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)^[2] or Predentata^[1] is an extinct clade 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 belong to the 'lizard-hipped' dinosaurs (the saurischians).

Some commonly known Ornithischians include the horned dinosaurs (ceratopsians), armored dinosaurs (stegosaurs and ankylosaurs), Iguanodon, and the "duck-billed" dinosaurs (hadrosaurids). Being herbivores that sometimes lived in herds, they were more numerous than the saurischians. Many were prey animals for the theropods and were usually smaller than the sauropods.

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.^[3]

Classification

Taxonomy

The simplified taxonomic list of ornithischian groups presented here follows a summary published by Thomas R. Holz, Jr. in 2011.^[4]

• 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.^[5] 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.^[6]

Cladogram after Butler et al., 2011. Ornithopoda includes Hypsilophodon, Jeholosaurus and others.^[7]

References

• 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

Wikispecies has information related to: Ornithischia

• Ornithischia, from Palæos. (cladogram, characteristics)

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