Hadrosauridae (from Ancient Greek ἁδρός (hadrós) 'stout, thick' and σαύρα (saúra) 'lizard'), is a family of ornithischian dinosaurs informally referred to as duck-billed dinosaurs. 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 Late Jurassic/Early 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, and South America. 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 latter on two caudal centra and a phalanx. 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 year as Hadrosaurus foulkii, giving a slightly better picture of the form of a hadrosaur. Leidy provided additional description in a 1865 paper. Among his 1858 work Leidy briefly suggested that the animal was likely amphibious in nature; this school of thought about hadrosaurs would come to be dominant for over a century to come.
Further discoveries such as "Hadrosaurus minor" and "Ornithotarsus immanis" would come from the East, and Edward Drinker Cope led an expedition to the Judith River Formation where Trachodon was found. Upon the fragments discovered he named seven new species in two genera, as well as assigning material to Hadrosaurus. Cope had studied the jaws of hadrosaurs and come to the conclusion that the teeth were fragile and could have been dislodged incredibly easily. As such, he supposed the animals must have fed largely on soft water plants; he presented this idea to the Philadelphia Academy in 1883, and this idea would come to be very influential on future study. Research would continue in the Judith River area for years to come, but the formation never yielded much more than fragmentary remains, and Cope's species as well as Trachodon itself would in time be seen as of doubtful validity. The Eastern states, too, would never yield particularly informative specimens. Instead, other sites in the American West would come to provide many very complete specimens that would form the backbone of hadrosaur research. One such specimen was the very complete AMNH 5060 (belonging to Edmontosaurus annectens), recovered in 1908 by the fossil collector Charles Hazelius Sternberg and his three sons in Converse County, Wyoming. It was described by Henry Osborn in 1912, who dubbed it the "Dinosaur mummy". This specimen's skin was almost completely preserved in the form of impressions. The skin around its hands, thought to represent webbing, was seen as further bolstering the idea that hadrosaurs were very aquatic animals. Cope had planned to write a monograph about the group Ornithopoda, but never made much progress towards it before his death. This unrealized endeavor would come to be the inspiration for Richard Swann Lull and Nelda Wright to work on a similar project decades later. Eventually they realized the whole of Ornithopoda was too broad of a scope, until eventually it was narrowed down to specifically North American hadrosaurs. Their monograph, Hadrosaurian Dinosaurs of North America, was published in 1942, and looked back at the whole of understanding about the family. It was designed as a definitive work, covering all aspects of their biology and evolution, and as part of it every known species was re-evaluated and many of them redescribed. They agreed with prior authors on the semi-aquatic nature of hadrosaurs, but re-evaluated Cope's idea of weak jaws and found quite the opposite. The teeth were rooted in strong batteries and would be continuously replaced to prevent them getting worn down. Such a system seemed incredibly overbuilt for the job of eating soft Mesozoic plants, and this fact confused the authors. Though they still proposed a diet of water plants, they considered it likely this would be supplemented by occasional forrays into browsing on land plants.
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