Ornithopods were the most numerous and diverse herbivores of Dinosauria, consisting of iguanodontians, hadrosaurids (duckbills), and a few distinctive, more primitive forms. Although equipped with robust back legs and long tails, and undoubtedly capable of sustained bipedal locomotion, hooves on certain fingers of hadrosaurids and iguanodontians suggest that these animals spent time in a quadrupedal stance as well.
Ornithopods ranged in size from very small (<2 m) to rather large (>15 m), and evidently colonized virtually every inhabitable region of the globe.
Ornithopods had very advanced chewing capabilities. The skull has an inset tooth row indicating cheeks, as well as the tripartite division of chewing herbivores (including a cropping beak, a diastem, and closely appressed cheek teeth for grinding), and in virtually all cases, a large coronoid process suggests strong jaw adductor muscles. Hadrosaurs took chewing to unprecedented heights with the evolution of a pleurokinetic skull combined with dental batteries. Coprolites and stomach contents suggest hadrosaurs needed all the teeth they had: their diet appears to have been coarse and fibrous.
Ornithopods were very social animals, none more so than hadrosaurs. The discovery of many bonebeds attests to this, as do a remarkable and complex variety of sexually dimorphic head and skull features found in many ornithopods, suggesting that life as an ornitho-pod involved intensive sexual selection. In hadrosaurs at least, communication may well have been enhanced by a variety of vocalizations.
The genus Maiasaura has given us a view of child-rearing, duckbill style. Complete growth series, from hatchlings at nests to adults, are known, and considerable evidence exists that hadrosaurs grew remarkably quickly. Nesting apparently was communal, and parental care was expended on altricial young: duckbills were likely K-strategists. Interestingly, other ornithopods, such as the genus Orodromeus, may have raised precocial young and favored an r-strategy of childrearing.
Ornithopod evolution was characterized by an increase in the sophistication of chewing specializations. Among the large ornithopods, it could be argued that, in North America and Asia at least, the highly advanced hadrosaurs ecologically replaced non-hadrosaurid iguanodontians.
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