Animalia Life

The hadrosaurids, or "duck-billed" dinosaurs, were large, bipedal dinosaurs, with body lengths up to fifteen meters, that lived during the Upper Cretaceous across North and Central America, South America, Europe, and Asia. It is thought that they originated in Asia, but once they arose, they spread and diversified worldwide, quickly becoming the primary constituent of herbivorous dinosaur faunas. The first hadrosaur remains to be found were represented by a few fragments from Montana and South Dakota, described by Joseph Leidy in 1856. Shortly afterward, a partial skeleton was found in New Jersey, the first nearly complete dinosaur found anywhere, and described by Leidy in 1858 as Hadrosaurus foulkii. Hadrosaurs are ornithopod (bird-footed) dinosaurs, a term that refers to their three-toed feet. They are thus part of the Ornithischia, one of the two major dinosaur subdivisions, and characterized by a pubis (one of the three bones of the pelvis) that is inclined backward. In cladistic terms, they are considered to be a monophyletic group; that is, they are all derived from a common ancestor. They are particularly characterized by a toothless front to the mouth, which is flared outward to form a broad, flat "bill", prompting early researchers to dub them the duck-billed dinosaurs. This bill was covered by a thick, horny sheath, and the rest of the jaws bore closely packed batteries of grinding teeth, up to four hundred on each side of each jaw. The postcranial anatomy of hadrosaurs is generally very similar and the really obvious differences lie in the crests that many species bear on the top of their heads. These are formed of outgrowths of the nasal bones, are frequently hollow, and are found particularly in the hadrosaurs known as lambeosaurines.

Hadrosaurid Lifestyle
More is known about the life history of hadrosaurids than about any other group of dinosaurs, due to information fromskin impressions, trackways, and eggs and nesting sites. They were originally thought to be amphibious animals, inhabiting marshy areas and swimming and diving for soft aquatic plants. This was partly based on skin impressions from mummified specimens which appeared to show webbing between the digits. It was even suggested that the crested forms used the crests as snorkels and were able to breathe through them while underwater. More recent analyses of stomach contents and the function of the tooth batteries suggest, however, that they were adapted to efficiently process fibrous vegetation of low nutritional value, such as coniferous needles and twigs. The crests arenowthought to have developed for a variety of purposes, including increased sensory area for an improved sense of smell, and to act as resonators for producing distinctive calls. In 1979, juvenile hadrosaurid bones were discovered in the Two Medicine Formation in Montana by Jack Horner. Subsequently, he discovered nesting sites with eggs that provide evidence for reproductive strategies in the hadrosaur Maiasaura. These animals appear to have built large, circular nesting mounds with concave centers in common nesting areas. There is evidence of site fidelity, indicating that the nesting sites were used from season to season. Up to twenty eggs were laid in the nest in a circular pattern, then covered by vegetation that kept the eggs warm as it decomposed. Bones of young have been found in some of the nests and these indicate from tooth wear that the animals had already been feeding on vegetation. Also, the ends of the bones were still poorly formed, showing that they were altricial or nest-bound for some period after they had hatched. During this period they would have been fed by adults, which in turn suggests social behavior in which the eggs were protected by adultswhothen fed and raised the nest-bound hatchlings.

Hadrosaur Facts

Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Chordata
Subphylum: Vertebrata
Class: Reptilia
Order: Ornithischia
Suborder: Ornithopoda (bipedal herbivores)
Geographical location: Western and eastern North America, central and southern South America, Europe, Asia
Habitat: Terrestrial habitats
Gestational period: Unknown
Life span: Estimated at twenty-five years
Special anatomy: Large, bipedal dinosaurs in which the snout was developed into a beak and a crest was often developed on the head

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