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Iguanodontia is ecognized as a stem-based clade today, but its name has a long history in the study of dinosaurs.

linked to iguanodontians reported in England. Soon after, Louis Dollo described numerous beautifully preserved Iguanodon bernissartensis skeletons from the Early Cretaceous of Belgium (Chapter 3). These studies led to iguanodontians as subjects of some of the first detailed scientific studies of dinosaurs.

Interpretations of Iguanodon and other iguanodontians have undergone much revision since then, as a result of new discoveries and re-analysis of previously discovered skeletal material. However, probably the most important hypothesis that emerged from the study of Iguanodon was of bipedal-ism in some dinosaurs, which Dollo discerned through the study of more than 30 skeletons of this species. More recently, iguanodontians have been recognized as dinosaurs that may have had complex sociality, which is evident from an extensive body and trace fossil record of hadrosaurids of the Late Cretaceous. Moreover, iguanodontians were important contributors to terrestrial ecosystems on most continents, from the Late Jurassic through to the Late Cretaceous, partially or completely replacing sauropods in ecological niches as the largest herbivores.

Probably the most primitive of iguanodontians is the Early Cretaceous Tenonto-saurus of western North America. Some paleontologists have interpreted this iguan-odontian as a hypsilophodontid or otherwise outside of Iguanodontia, which illustrates its seemingly basal status. Other basal iguanodontians include an Early Cretaceous iguanodontian from Australia, the colorfully named Muttaburrasaurus, as well as the Late Cretaceous Zalmoxes of Romania and Rhabdodon of France.

Some iguandontians show how cladistic interpretations of "primitive" (more basal) versus "advanced" (more derived) forms can be at odds with actual geologic ages. For example, the Late Jurassic Dryosaurus and Camptosaurus of western North America show relatively more advanced traits than Tenontosaurus and the other aforementioned basal iguanodontians. This contradiction is explained by paleontologists not yet knowing which iguanodontian was the most ancestral. This hypothetical common ancestor of Iguanodontia probably lived during the Middle or Late Jurassic, although later descendants, represented by Tenontosaurus and other species, retained its traits. Furthermore, Dryosaurus and Camptosaurus are still more primitive than the Early Cretaceous Iguanodon, suggesting that some iguanodontian lineages evolved more quickly than others. Interestingly, Camptosaurus shows some gross morphological convergence with prosauropods (Chapter 10), in that it had a small head and long neck in comparison to other ornithopods (Fig. 11.6). Some of the other Jurassic and Early Cretaceous iguanodontians looked similar to their non-iguanodontian cousins in many ways, but they did differ in one important respect - size. Some specimens of Iguanodon are as long as 10 meters, and by the Late Cretaceous a few hadrosaurids, such as Shantungosaurus of China, reached lengths of 15 meters.

Along the evolutionary journey of ornithopod lineages to Hadrosauridae is the stem-based clade Iguanodontoidea, which has Iguanodon as its most basal genus. Ouranosaurus was an Early Cretaceous iguanodontoidean of Niger that was closely related to Iguanodon, but it was strikingly different because of its elongate and robust processes on its dorsal vertebrae. These processes are interpreted as supports for either sail-like flaps of skin or humps like those interpreted for spinosaurs, which were theropods with similar features (Chapter 9). Despite such a novel trait, it is still considered a predecessor to the hadrosaurids.

The Late Cretaceous Telmatosaurus has what are considered the most ancestral traits for the node-based clade Hadrosauridae, so it is used for comparison to all other probable hadrosaurid descendants. The following characters distinguish Hadrosauridae:

FIGURE 11.6 Camptosaurus, a common iguanodontian of the Late Jurassic in western North America. Specimen in the College of Eastern Utah Prehistoric Museum, Price, Utah.

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