Ornithischia

Ornithischians Temporal range: Late Triassic–Late Cretaceous, 231.4–66 Ma PreЄ Є O S D C P T J K Pg N
Edmontosaurus pelvis (showing ornithischian structure – left side) Oxford University Museum of Natural History
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia Seeley, 1888
Subgroups
†Eocursor †Fabrosaurus †Pisanosaurus †Taveirosaurus †Trimucrodon †Heterodontosauridae †Genasauria
Synonyms
Predentata[1] Marsh, 1894

Ornithischia (/ɔːrnᵻˈθɪskiə/ or-ni-thiss-kee-ə) is an extinct clade of herbivorous dinosaurs with a pelvic structure similar to that of birds.^[2] The name ornithischia, or "bird-hipped, reflects this similarity and is derived from the Greek ornitheos (ορνιθειος) meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint'. It is important to note, however, that birds are actually members of the saurischian, or "lizard-hipped" dinosaurs.

Ornithischia is a "extremely well supported" clade^[2] that plays a significant role in the dinosaur phylogeny as one of the two major clades. Many adaptations define the group though most are thought to be linked to Ornithischia's herbivory. Additionally, many Ornithischians are beaked.

Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs such as stegosaurs and ankylosaurs, pachycephalosaurs and the ornithopods.^[2] There is strong evidence that certain members of ornithischia lived in herds^[2][3] and there is even some evidence to suggest certain species may have cared for their young.^[4] Additionally, there is much debate over whether filamentous integumentary structures displayed in specimens of the genus Tianyulong^[5] and bristle-like integumentary structures displayed in specimens of the genus Psittacosaurus^[6] may have been precursors to feathers and, if so, whether later ornithischians may have been feathered.

Characteristics

Ornithischian pelvic structure (left side)

The clade Dinosauria was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. This division, which has generally been accepted, is based in part on the evolution of the pelvis into a more bird-like structure (although birds did not descend from ornithischian dinosaurs), details in the vertebrae and armor, and the possession of a predentary bone. The predentary is a single toothless bone that lies in the front of the dentaries, extending the lower jaw. The predentary coincides with the premaxilla in the upper jaw. Together they form a beak-like apparatus used to clip off plant material.

The ornithischian pubis bone points down and towards the tail (posterior), parallel with the ischium, with a forward-pointing process to support the abdomen. This makes a four-pronged pelvic structure. In contrast to this, the saurischian pubis points downward and toward the head (anterior), as in ancestral reptiles. Ornithischians also typically possess smaller antorbital fenestra (openings in the skull in front of their eye sockets) than saurischians and a wider, more stable pelvis. A bird-like pubis arrangement, parallel to the vertebral column, evolved independently three times in dinosaur evolution, namely in the ornithischians, therizinosauroids and bird-like dromaeosaurids.

Ornithischians shifted from bipedal to quadrupedal posture at least three times in their evolutionary history and have been shown to have been capable of adopting both postures early in their evolutionary history.^[7]

Classification

Taxonomy

The simplified taxonomic list of ornithischian groups presented here follows a summary published by Thomas R. Holz, Jr. in 2011.^[8]

• Ornithischia
  • Eocursor
  • Fabrosaurus
  • Pisanosaurus
  • Taveirosaurus
  • Trimucrodon
  • Heterodontosauridae – (strong-snouted dinosaurs)
  • Thyreophora – (armored dinosaurs)
    • Stegosauria
      • Huayangosauridae
      • Stegosauridae
    • Ankylosauria
      • Nodosauridae
      • Ankylosauridae
  • Neornithischia
    • Ornithopoda
      • Thescelosauridae – (last primitive beaked dinosaurs)
      • Iguanodontia
        • Rhabdodontidae – (advanced European beaked dinosaurs)
        • Dryosauridae – (small advanced beaked dinosaurs)
        • Camptosauridae – (mid-sized advanced beaked dinosaurs)
        • Hadrosauridae – (duckbilled dinosaurs)
    • Marginocephalia
      • Pachycephalosauria – (boneheaded dinosaurs)
      • Ceratopsia
        • Chaoyangsauridae – (early parrot-beaked dinosaurs)
        • Psittacosauridae – (parrot dinosaurs)
        • Neoceratopsia
          • Leptoceratopsidae – (small-frilled dinosaurs)
          • Bagaceratopsidae – (lump-nosed frilled dinosaurs)
          • Protoceratopsidae – (deep-tailed frilled dinosaurs)
          • Ceratopsidae – (horned dinosaurs)

Phylogeny

Ornithischia is a branch-based taxon defined as all dinosaurs more closely related to Triceratops horridus Marsh, 1889 than to either Passer domesticus (Linnaeus, 1758) or Saltasaurus loricatus Bonaparte & Powell, 1980.^[9] Genasauria comprises the clades Thyreophora and Neornithischia. Thyreophora includes Stegosauria (like the armored Stegosaurus) and Ankylosauria (like Ankylosaurus). Neornithischia comprises several basal taxa, Marginocephalia (Ceratopsia and Pachycephalosauria), and Ornithopoda (including duck-bills (hadrosaurs), such as Edmontosaurus). Cerapoda is a relatively recent concept (Sereno, 1986).

