Period: Late Cretaceous

Order, Suborder, Family: Ornithischia, Ornithopoda, Hadrosauridae

Location: North America (United States)

Length: 30 feet (9 meters)

Hadrosaurus ("thick reptile") was the first hadrosaur skeleton to be discovered. It was named and described in 1858 by Joseph Leidy, the father of American paleontology. Most of the skeleton was found in marine rocks of Late Cretaceous age in southern New Jersey near Philadelphia. It included many vertebrae, teeth, parts of the front and back limbs, and pelvis.

Hadrosaurus was the first dinosaur to have its skeleton mounted. Mounts of this dinosaur were displayed first at the Academy of Natural Sciences in Philadelphia. It was later displayed at Princeton, the Smithsonian, and at the Field-Columbia Museum in Chicago (now the Field Museum of Natural History).

Leidy was the first scientist to realize that plant-eating dinosaurs such as Hadrosaurus and Iguanodon did not resemble large, sprawling lizards. Instead they stood upright; Leidy thought they stood like kangaroos.

Hadrosaurus did stand on its back legs. Its front limbs were shorter than its legs. It probably had a horny beak for cropping plants and leaves, and behind its beak it had a complex tooth system for chewing.

For many years, scientists had little idea what Hadrosaurus looked like. The skull was not found with the rest of the skeleton. Recent studies have shown that it looked like the hook-nosed hadrosaurids such as Gryposaurus and the more poorly known Kritosaurus. Both of these animals had a deep, narrow face with a rounded arch above the nostrils (it is not known if Hadrosaurus had this arch). This arched snout was probably covered with thick skin and may have been used both as a display and as a fighting structure. If so, it may have been more prominent in males than in females.

These hook-nosed hadrosaurids were close relatives of Hadrosaurus. Because they are similar, they have often been confused. Hadrosaurus probably lived only in the eastern part of the United States. Aralosaurus from the Kazakhstan was also a close relative of these hook-nosed duckbilled dinosaurs.