Ornithischia

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Ornithischia
Temporal range:
Ma
(Possible Triassic record)
A collection of ornithischian fossil skeletons. Clockwise from upper left: Heterodontosaurus (Heterodontosauridae), Nipponosaurus (Ornithopoda), Borealopelta (Ankylosauria), Triceratops (Ceratopsia), Stegoceras (Pachycephalosauria), and Stegosaurus (Stegosauria).
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: †Ornithischia
Seeley
, 1888
Subgroups[2]
Synonyms

Ornithischia (

theropod dinosaurs.[4]
Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, and the ornithopods.[4] There is strong evidence that certain groups of ornithischians lived in herds,[4][6] often segregated by age group, with juveniles forming their own flocks separate from adults.[7] Some were at least partially covered in filamentous (hair- or feather- like) pelts, and there is much debate over whether these filaments found in specimens of Tianyulong, Psittacosaurus,[8] and Kulindadromeus may have been primitive feathers.[9]

Description

In 1887, Harry Seeley divided Dinosauria into two clades: Ornithischia and Saurischia. Ornithischia is a strongly supported clade with an abundance of diagnostic characters (common traits).[4] The two most notable traits are a "bird-like" hip and beak-like predentary structure, though they shared other features as well.[4]

Early ornithischians were relatively small dinosaurs, averaging about 1–2 meters in body length, with a triangular skull that had large circular orbits on the sides. This suggests that early ornithischians had relatively huge eyes that faced laterally. The forelimbs of early ornithischians are considerably shorter than their hindlimbs. A small forelimb such as those present in early ornithischians would not have been useful for locomotion, and it is evident that early ornithischians were bipedal dinosaurs. The entire skeleton was lightly built, with a largely fenestrated skull and a very stout neck and trunk. The tail is nearly half of the dinosaurs' overall length. The long tail presumably acted as a counterbalance and as a compensating mechanism for shifts in the creature's center of gravity. The hindlimbs of early ornithischians show that the tibia is considerably longer than the femur, a feature that suggests that early ornithischians were adapted for bipedality, and were fast runners.[10]

"Bird-hip"

The ornithischian pelvis was "opisthopubic", meaning that the pubis pointed down and backwards (

anterior), as in ancestral reptiles (Figure 1b).[4]

  • Figure 1a - Ornithischian opisthopubic pelvic structure (left side)[4]
    Figure 1a - Ornithischian opisthopubic pelvic structure (left side)[4]
  • Figure 1b - Saurischian propubic pelvic structure (left side)[4]
    Figure 1b - Saurischian propubic pelvic structure (left side)[4]

The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and

therizinosauroids).[11] Some argue that the opisthopubic pelvis evolved a fourth time, in the clade Dromaeosauridae, but this is controversial, as other authors argue that dromaeosaurids are mesopubic.[11] It has also been argued that the opisthopubic condition is basal to maniraptorans (including among others birds, therizinosauroids and dromaeosaurids), with some clades having later experienced a reversal to the propubic condition.[12]

Figure 2 - Heterodontosaurus skull with palpebral bone (colored red), antorbital fenestra (yellow) and predentary (green).[4]

Predentary

Ornithischians shared a unique bone called the predentary (Figure 2).

rostral bone.[13]

In 2017 Baron & Barrett suggested that Chilesaurus may represent an early diverging ornithischian that had not yet acquired the predentary of all other ornithischians.[14]

Other characteristics

  • Ornithischians had paired premaxillary bones that were toothless and roughened at the tip of the snout (presumably due to the attachment of a keratinous beak).[4]
  • Ornithischians developed a narrow "eyebrow", or palpebral bone, across the outside of the eye socket.[4]
  • Ornithischians had reduced, or even closed-off, antorbital fenestrae (the fenestra in front of the eye socket).[4]
  • Ornithischian jaw joints were lowered below the level of the teeth, bringing the teeth into simultaneous occlusion.[4]
  • Ornithischians had "leaf-shaped" cheek teeth.[4]
  • Ornithischian backbones were stiffened near the pelvis by the ossification of tendons above the sacrum. Additionally, ornithischians had at least five sacral vertebrae attaching to the pelvis.[4]

Classification

Ornithischia is a branch-based clade defined as all dinosaurs more closely related to Iguanodon than to either Allosaurus or Camarasaurus.[2]

