Ornithischia

Ornithischians Temporal range: Late Triassic–Late Cretaceous, 228–65 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
Class:	Reptilia
Superorder:	Dinosauria
Order:	†Ornithischia Seeley, 1888
Subgroups
†Fabrosauridae †Heterodontosauridae †Lesothosauridae †Pisanosauridae †Genasauria †Cerapoda †Thyreophora

Ornithischia ( /ɔrnɨˈθɪskiə/ or-ni-thiss-kee-ə)^[1] or Predentata is an extinct order of beaked, herbivorous dinosaurs. The name ornithischia is derived from the Greek ornitheos (ορνιθειος) meaning 'of a bird' and ischion (ισχιον) meaning 'hip joint'. They are known as the 'bird-hipped' dinosaurs because of their bird-like hip structure, even though birds actually descended from the 'lizard-hipped' dinosaurs (the saurischians). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians. They were prey animals for the theropods and were smaller than the sauropods.

Characteristics

Ornithischian pelvic structure (left side)

The Dinosauria superorder was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. This division, which has generally been accepted, is based on the evolution of the pelvis into a more bird-like structure (although birds did not descend from these dinosaurs), details in the vertebrae and armor and the possession of a 'predentary' bone. The predentary is an extra bone in the front of the lower jaw, which extends the dentary (the main lower jaw bone). 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 downward and toward the tail (backwards), 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 (forwards), as in ancestral lizard types. Ornithischians also had smaller antorbital fenestrae (holes in front of their eye sockets) than did 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, in the therizinosauroids and in 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.^[2]

Classification

Taxonomy

Linnaean ranks after Benton (2004):

• Order Ornithischia
  • Genus Eocursor
  • Genus Pisanosaurus
  • Family Fabrosauridae
  • Family Heterodontosauridae
  • Family Lesothosauridae
  • Suborder Thyreophora – (armored dinosaurs)
    • Family Scelidosauridae
    • Infraorder Stegosauria
    • Infraorder Ankylosauria
  • Suborder Cerapoda
    • Infraorder Ornithopoda
      • Family Hypsilophodontidae*
      • Family Hadrosauridae – (duck-billed dinosaurs)
    • Infraorder Pachycephalosauria
    • Infraorder Ceratopsia – (horned dinosaurs)

Phylogeny

Genasaurian ornithischians are divided into two clades: the Thyreophora and the Cerapoda. The Thyreophora include the Stegosauria (like the armored Stegosaurus) and the Ankylosauria (like Ankylosaurus). The Cerapoda include the Marginocephalia (Ceratopsia like the frilled ceratopsidae and Pachycephalosauria) and the Ornithopoda (including duck-bills (hadrosaurs) such as Edmontosaurus). The Cerapoda are 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.^[3]

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

Cladogram after Butler et al., 2011. Ornithopoda includes Hypsilophodon, Jeholosaurus and others.^[4]

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

References

1. ^ OED
2. ^ http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0007331;jsessionid=CE8F3EE637FFD712F6BF85FF02711889
3. ^ 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. 
4. ^ Richard J. Butler, Jin Liyong, Chen Jun, Pascal Godefroit (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. 

• Butler, R.J. 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)

Dinosaurs portal