The advanced iguanodontian ornithopods known as the Hadrosau-ridae ("bulky lizards") include some of the best-understood dinosaur taxa because of the abundance of their specimens and often because of excellent preservation. Hadrosaurs evolved some of the most effective methods for eating plants ever seen. These dinosaurs had more teeth than any other known land animals. There are more than 35 scientifically accepted taxa of hadrosaurs.
Hadrosaurs have also played an important role in the understanding of dinosaur anatomy and behavior. The first irrefutable evidence that some dinosaurs walked on two legs was the result of Joseph Leidy's description in 1868 of Hadrosaurus (Late Cretaceous, New Jersey), the first dinosaur discovered with forelimbs that were clearly not adapted for walking. The elaborate, often hollow crests observed in hadrosaurs have implications for vocalization and behavior, and also provide evidence of sexual dimorphism in dinosaurs. The discovery of eggs, nests, and hatchlings of Maiasaura (Late Cretaceous, Montana) and other hadrosaurs has been interpreted as evidence for parental care in the early stages of these dinosaurs' lives. The existence of excellent growth series of several hadrosaur species has revealed much about the metabolism and growth rate of dinosaurs. Some unique, naturally "mummified" specimens of hadrosaurs contain large patches of skin impressions, revealing much about the pattern and size of dinosaur scales. Some of these duckbill "mummies" may even preserve remnants of muscles, internal organs, and stomach contents.
The hadrosaurs were the last significant group of ornithopods to evolve. North American hadrosaurs such as Edmontosaurus lived in a world occupied by other familiar dinosaurs, including Ankylosau-rus, Triceratops, and Tyrannosaurus, and were among the last kinds of dinosaurs to exist before the mass-extinction event at the end of the Cretaceous Period.
The skulls and jaws of hadrosaurs are their most diagnostic features, or traits. Without a skull, it is sometimes difficult to tell one hadrosaur taxon from another because of the similarities in their postcranial skeletons. Hadrosaurs are classified into two main groups based principally on the anatomy of the skull: Hadrosauri-nae and Lambeosaurinae.
The hadrosaurines had solid head crests or no head crest at all. Representative hadrosaurines included some giants of the duckbilled dinosaurs, such as Edmontosaurus (Late Cretaceous, western North America) and Shantungosaurus. Hadrosaurine skulls were boxlike with a large nasal opening in the front. A few, such as Sau-rolophus (Late Cretaceous, Alberta and Mongolia) had a solid bony crest on the top of the head.
The lambeosaurines had long, narrow skulls with a hollow crest on top of the head. The morphology of their crests differed from one taxon to another, but all of the crests functioned as extensions of the nasal passages and may have been involved in display, sound production, or olfaction. Familiar lambeosaurines include Parasaurolophus, Corythosaurus, and Lambeosaurus, all from the Late Cretaceous of western North America. A host of
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Saurolophus skull with a bony crest at the top of its head new, remarkable lambeosaurines have recently come to light from the latest Cretaceous of far eastern Russia and China, such as Charonosaurus, Olorotitan, and Sahaliyania.
The jaws of hadrosaurs contained advanced dental batteries for grinding plants. Like the iguanodontids, duckbills could move their upper jaws sideways to rub their upper teeth across the full surface of the lower teeth. Rather than having the chisel-like teeth found in other ornithopods, hadrosaurs developed tightly packed rows of interlocking teeth. The teeth were layered so that new teeth continuously replaced old teeth. A duck-billed jaw had between 550 and 1,400 such teeth, most of which lay below the grinding surface and became active as the tooth layer at the surface was worn away. Had-rosaur teeth were so tightly packed that they formed a long, smooth pavement for chewing; this surface functioned as a magnified version of a metal file. These teeth were strong enough to pulverize the toughest kinds of plants with ease yet were adaptable to the kinds of softer, flowering plants that were beginning to populate their world.
Other features shared by hadrosaurs include a functionally bipedal posture and shorter forelimbs than hind limbs; three weight-supporting toes on their hind feet; long, stiffened tails held off of the ground; a relatively long and slender neck; and four-fingered hands, some fingers of which were joined together by a covering of skin, making a kind of mittenlike structure. Although
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