebed with a considerable density of juvenile individuals—out of 14, 11 were juveniles—, suggesting agregarious behavior for an increased protection from predators. The notable abundance of juveniles indicates a high mortality in them or that a large mass-mortality event of an entire group occurred, with more susceptibility in juveniles. Additionally, the increase in the tibia-femur ratio through theontogeny ofSinornithomimus may indicate highercursorial capacities in adults than in juveniles.[23] Moreover, and also contrary to theSinornithomimus bonebed, a high concentration of ornithomimosaur specimens from the Bayshi Tsav locality was collected in a single multitaxic bonebed that is composed of at least five individuals at different ontogenetic stages. It is unlikely that the individuals of this bonebed represent a strategicalsocial behaviour of a single species given the identification of at least two different taxa. Under this consideration, it is possible that a small pack of more than 10 individuals of different ornithomimosaurianherds was travellingtogether in optimal areas tofind food resources,nesting sites or something else.[24][25]
A right second metatarsal belonging to a large-bodied ornithomimosaur weighing approximately 432 kg has been described from Mississippi with a "butterfly" fragment fracture pattern characteristic of blunt force trauma, likely as a result of an interaction with a predator or a violent bout of intraspecific competition.[26]
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