Hadrosaurs - The Duck-Billed Dinosaurs

The Evolution and Behavior of Hadrosaur Dinosaurs

A fossil specimen of the obscure hadrosaur Procheneosaurus (American Museum of Natural History).

It's a common theme of evolution that, during different geological epochs, 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 small-brained, quadrupedal plant-eaters can (in many respects) can be considered the prehistoric equivalent of cattle--but not ducks, which lay on an entirely different evolutionary branch!

(See a gallery of duck-billed dinosaur pictures and profiles.)

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 in herds of hundreds or thousands of individuals, and some signaling to each other from afar by funneling blasts of air through the large, ornate crests on their heads, a characteristic hadrosaur feature (albeit more developed in some genera than in others).

The Anatomy of Duck-Billed Dinosaurs

Hadrosaurs (Greek for "bulky lizards") were far from the sleekest, or most attractive, dinosaurs ever to walk 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 even larger, multi-ton species 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 truly enormous sizes--about 20 tons, or ten times as big as a modern elephant!

Duck-Billed Dinosaur Family Life

Duck-billed dinosaurs seem to have shared more in common 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. (The crests may have served an additional function during mating season, when males with bigger, more ornate headgear won the right to breed.)

Maiasaura (one of the few dinosaurs to be named after the female, rather than the male, of the genus) is an especially important duck-billed dinosaur, thanks to the discovery of an extensive North American nesting ground bearing the fossilized remains of adult and juvenile individuals, as well as numerous eggs arranged in bird-like clutches. Clearly, this "good mother lizard" kept close watch over its 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 child-rearing is Hypacrosaurus).

Duck-Billed Dinosaur 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 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 puzzling mixture of hadrosaur and "iguanodont" traits; one late genus, Telmatosaurus, maintained its Iguanodon-like profile even during the closing stages of the Cretaceous period, probably because this dinosaur was isolated on a European island and thus cut off from the mainstream of evolution.

By the end of the Cretaceous period, hadrosaurs were the most populous dinosaurs on earth, an essential part of the food chain in that they consumed the thick, overflowing vegetation of North America and Eurasia and were eaten in turn by carnivorous raptors and tyrannosaurs. 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--but given the way events turned out, we'll never know for sure.