Hadrosaurids (), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod family, which includes genera such as Edmontosaurus and Parasaurolophus, was a common group of herbivores during the Late Cretaceous Period. Hadrosaurids are descendants of the Upper Jurassic/Lower Cretaceous iguanodontian dinosaurs and had a similar body layout. Hadrosaurs were among the most dominant herbivores during the Late Cretaceous in Asia and North America, and during the close of the Cretaceous several lineages dispersed into Europe, Africa, South America and Antarctica. Like other ornithischians, hadrosaurids had a predentary bone and a pubic bone which was positioned backwards in the pelvis. Unlike more primitive iguanodonts, the teeth of hadrosaurids are stacked into complex structures known as dental batteries, which acted as effective grinding surfaces. Hadrosauridae is divided into two principal subfamilies: the lambeosaurines (Lambeosaurinae), which had hollow cranial crests or tubes; and the saurolophines (Saurolophinae), identified as hadrosaurines (Hadrosaurinae) in most pre-2010 works, which lacked hollow cranial crests (solid crests were present in some forms). Saurolophines tended to be bulkier than lambeosaurines. Lambeosaurines included the aralosaurins, tsintaosaurins, lambeosaurins and parasaurolophins, while saurolophines included the brachylophosaurins, kritosaurins, saurolophins and edmontosaurins. Hadrosaurids were facultative bipeds, with the young of some species walking mostly on two legs and the adults walking mostly on four. History of discovery Ferdinand Vandeveer Hayden, during expeditions near the Judith River in 1854 through 1856, discovered the very first dinosaur fossils recognized from North America. These specimens were obtained by Joseph Leidy, who described and named them in 1856; two of the several species named were Trachodon mirabilis of the Judith River Formation and Thespesius occidentalis of the "Great Lignite Formation". The former was based on a collection of teeth whilst the later on two and a . Although most of the Trachodon teeth turned out to belong to ceratopsids, the holotype and remains of T. occidentalis would come to be recognized as the first recognized hadrosaur specimens. Around the same time in Philadelphia, on the other side of the continent, geologist William Parker Foulke was informed of numerous large bones accidentally uncovered by farmer John E. Hopkins some twenty years earlier. Foulke obtained permission to investigate the now scattered fossils in 1858, and these specimens as well were given to Leidy. They were described in the same