The cladogram below follows a 2009 analysis by Zheng and colleagues. All tested members of Heterodontosauridae form a polytomy.^[10]

Ornithischia	Pisanosaurus Heterodontosauridae Genasauria Thyreophora Lesothosaurus Scutellosaurus Emausaurus Scelidosaurus Stegosauria Ankylosauria Neornithischia Stormbergia Agilisaurus Hexinlusaurus Cerapoda Othnielia Hypsilophodon Jeholosaurus Yandusaurus Orodromeus Zephyrosaurus Ornithopoda Marginocephalia Pachycephalosauria Ceratopsia

Cladogram after Butler et al., 2011. Ornithopoda includes Hypsilophodon, Jeholosaurus and others.^[5]

Ornithischia	Pisanosaurus Heterodontosauridae Eocursor Genasauria Lesothosaurus Thyreophora Scutellosaurus Emausaurus Scelidosaurus Stegosauria Ankylosauria Neornithischia Stormbergia Agilisaurus Hexinlusaurus Othnielosaurus Cerapoda Ornithopoda Marginocephalia Pachycephalosauria Ceratopsia

References

1. Ferigolo, J.; Langer, M. C. (2007). "A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone". Historical Biology 19: 23. doi:10.1080/08912960600845767.
2. Fastovsky, David E.; Weishampel, David B. (2012). Dinosaurs: A Concise Natural History. Cambridge: Cambridge University Press. ISBN 1107276462.
3. Qi, Zhao; Barrett, Paul M.; Eberth, David A. (2007-09-01). "Social Behaviour and Mass Mortality in the Basal Ceratopsian Dinosaur Psittacosaurus (early Cretaceous, People's Republic of China)". Palaeontology 50 (5): 1023–1029. doi:10.1111/j.1475-4983.2007.00709.x. ISSN 1475-4983.
4. Meng, Qingjin; Liu, Jinyuan; Varricchio, David J.; Huang, Timothy; Gao, Chunling. "Palaeontology: Parental care in an ornithischian dinosaur". Nature 431 (7005): 145–146. doi:10.1038/431145a.
5. Richard J. Butler, Jin Liyong, Chen Jun, Pascal Godefroit (May 2011). "The postcranial osteology and phylogenetic position of the small ornithischian dinosaur Changchunsaurus parvus from the Quantou Formation (Cretaceous: Aptian–Cenomanian) of Jilin Province, north-eastern China". Palaeontology 54 (3): 667–683. doi:10.1111/j.1475-4983.2011.01046.x.
6. Mayr, Gerald; Peters, Stefan D.; Plodowski, Gerhard; Vogel, Olaf (2002-08-01). "Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus". Naturwissenschaften 89 (8): 361–365. doi:10.1007/s00114-002-0339-6. ISSN 0028-1042.
7. Jeffrey A. Wilson; Claudia A. Marsicano; Roger M. H. Smith (6 October 2009). "Dynamic Locomotor Capabilities Revealed by Early Dinosaur Trackmakers from Southern Africa". PLOS ONE.
8. Holtz, Thomas R. Jr. (2012) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2011 Appendix.
9. Butler, Richard; Upchurch, Paul; Norman, David (2008). "The phylogeny of ornithischian dinosaurs". Journal of Systematic Paleontology 6 (1): 1–40. doi:10.1017/S1477201907002271.
10. Zheng, Xiao-Ting; You, Hai-Lu; Xu, Xing; Dong, Zhi-Ming (19 March 2009). "An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures". Nature 458 (7236): 333–336. doi:10.1038/nature07856. PMID 19295609.

• Butler, R.J. (October 2005). The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho. Zoological Journal of the Linnean Society 145(2):175–218.
• Sereno, P.C. 1986. Phylogeny of the bird-hipped dinosaurs (order Ornithischia). National Geographic Research 2(2):234–256.

External links

• Ornithischia, from Palæos. (cladogram, characteristics)