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Clades and Species of Ornithopoda Paleobiogeography and Evolutionary History of Ornithopoda Ornithopods as Living Animals Summary Discussion Questions Bibliography

Ornithopoda was one of the most diverse and geographically widespread of all dinosaur clades. Their success originated in the Early

Why Study Ornithopods?

Jurassic and continued until the end of the Cretaceous when, as with other non-avian dinosaurs, they became extinct. Their diversity can be best illustrated by their size range - some species were as small as 1 meter but others were about 15 meters long, placing them well within the size range of theropods and around the same mass as some sauropods. They are sometimes known as the "cows of the Mesozoic" in recognition of their specialized herbivore diets, perceived passivity, and probable gregarious nature. However, some of their members, especially the hadro-saurids, had complex intraspecific behaviors more similar to birds than bovines. The most innovative adaptation ornithopods had in common with one another was the development of teeth and jaws that were arguably the most efficient grinders of plant food ever devised in terrestrial herbivores. Indeed, some ornithopods had large arrays of teeth (dental batteries) and moveable skull parts that would have surpassed those of any modern mammalian plant eater.

Ornithopods have the most complete geologic record of any major dinosaur clade. In terms of body fossils, they are represented by:

■ complete skeletons ranging from juveniles to adults in some species;

■ many eggs, some with embryos identifiable to ornithopod species;

■ skin impressions;

■ former gut contents; and

■ possible internal organ preservation.

Their trace fossil record is also rich, consisting of numerous trackways, nests, and coprolites. They became so widespread throughout the Mesozoic that they left their bodily remains or traces of their behavior on every continent by the end of the Cretaceous. Perhaps the most interesting aspect of this wide geographic range was that some of their skeletal material has been found in both Arctic and Antarctic regions. Such a latitudinal distribution has been part of the basis for hypothesizing ornithopod migrations, conjuring once odd but now-familiar recreations of great herds of these dinosaurs moving with the seasons, as with modern mammalian herbivores.

Perhaps the best reason to study ornithopods is because their fossil record provides the most complete and compelling evidence available about the social lives of dinosaurs, which is only rivaled by sauropods. Preliminary data indicate that at least one species of ornithopod lived in large communal nesting grounds, where the juveniles were restricted to their nests for their formative years while the parents took care of them. Other ornithopods show elaborate head structures that probably served as resonating chambers for calling to one another, but also may have been prominent cues for gender identification. Some ornithopods had easily visible tusks, and others had large spiked thumbs and hands that could grasp, all giving rise to hypotheses about the functions of such features.

Ornithopods certainly comprised one of the most interesting of dinosaur clades and have been the source of some revolutionary hypotheses about dinosaurs in general. Their study has been at the center of the significant changes in perceptions about intraspecific behavior of dinosaurs that have taken place in only the past 25 years.

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