During its long history of study, numerous different subgroups of ornithischians have been recognized. Since the 1980s, many have been given

International Code of Phylogenetic Nomenclature (also known as the ICPN or PhyloCode) was formalized, which treats all the definitions made under the ICZN as informal and inapplicable for scientific use unless re-registered under the ICPN. In 2021, an international group of researchers led by Daniel Madzia registered almost all of the most commonly used ornithischian clades under the ICPN, with the intent of standardizing their definitions. A composite cladogram showing most of their clades and specifiers is shown below.[2]

Due to the complexities of early ornithischian evolution, Madzia and colleagues named the new clade Saphornithischia to encompass the taxa traditionally considered ornithischians, to the exclusion of potentially early forms like Silesauridae, Pisanosaurus or Chilesaurus, which some authors have considered basal ornithischians while others consider them non-ornithischians or non-dinosaurs.[2]

In 2022, Norman et al. redefined the clade Prionodontia, proposed by Richard Owen in 1874 and often seen as a synonym of Ornithischia, to be the least inclusive clade including Iguanodon, Scelidosaurus, and Echinodon, similar to its original proposal. They note that it would represent the same clade as Saphornithischia, and despite the latter being registered under the ICPN, Prionodontia may receive priority as it was coined over a century earlier. Their analysis also treats "silesaurids" as a paraphyletic grade of basal ornithischians forming no distinct family, so they coin the clade Parapredentata to include all dinosaurs in the least inclusive clade containing Silesaurus and Iguanodon.[1]

Allosaurus (Theropoda)

Camarasaurus (Sauropodomorpha)

Ornithischia

Heterodontosaurus tucki (Heterodontosauridae)

Genasauria
Thyreophora
Stegosauria

Huayangosaurus taibaii (Huayangosauridae
)

Stegosaurus stenops (Stegosauridae)

Ankylosauria

Mymoorapelta maysi

Nodosauridae

Polacanthus foxii (Polacanthinae)

Nodosaurinae

Nodosaurus textilis

Panoplosaurus mirus (Panoplosaurini
)

Struthiosaurus austriacus (Struthiosaurini
)

Ankylosauridae

Hylaeosaurus armatus

Shamosaurinae

Gobisaurus domoculus

Shamosaurus scutatus

Ankylosaurinae

Pinacosaurus grangeri

Saichania chulsanensis

Ankylosaurini)

Neornithischia

Jeholosaurus shangyuanensis (Jeholosauridae
)

Thescelosauridae

Orodromeus makelai (Orodrominae
)

Thescelosaurus neglectus (Thescelosaurinae
)

Hypsilophodon foxii (Hypsilophodontidae)

Cerapoda
Marginocephalia
Pachycephalosauria

Stegoceras validum

Pachycephalosaurinae

Sphaerotholus goodwini

Pachycephalosaurini
)

Ceratopsia

Psittacosaurus mongoliensis

Chaoyangsaurus youngi (Chaoyangsauridae
)

Neoceratopsia

Leptoceratops gracilis (Leptoceratopsidae
)

Coronosauria

Protoceratops andrewsi (Protoceratopsidae)

Ceratopsidae

Ceratops montanus

Chasmosaurinae

Chasmosaurus belli

Anchiceratops ornatus

Arrhinoceratops brachyops

Triceratops horridus (Triceratopsini
)

Centrosaurinae

Nasutoceratopsini
)

Eucentrosaura
Centrosaurini)
Pachyrhinosaurini

Achelousaurus horneri

Pachyrhinosaurus canadensis

Ornithopoda
Elasmaria

Gasparinisaura cincosaltensis

Macrogryphosaurus gondwanicus

Talenkauen santacrucensis

Iguanodontia
Rhabdodontidae

Rhabdodon priscus

Zalmoxes robustus

Tenontosaurus tilletti

Dryomorpha

Dryosaurus altus (Dryosauridae
)

Ankylopollexia

Camptosauridae)

Styracosterna

Hypselospinus fittoni

Hadrosauriformes

Iguanodon bernissartensis (Iguanodontidae)

Hadrosauroidea

Probactrosaurus gobiensis

Hadrosauridae

Hadrosaurinae
)

Saurolophus osborni (Saurolophinae)

Lambeosaurus lambei (Lambeosaurinae
)

