ornithischian

/awr'neuh this"kee euhn/, n.

1. any herbivorous dinosaur of the order Ornithischia, having a pelvis resembling that of a bird. Cf. saurischian.

adj.

2. belonging or pertaining to the Ornithischia.

[1900-05; < NL Ornithischi(a) (ornith- ORNITH- + -ischia < Gk ischíon ISCHIUM) + -AN]

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Any of the "bird-hipped" dinosaur species (order Ornithischia), herbivores with hip bones arranged like those of modern birds, with the pubis bone pointed backward.

Many species had a toothless, horny beak and powerful cheek teeth. Ornithischians flourished from the Late Triassic to the Late Cretaceous Epoch (227–65 million years ago). Many species (in the suborder Cerapoda) were bipedal and had ornate skull structures. Some were quadrupedal, with a massive head culminating in a great bony frill. Other quadrupeds (in the suborder Thyreophora) had small heads and were heavily plated and armoured along the back, and some had armour on the flanks and head as well. See also protoceratops; saurischian; stegosaur; triceratops.

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▪ dinosaur order

 any member of the large taxonomic group of herbivorous dinosaurs (dinosaur) comprising Triceratops and all dinosaurs more closely related to it than to birds (bird). The ornithischians (meaning “bird-hipped”) are one of the two major groups of dinosaurs, the other being the saurischians (saurischian). Ornithischians are so called because their hip bones were superficially arranged like those of birds in that a large portion of the pubic bone pointed down and toward the tail instead of down and forward as in other reptiles. Despite their name, ornithischians are not related to birds, which evolved from theropod saurischian dinosaurs. Ornithischians were also distinguished by an extra bone at the tip of the lower jaw called the predentary. Ornithischian teeth were leaf-shaped, and the jaw joint was located well below the occlusal plane (where the teeth met during chewing). Most species had a toothless horny beak useful for nipping off vegetation, along with powerful cheek teeth for grinding plant matter. Most ornithischians also possessed a lattice of bony tendons that crisscrossed adjacent projections of the vertebrae column. Some lineages evolved jaws with dozens of close-set rows of teeth on which the enamel was more developed on either the inside or the outside. As these teeth came into use, the side with thinner enamel was worn down, so the upper and lower jaws formed an efficient grinding surface. All ornithischians were herbivorous, which made ornithischian dinosaurs the first major group of terrestrial vertebrates (vertebrate) to have been dominated by plant eaters.

      The ornithischians can be divided into two distinct subgroups: Cerapoda (dinosaur) and Thyreophora (dinosaur). The cerapods are further subdivided into the Ornithopoda (ornithopod), Pachycephalosauria (dinosaur), and Ceratopsia (ceratopsian), though some authorities link the Pachycephalosauria and Ceratopsia together in a group called Marginocephalia. The Thyreophora consisted of the Stegosauria (dinosaur) and Ankylosauria (dinosaur) and their relatives.

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