Ornithopods have arguably the best fossil record of any major dinosaur clade and are well represented by body fossils, including eggs and growth series from \ embryos to full-sized adults in some species; and trace fossils, such as trackways, nests, and coprolites. As a result of this combination of quality and quantity of fossil material, lifestyles of some species of ornithopods are among the best understood of all dinosaurs. This understanding has lead to revisions of some stereotypes of dinosaurs in general. Recent conceptions about dinosaurs resulted from the study of ornithopods and include intraspecific visual and vocal communication, nurturing behavior, formation of communal nesting grounds, and herding.
The fossil record for primitive ornithischians, which may have been linked to ornithopods begins during the Late Triassic, but the first undoubted ornithopods are the Early Jurassic heterodontosaurids of South Africa. These ornithopods, exemplified by Heterodontosaurus, were small and probably quick-moving bipedal herbivores with differentiated teeth as their most distinctive characteristic. Soon after the arrival of these primitive ornithopods on the Jurassic scene, other non-iguanodontians, such as Hypsilophodon and Orodromeus, dispersed throughout most continents from the Middle Jurassic through to the Late Cretaceous. These relatively larger bipedal ornithopods are mostly understood through a limited body fossil record, and their most striking evolutionary development was pleurokinesis, which aided considerably in processing vegetation.
By far the most successful clade of ornithopods in diversity, evolutionary specializations, and representation in the fossil record through sheer numbers was Iguanodontia. Iguanodontians eventually lived in most terrestrial environments on all continents during the Late Jurassic through to the Late Cretaceous. The Early Cretaceous Iguanodon is the most famous genus of the iguanodontians, but the hadrosaurids, which probably also began in the Early Cretaceous, are the best known of all ornithopods. Hadrosaurids, which include hadrosaurines (Hadrosaurus, Maiasaura) and lambeosaurines (Lambeosaurus, Parasaurolophus), are some of the most ornate and socially complex of dinosaurs. These dinosaurs were large (some as long as 15 meters), quadrupedal or bipedal animals that developed specialized adaptations to feeding, such as extensive and continually replaced dental batteries. They also have the best-understood reproductive cycles of all dinosaurs, epitomized by the nesting grounds interpreted for Maiasaura and Hypacrosaurus.
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