It's a common theme of nature that, during different historical periods, different types of animals occupy the same ecological niche. Today, the job of "four-legged herbivore" is filled by creatures like deer, sheep, horses and cows; 75 to 65 million years ago, toward the end of the Cretaceous period, this niche was taken up by the hadrosaurs--quadrupedal, duck-billed dinosaurs that (in many respects) can be considered the prehistoric equivalent of cattle. (See a gallery of hadrosaur pictures.)
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Based on the large number of fossil remains found, it's likely that more hadrosaurs existed during the latter stages of the Cretaceous than any other type of dinosaur (including tyrannosaurs, ceratopsians, and raptors). These creatures roamed the woodlands of North America, Europe and Asia, some of them in herds of hundreds or thousands of individuals, and some of them signaling to each other by funneling blasts of air through the large crests on their heads.
Hadrosaur Anatomy
Hadrosaurs ("bulky lizards") were far from the sleekest, or most attractive, dinosaurs to roam the earth. These plant-eaters were characterized by their thick, squat torsos, massive, inflexible tails, tough beaks and numerous cheek teeth (up to 1,000 in some species); some of them (the lambeosaurinae) had crests on top of their head, while others (the hadrosaurinae) didn’t. Like cows and horses, these dinosaurs are believed to have grazed on all fours, but some genuses may have been capable of running away on two feet to escape predators.
Hadrosaurs were the largest of all the ornithischian, or bird-hipped, dinosaurs (the other major class of dinosaurs, saurischians, included truly enormous herbivores like Apatosaurus and Argentinosaurus). Most duck-billed dinosaurs, like Anatotitan or Hypacrosaurus, weighed in the neighborhood of a few tons, but a few, like Shantungosaurus, attained sauropod-like sizes--almost 20 tons, or ten times as big as an elephant!
Hadrosaur Lifestyles
Duck-billed dinosaurs seem to have shared more with modern cows and horses than their grazing habits (though it's important to understand that grass had yet to evolve in the Cretaceous period; rather, these dinosaurs nibbled on low-lying plants). At least some hadrosaurs, such as Edmontosaurus, appear to have roamed the woodlands in large herds, probably as protection against predators. The crests atop hadrosaurs like Charonosaurus and Parasaurolophus were probably used to signal other herd members; studies have shown that these structures produce loud sounds when blasted with air.
Maiasaura (one of the few dinosaurs to be named after the female, rather than the male, of the genus) is an especially important hadrosaur, thanks to the discovery of an extensive North American nesting ground with the fossilized remains of adult and juvenile specimens, as well as numerous eggs. Clearly, this "good mother lizard" kept close watch over her children even after they were hatched, so it's at least possible that other duck-billed dinosaurs did the same.
Hadrosaur Evolution
Hadrosaurs are one of the few families of dinosaurs to have lived entirely in one historical period, the middle to late Cretaceous (other dinosaurs, like tyrannosaurs, flourished during the Cretaceous as well, but there's evidence for immediate forebears dating back to the Jurassic). As far as paleontologists can tell, the closest relatives of the hadrosaurs were the iguanodonts, which included the eponymous Iguanodon. Some early hadrosaurs showed a mix of hadrosaur and iguanodont traits, and one late genus, Telmatosaurus, maintained its Iguanodon-like characteristics even in the late Cretaceous, probably because it lived on an island, out of the mainstream of dinosaur evolution.
By the end of the Cretaceous, the hadrosaurs were the most populous dinosaurs on earth, an important part of the food chain in that they ate the overflowing vegetation and were eaten in turn by carnivores. If the dinosaurs hadn't been wiped out by the K/T asteroid, 65 million years ago, it's possible that some duck-billed dinos might have evolved to truly gigantic, Brachiosaurus-like sizes. Sadly, we can never know for sure.
Here's a list of the most notable genuses of duck-billed dinosaurs; you can click on the links for more information.
Anatotitan This hadrosaur's name means "giant duck."
Bactrosaurus One of the earliest of the duck-billed dinosaurs.
Brachylophosaurus This duck-billed dinosaur's beak looked more like a parrot's.
Charonosaurus This duck-billed dinosaur was much bigger than an elephant.
Corythosaurus This "Corinthian-helmeted" dino had a distinctive mating call.
Edmontosaurus This large, duck-billed herbivore was a contemporary of T. Rex.
Gryposaurus One of the most common of the duck-billed dinosaurs.
Hadrosaurus The official state dinosaur of New Jersey.
Hypacrosaurus We know a lot about this duck-billed dinosaur's family life.
Lambeosaurus This herbivore had a hatchet-shaped crest on its noggin.
Maiasaura This "good mother lizard" kept close tabs on her young.
Olorotitan One of the most complete dinosaur fossils ever found in Russia.
Ouranosaurus Scientists can't decide if this hadrosaur had a sail or a hump.
Parasaurolophus Probably the loudest dinosaur ever to roam the earth.
Prosaurolophus The likely ancestor of both Saurolophus and Parasaurolophus.
Saurolophus One of the few hadrosaurs known to have lived on two continents.
Shantungosaurus The biggest of all the duck-billed dinosaurs.
Telmatosaurus This duck-billed dinosaur was discovered in Transylvania.