Evolution

As of the late 2010s, the exact placement of Ornithischia within the dinosaur lineage is a contentious issue.[15] Traditionally, Ornithischia is considered the sister group of Saurischia (which contains Theropoda and Sauropodomorpha).[16] However, in the alternative hypothesis of dinosaur relationships that was proposed by Baron, Norman & Barrett in the journal Nature in 2017, Ornithischia was recovered as the sister group to the Theropoda, which grouped together in the clade Ornithoscelida.[17][18] This hypothesis was recently challenged by an international consortium of early dinosaur experts led by Max Langer. However, the data that supported the more traditional placement of Ornithischia, as sister taxon of Saurischia, was found not to be statistically significant from the evidence that supported the Ornithoscelida hypothesis, in both the study by Langer et al. and the reply to the study by Baron et al.[19][20] A further 2017 study found some support for the previously abandoned Phytodinosauria model, which classifies ornithischians together with sauropodomorphs.[21]

Palaeoecology

Ornithischians shifted from bipedal to quadrupedal posture at least three times in their evolutionary history and it has been shown primitive members may have been capable of both forms of movement.[22]

Most ornithischians were herbivorous.[4] In fact, most of the unifying characters of Ornithischia are thought to be related to this herbivory.[4] For example, the shift to an opisthopubic pelvis is thought to be related to the development of a large stomach or stomachs and gut which would allow ornithischians to more effectively digest plant matter.[4] The smallest known ornithischian is Fruitadens haagarorum.[23] The largest Fruitadens individuals reached just 65–75 cm. Previously, only carnivorous, saurischian theropods were known to reach such small sizes.[23] At the other end of the spectrum, the largest known ornithischians reach about 15 meters (smaller than the largest saurischians).[24]

However, not all ornithischians were strictly herbivorous. Some groups, like the

Liaoningosaurus paradoxus, appears to have been at least partially carnivorous, with hooked claws, fork-like teeth, and stomach contents suggesting that it may have fed on fish.[26]

There is strong evidence that some ornithischians lived in herds.[4][6] This evidence consists of multiple bone beds where large numbers of individuals of the same species and of different age groups died simultaneously.[4][6]

See also

Notes

  1. ^ plural ischia

References

  1. ^
    doi:10.1093/zoolinnean/zlac062
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  2. ^
    PMID 34966571
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  3. S2CID 85819339
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  4. ^
    ISBN 978-1107276468
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  5. ^ Colbert, Edwin H. (Edwin Harris); Knight, Charles Robert (1951). The dinosaur book: the ruling reptiles and their relatives. New York: McGraw-Hill. p. 152.
  6. ^
    S2CID 128781816
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  7. doi:10.4202/app.2012.0128
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  8. S2CID 17781405
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  9. S2CID 206556907. Archived from the original
    (PDF) on 2019-02-09. Retrieved 2016-08-28.
  10. ^ Colbert, E. H. (1981). A primitive ornithischian dinosaur from the Kayenta Formation of Arizona. Museum Northern Arizona Bull. 53, 1-61
  11. ^
    ISBN 9780080494746
    .
  12. ^ Holtz, T.R. and Osmólska, H. (2004). "Saurischia." In Weishampel, Dodson and Osmólska (eds.), The Dinosauria, second edition. Berkeley: University of California Press.
  13. ISSN 1932-8486
    .
  14. PMID 28814574
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  15. S2CID 89924902
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  16. doi:10.1098/rspl.1887.0117
    .
  17. S2CID 205254710
    .
  18. ^ "New study shakes the roots of the dinosaur family tree". 2017-03-22.
  19. S2CID 205260354
    .
  20. S2CID 205260360
    .
  21. PMID 29134086
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  22. PMID 19806213
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  23. ^
    PMID 19846460
    .
  24. S2CID 85351874
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  25. PMID 16701088
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  26. ^ Ji, Q.; Wu, X.; Cheng, Y.; Ten, F.; Wang, X.; Ji, Y. (2016). "Fish-hunting ankylosaurs (Dinosauria, Ornithischia) from the Cretaceous of China". Journal of Geology. 40: 2.
  • Sereno, P.C. (1986). "Phylogeny of the bird-hipped dinosaurs (order Ornithischia)". National Geographic Research. 2 (2): 234–256.

External links

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