It's a common theme of evolution that, during different historical periods, different types of animals tend to occupy the same ecological niches. Today, the job of "slow-witted, four-legged herbivore" is filled by mammals 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, or duck-billed dinosaurs. These quadrupedal plant-eaters can (in many respects) can be considered the prehistoric equivalent of cattle--but not ducks, which are an entirely different story! (See a gallery of hadrosaur pictures.)
Given their extensive fossil remains, it's likely that more hadrosaurs existed during the latter stages of the Cretaceous period than any other type of dinosaur (including tyrannosaurs, ceratopsians, and raptors). These gentle creatures roamed the woodlands and plains of North America, Europe and Asia, some genera in herds of hundreds or thousands of individuals, and some signaling to each other by funneling blasts of air through the large, ornate crests on their heads, a characteristic hadrosaur feature.
Hadrosaur Anatomy and Evolution
Hadrosaurs (Greek for "bulky lizards") were far from the sleekest, or most attractive, dinosaurs ever to roam the earth. These plant-eaters were characterized by their thick, squat torsos, massive, inflexible tails, and tough beaks and numerous cheek teeth (up to 1,000 in some species) designed for breaking down tough vegetation; some of them (the "lambeosaurinae") had crests on top of their heads, while others (the "hadrosaurinae") didn’t. Like cows and horses, hadrosaurs grazed on all fours, but some may have been capable of running clumsily 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 giant, plant-eating sauropods and carnivorous theropods). Confusingly, hadrosaurs are technically classified as ornithopods, a larger family of ornithischian dinosaurs that included Iguanodon and Tenontosaurus; in fact, it can be hard to draw a firm line between the most advanced ornithopods and the earliest true hadrosaurs. Most duck-billed dinosaurs, including Anatotitan and Hypacrosaurus, weighed in the neighborhood of a few tons, but a few, like Shantungosaurus, attained gigantic sizes--about 20 tons, or ten times as big as an elephant!
Hadrosaur Family Life
Duck-billed dinosaurs seem to have shared more with modern cows and horses than just their grazing habits (though it's important to understand that grass had yet to evolve in the Cretaceous period; rather, hadrosaurs nibbled on low-lying plants). At least some hadrosaurs, such as Edmontosaurus, roamed the North American woodlands in large herds, doubtless as a form of defense against menacing raptors and tyrannosaurs. The gigantic, curved crests atop the noggins of hadrosaurs like Charonosaurus and Parasaurolophus were probably used to signal other herd members; studies have shown that these structures produced 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 bearing 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 (one other genus for which we possess definitive proof of this is Hypacrosaurus).
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 late Cretaceous as well, but there's evidence for distant ancestors dating as far back as the Jurassic period). As mentioned above, some early duck-billed dinosaurs evidenced a mix of hadrosaur and "iguanodont" traits; one late genus, Telmatosaurus, maintained its Iguanodon-like characteristics even during the closing stages of the Cretaceous period, probably because this dinosaur was confined to a European island and thus out of the mainstream of evolution.
By the end of the Cretaceous period, the hadrosaurs were the most populous dinosaurs on earth, an essential part of the food chain in that they ate the overflowing vegetation of North America and Eurasia and were eaten in turn by carnivorous dinosaurs. If the dinosaurs as a whole hadn't been wiped out in the K/T Extinction Event, 65 million years ago, it's conceivable that some hadrosaurs might have evolved to truly gigantic, Brachiosaurus-like sizes, bigger even than Shantungosaurus. Sadly, we'll never know for sure.
Here's a list of the most notable hadrosaurs; just click on the links for more information.
Acristavus This early hadrosaur lacked any ornamentation on its skull.
Amurosaurus The most complete hadrosaur to be discovered in Russia.
Anatosaurus This dinosaur is now known as either Anatotitan or Edmontosaurus.
Anatotitan This hadrosaur's name means "giant duck."
Angulomastacator This dinosaur had a strangely shaped upper jaw.
Aralosaurus Not much is known about this central Asian duckbill.
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.
Claosaurus This "broken lizard" was a primitive hadrosaur.
Corythosaurus This "Corinthian-helmeted" dino had a distinctive mating call.
Edmontosaurus This large, duck-billed herbivore was a contemporary of T. Rex.
Eolambia An early hadrosaur from North America.
Equiijubus Its name is Greek for "horse mane."
Gryposaurus One of the most common of the duck-billed dinosaurs.
Hadrosaurus The official state dinosaur of New Jersey.
Huehuecanauhtlus Its name is Aztec for "ancient duck."
Hypacrosaurus We know a lot about this duck-billed dinosaur's family life.
Jaxartosaurus A poorly known hadrosaur from central Asia.
Jinzhousaurus This Asian dinosaur was one of the first hadrosaurs.
Kundurosaurus This hadrosaur was discovered in the far east of Russia.
Lambeosaurus This herbivore had a hatchet-shaped crest on its noggin.
Latirhinus This duck-billed dinosaur had an enormous nose.
Magnapaulia The largest lambeosaurine hadrosaur yet identified.
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.
Probactrosaurus An early stage in hadrosaur evolution.
Prosaurolophus The likely ancestor of both Saurolophus and Parasaurolophus.
Saurolophus One of the few hadrosaurs known to have lived on two continents.
Secernosaurus The first hadrosaur to be discovered in South America.
Shantungosaurus The biggest of all the duck-billed dinosaurs.
Tanius Not much is known about this Chinese hadrosaur.
Telmatosaurus This duck-billed dinosaur was discovered in Transylvania.
Velafrons This hadrosaur was recently discovered in North America.
Zhuchengosaurus This hadrosaur was even bigger than Shantungosaurus.