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Figure 10.20. Cladogram of basal Iguanodontia. Derived characters include: at I premaxilla with a transversely expanded and edentulous margin, reduction of the antorbital opening, denticulate margin of the predentary, deep dentary ramus; loss of sternal rib ossification, loss of a phalanx in digit III of the hand, compressed and blade-shaped prepubic process; at 2 strong offset of premaxilla margin relative to the maxilla, peg-in-socket articulation between maxilla and jugal, development of a pronounced diastema between the beak and mesial dentition, mammillations on marginal denticles of teeth, maxillary crowns narrower and more lanceolate than dentary crowns, closely appressed metacarpals ll-IV, deep triangular fourth trochanter deep extensor groove on femur radically reorganize what we've just described. For now, however, we must regard Atlascopcosaurus, Leaellynasaura, Qantasaurus, Notohypsilophodon, Fulgurotherium, Drinkeria, and Yandusaurus, as euornithopods without a more exact phylogenetic home.

Iguanodontia By far, Iguanodontia is the bushiest of ornithopod clades, claiming not only the very diverse Hadrosauridae, but also a variety of more basal forms (Figure 10.20). Iguanodontians uniquely share the following features: transversely expanded and toothless premaxilla, smoothly round oral margin of the predentary, deep dentary with parallel dorsal and ventral borders, loss of a phalanx in the third digit of the hand, and a compressed, blade-shaped prepubic process.

Tenontosaurus is the most basal iguanodontian ornithopod; remaining iguanodontians are called Dryomorpha after the most basal member, Dryosaurus. Climbing slightly higher, we encounter the more restrictive clade called Ankylopollexia - the "fused thumbs." These forms share a number of derived features, among them relatively close packing of the teeth, upper teeth with a prominent ridge on their outer side, fusion of the wrist bones, and the formation of the spiked thumb. Within this clade, Camptosaurus (Figure 10.21) is the most basal, while Lurdusaurus is the next most derived.

Figure 10.21. The Late Jurassic iguanodontian Camptosaurus from the Western Interior of the USA. (Photograph courtesy of the Royal Ontario Museum.)

The next included clade - Iguanodontoidea - includes species of Iguanodon and all more derived iguanodontians and is diagnosed by numerous changes in the skull and teeth that relate to the feeding mechanics of this group, as well as modifications of the hand, pelvis, femur, and foot. Contained within Iguanodontoidea, successively are Iguanodon, Ouranosaurus, Altirhinus, Probactrosaurus, Eolambia (which may be closely related to Altirhinus), Protohadros, and Hadrosauridae. Jinzhousaurus and Nanyangosaurus, both from the Early Cretaceous of China, also appear to be closely related to Hadrosauridae, although their exact positions are not yet clear.

Hadrosauridae Until the 1990s, the monophyly of Hadrosauridae had long been assumed, but had not been demonstrated by cladistic analysis. One of the first of these recent forays into hadrosaurid phylogeny was conducted by Horner, who provocatively suggested a diphyletic origin for Hadrosauridae, with lambeosaurines deriving from an Ouranosaurus-like iguanodontian, and hadrosaurines from a more Iguanodon-like iguanodontian. These conclusions spawned a number of more recent cladistic analyses, including those by Horner himself, all of which now strongly support the monophyly of Hadrosauridae (Figure 10.21), defined as that clade of ornithopod dinosaurs consisting of the most recent common ancestor ofTelmatosaurus and Parasaurolophus, plus all the descendants of this common ancestor. Diagnostic of Hadrosauridae are numerous modifications of the feeding apparatus (including the develop-ment of a dental battery and modification of the jawjoint and its support), and changes in the shoulder and knee joints.

Two major clades - Lambeosaurinae and Hadrosaurinae - constitute most of Hadrosauridae (Figure 10.22) and they are each other's sister-group,

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