The Hadrosauridae or duckbilled dinosaurs was the last family of Ornitho-poda to evolve. Hadrosaurids first appeared in the Middle Cretaceous and died out at the end of the period, about 30 my later. Although their bodies were all remarkably similar, like those of early iguanodontids, their heads differed a great deal. Figure 96 illustrates some crested types and a variety of other shapes are shown in Fig. 112.In their day, the duckbilled hadrosaurids were the most common and varied group of ornithopods to evolve. This group probably developed in central Asia and spread throughout the northern hemisphere.
Very few genera, apart from the primitive flatheaded Secernosaurus from Argentina, found their way into Gondwanaland. This great mass of land had already broken up and, by Upper Cretaceous times, the continents were drifting apart.
The hadrosaurids fed on very tough plant material such as conifer needles and twigs. Like the iguanodontids (Sect. 10.5.3), they had long snouts and heavy jaws with numerous teeth. In the hadrosaurids, these were cemented together by bony tissue to form batteries that ground the food with up and down rather than backward and forward movements. The tough, sharp and horny beaks, from which the common name duckbilled dinosaurs is acquired, would have been well suited to nipping off twigs and stripping leaves from branches. The beaks grew continuously, thus compensating for the wear associated with hard, tough vegetable food. It seems probably that the increase in abundance of hadrosaurids in the Late Cretaceous was associated with the adaptation of their highly efficient jaws to the appearance of flowering plants and disappearance of more primitive types such as seed ferns and bennettitales (Ostrom 1964; Norman 1985).
As reported in the last chapter (Sect. 9.3.3), hadrosaurids nested in large colonies where the young dinosaurs were fed by their parents who returned to the same nest site year after year. The functions of the crests (Fig. 46) and complex narial passages in visual and vocal display have been discussed in Section 9.3.1. The family Hadrosauridae is divided into two subfamilies. In one, the hadrosaurines, the heads were flat and either surmounted by solid, bony crests or there were no crests at all (Fig. 111). In the other subfamily, high domed heads bore flamboyant hollow crests (Fig. 96). These hadrosaurids were known as lambeosaurine duckbills, and were largely confined to North America. The first subfamily, the hadrosaurine duckbills, was the more successful. Distributed throughout North America, Europe and Asia, some of its members were among the last dinosaurs to survive.
The earliest, and one of the smallest hadrosaurids so far discovered, is Bactrosaurus (Fig. 112a) from Mongolia and China. This animal had a low, flat head without a crest and a narrow bill like those of the hadrosaurines, but the build of its body was like that of a lambeosaurine duckbill. It may represent an ancestral hadrosaur which had evolved from the iguanodontids. Bactrosaurus had typical hadrosaurid dentition capable of grinding tough vegetation. Hadrosaurus (Fig. 112b) was the first dinosaur to be discovered in North America - by Joseph Leidy in 1858. Leidy recognised that it was structurally related to Iguanodon, (discovered in England and described some 30 years earlier by Gideon Mantell; Sect. 10.5.3). He inferred that Hadrosaurus was bipedal. A typical hadrosaurine, it had no crest on its long, low head but, as in Kritosaurus (Fig. 112a), there was a large lump on its snout that may have been covered with skin. It has been suggested that this might have been a male sexual recognition character. Edmontosaurus (Fig. 112d) was an exceptionally large hadrosaurine duckbill, reaching a length of 13 m. Behind its toothless beak were banks of tightly packed teeth forming 'a veritable grinding pavement in both jaws (Palmer 1999).As these teeth were worn down, they would have been replaced by new ones from beneath. At any one time, there could have been more than 1,000 teeth in the jaws of Edmontosaurus. These teeth ground against each other as did those of Iguanodon (Sect. 10.5.3). Edmontosaurus and its relatives such as Anatosaurus would have been able to shred the very toughest plant material.
The last group of bipedal herbivorous dinosaurs to be discussed is the family Psittacosauridae. For many years, the psittacosaurids were thought to be orni-thopods and related to Hypsilophodon (Fig. 111). Psittacosaurus ('parrot lizard'; Fig. 113) from Asia was a small lightly built bipedal dinosaur with a long tail which counterbalanced its body over the hips. Its forelegs were short, and its hands had four fingers with blunt claws, whereas the ceratopsids (Sect. 10.6.2), with which it is believed to have shared a common ancestor, had five. Furthermore, whilst Psittacosaurus had no teeth in its beak, the early
ceratopsids (Protoceratopsidae) had teeth in the upper half of the beak. Like the hypsilophodonts (Sect. 10.5.2), the psittacosaurids probably walked quadrupedally for much of the time, but would have been able to run faster on two legs than on four (Palmer 1999). Their parrot-like beaks, with a peculiar rostral bone, suggests that either a new type of plant had evolved that required this type of beak to crop it, or else the development of a parrot-like beak enabled the psittacosaurids and later ceratopsians to feed on plants previously inedible for most dinosaurs (Norman 1985). The appearance of the ceratop-sians coincided very roughly with the first appearance of the angiosperms.
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