ORNITHOPOD

any member of an infraorder (Ornithopoda) of ornithischian dinosaurs characterized by a bipedal (two-legged) stance, from which is derived their name (bird feet). Ornithopods, along with pachycephalosaurids and ceratopsians, made up the cerapod suborder of the ornithischians. Unlike most other ornithischians, ornithopods could walk and run on their hind legs, with the tail held off the ground, though they may have spent much of their time moving more slowly on all fours. Many were good runners who made use of their speed to escape predators. Ornithopods were the dinosaur equivalent of such modern ruminants as cattle and deer; their toothless, horny beaks were designed for cropping vegetation, which they ground up in their molarlike cheek teeth. The ornithopods flourished from the Late Triassic to the Late Cretaceous period (230 to 66.4 million years ago) and were one of the most successful and longest-lived dinosaur groups. They consisted of five families: Fabrosauridae, Heterodontosauridae, Hypsilophodontidae, Iguanodontidae, and Hadrosauridae. The fabrosaurs were the earliest and most primitive of the ornithopods; these small, lightly built, lizardlike creatures reached lengths of 0.61.2 m (24 feet). The heterodonts were more advanced and began to show the horny beaks and specialized teeth typical of ornithischians. The hypsilophodonts flourished from the Late Jurassic to the Late Cretaceous (163 to 66.4 million years) and were one of the most widespread and long-lived families of dinosaurs. Their members ranged in size from 1.5 to 7 m and show evidence of having been speedy runners. They may have given rise to the more advanced iguanodonts and hadrosaurs, who spent most of their time on all fours. The iguanodonts were medium-sized to large dinosaurs with the specialized grinding teeth typical of advanced ornithopods. The largest and best-known species, Iguanodon, reached a length of 9 m. The hadrosaurs, or duck-billed dinosaurs, received their name from their broad, flattened, elongated snouts and their toothless beaks. Their sets of grinding teeth and their cheek pouches were extremely well adapted to browsing on vegetation. The hadrosaurs commonly reached lengths of 911 m and were among the most abundant dinosaurs in North America by the end of the Cretaceous Period.