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Eolambia, an Early Cretaceous iguanodontoidean of the western USA, was briefly considered the oldest member of the Hadrosauridae, but more detailed study revealed that it lies outside of that clade. For example, it has only seven sacral vertebrae, whereas every member of Hadrosauridae has eight.

The body and trace fossil record for hadrosaurids begins in the Early Cretaceous, setting the stage for their excellent Late Cretaceous fossil record. Hadrosaurids are among the best studied and well known of dinosaurs, partially because of their extensive fossil record but also because they have been studied for nearly 150 years. For example, the first discovery of a near-complete skeleton of a dinosaur was Hadrosaurus foulkii in New Jersey in 1857. Joseph Leidy described this specimen soon afterwards and inspired the then-new idea of bipedalism in dinosaurs, later confirmed by Dollo (Chapter 3). Furthermore, the excellent fossil record for hadrosaurids resulted in the naming of numerous species and a probable (or at least apparent) high diversity in comparison to other dinosaur clades. Hadrosaurids provide an approximate mirror image of earlier iguanodontian diversity in that they

FIGURE 11.7 Representative genera of a hadrosaurine and lambeosaurine from western North America and China.

(A) Edmontosaurus, a Late Cretaceous hadrosaurine of western North America; Denver Museum of Natural History, Denver, Colorado.

(B) Tsintaosaurus, a Late Cretaceous lambeosaurine of China; Fernbank Museum of Natural History, Atlanta, Georgia

flourished during the Late Cretaceous, whereas more primitive iguanodontians showed a decline in both numbers and species.

Because of this abundance and diversity, the classification of hadrosaurids is nearly as complicated, detailed, and contentious as that devised for theropods (Chapter 9). To make sense of the large amount of fossil data for hadrosaurids, they will be discussed through their two most well-represented clades, Hadrosaurinae and Lambeosaurinae.

Hadrosaurines are nicknamed the "flat-headed" hadrosaurids because they have skulls that are wider than tall. Representative species include the Late Cretaceous Anatotitan, Brachylophosaurus, Edmontosaurus (Fig. 11.7A), Gryposaurus, Maiasaura, Prosaurolophus, Saurolophus, Shantungosaurus, and of course Hadrosaurus. In contrast, lambeosaurines are called the "hollow-crested" hadrosaurids owing to their enlarged and separated nasal chambers enclosed by bones that formed dorsal crests on their skulls. Lambeosaurines, which were also common in the Late Cretaceous, include Parasaurolophus, Corythosaurus, Hypacrosaurus, and Lambeosaurus of North America, as well as Tsintaosaurus of China (Fig. 11.7B). Interestingly, these two groupings of hadrosaurids were recognized long before the advent of cladistics, thus indicating how previous generations of paleontologists correctly interpreted their common ancestry despite the differences of a few anatomical features.

FIGURE 11.7 Representative genera of a hadrosaurine and lambeosaurine from western North America and China.

(A) Edmontosaurus, a Late Cretaceous hadrosaurine of western North America; Denver Museum of Natural History, Denver, Colorado.

(B) Tsintaosaurus, a Late Cretaceous lambeosaurine of China; Fernbank Museum of Natural History, Atlanta, Georgia

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