Ornithischia

Ornithischians Temporal range: Late Triassic–Late Cretaceous, 231.4–66 Ma PreЄ Є O S D C P T J K Pg N
Diversity of ornithischian dinosaurs.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
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 mainly herbivorous dinosaurs characterized by 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". Birds are members of the saurischian, or "lizard-hipped", dinosaurs.^[2]

Ornithischia is well-supported as one of the two major groups in the Dinosauria.^[2] Many characteristics define the group though most are thought to be linked to herbivory.

Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophera) such as stegosaurs and ankylosaurs, pachycephalosaurids and the ornithopods.^[2] There is strong evidence that certain groups of ornithischians lived in herds,^[2][3] often segregated by age group, with juveniles forming their own flocks separate from adults.^[4]

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,^[5] Psittacosaurus,^[6] and Kulindadromeus may have been primitive feathers.^[7]

Anatomy & Diagnostic Characters

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).^[2] The two most notable traits are a "bird-like" hip and beak-like predentary structure though they shared other features as well.^[2]

"Bird-hip"

The ornithischian pelvis is "opisthopubic" meaning that the pubis points down and back (posterior) parallel with the ischium (Figure 1a).^[2] Additionally, the pelvis has a forward-pointing process to support the abdomen.^[2] This results in a four-pronged pelvic structure. In contrast to this, the saurischian pelvis is "propubic" meaning the pubis points toward the head (anterior), as in ancestral reptiles (Figure 1b).^[2]

The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and therizinosauroids).^[8] Some argue that the opisthopubic pelvis evolved a fourth time in the clade Dromaeosauridae but this is controversial as other authors argue that dromeosaurids are mesopubic.^[8]

Predentary

Ornithischians share a unique bone called the predentary (Figure 2).^[2] This unpaired bone is found at the front of the lower jaws and presumably aided ornithischians in cropping vegetation.

Figure 1a - Ornithischian propisthopubic pelvic structure (left side)	Figure 1b - Saurischian propubic pelvic structure (left side)	Figure 2 - Heterodontosaurus skull with palpebral bone (red), antorbital fenestra (yellow) and predentary (green) colored.

Other Characteristics

Beak attachment

Ornithischians had paired premaxillary bones (not unique). At the tip of the snout, these are toothless and roughened (presumably due to the attachment of a keratinous beak).^[2]

Palpebral Bone

Development of a narrow "eyebrow" bone across the outside of the orbital fenestra (eye socket).^[2]

Reduced Antorbital Opening

A reduced, or even closed-off, antorbital fenestra (the hole just before the eye socket).^[2] The purpose of the antorbital opening is unclear though researchers are leaning towards the "Pneumatic Hypothesis" which states that the opening is related to airflow.^[9]

Jaw Joint

Ornithischian jaw joints were lowered below the level of the teeth.^[2] Jaw joints at the same level of the teeth result in a slicing motion of the jaw - useful for animals who shred their food. Herbivores, however, often grind their food to make digestion easier. Jaw joints that are raised above or lowered below the level of the teeth bringing the teeth into simultaneous occlusion.^[2]

Cheek Teeth

"Leaf-shaped" cheek teeth.^[2]

Vertebral Adaptations

Ornithischian backbones were stiffened near the pelvis by the ossification of tendons above the sacrum.^[2] Additionally, ornithischians had at least five sacral vertebrae attaching to the pelvis.

Ecology

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.^[10]

Most ornithischians were herbivorous.^[2] In fact, most of the unifying characters of Ornithischia are thought to be related to this herbivory.^[2] 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 digest plant matter better.^[2] The smallest known Ornithischians are Fruitadens haagarorum.^[11] The largest Fruitadens individuals reached just 65–75 cm. Previously, only carnivorous, saurischian theropods were known to reach such small sizes.^[11] At the other end of the spectrum, the largest known ornithischians reach about 15 meters (smaller than the largest saurischians).^[12]

However, not all ornithischians were strictly herbivorous. Some groups, like the heterodontosaurids, were likely omnivores.^[13] At least one species of ankylosaurian, 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.^[14]

There is strong evidence that some ornithischians lived in herds.^[2][3] This evidence consists of multiple bones beds with large numbers of the same species in different age classes who died simultaneously.^[2][3]

Classification

Taxonomy

The simplified taxonomic list of ornithischian groups presented here follows a summary published by Thomas R. Holtz, Jr. in 2011.^[15]

• 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.^[16] 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.^[17]

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

References

• 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

Wikispecies has information related to: Ornithischia

• Ornithischia, from Palæos. (cladogram, characteristics)

Basal archosauromorphs
Kingdom: Animalia Phylum: Chordata Class: Reptilia
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Related categories Archosaurs