Hadrosauridae

Hadrosauridae, whose name comes from the Greek words for "stout lizard," are a family of ornithischian dinosaurs commonly called duck-billed dinosaurs because the bones in their snouts form a flat shape like a duck's bill. This group of ornithopods, which includes well-known genera like Edmontosaurus and Parasaurolophus, were widespread herbivores in the Late Cretaceous Period. They evolved from iguanodontian dinosaurs of the Late Jurassic and Early Cretaceous, keeping a similar overall body shape. During the Late Cretaceous, hadrosaurs were among the most successful plant-eaters in Asia and North America, and near the end of the period, some groups spread into Europe, Africa, and South America.

Like all ornithischians, hadrosaurids had a predentary bone at the front of the lower jaw and a pubic bone that pointed backward in the pelvis. Their teeth, however, were more advanced than those of primitive iguanodonts: they were packed into complex structures called dental batteries, which worked as efficient grinding surfaces. The family splits into two main subfamilies. Lambeosaurinae (lambeosaurines) had hollow crests or tubes on their heads. Saurolophinae (saurolophines), often called hadrosaurines in works before 2010, lacked hollow crests (though some had solid ones). Saurolophines were generally heavier-bodied than lambeosaurines. Within lambeosaurines are the aralosaurins, tsintaosaurins, lambeosaurins, and parasaurolophins. Within saurolophines are the brachylophosaurins, kritosaurins, saurolophins, and edmontosaurins.

Hadrosaurids were facultative bipeds: the young of some species walked mostly on two legs, while adults walked mostly on four.

**History of discovery**

The first dinosaur fossils ever recognized from North America were found by Ferdinand Vandeveer Hayden during trips near the Judith River between 1854 and 1856. Joseph Leidy obtained these specimens, describing and naming them in 1856. Among the several species he named were *Trachodon mirabilis* from the Judith River Formation and *Thespesius occidentalis* from the "Great Lignite Formation." The first was based on a collection of teeth, the second on two tail vertebrae and a toe bone. Most of the *Trachodon* teeth later turned out to belong to ceratopsids, but the holotype and remains of *T. occidentalis* became recognized as the first known hadrosaur specimens. Around the same time, on the opposite side of the continent in Philadelphia, geologist William Parker Foulke learned that farmer John E. Hopkins had accidentally uncovered many large bones about twenty years earlier. Foulke got permission to study the now-scattered fossils in 1858, and these too were given to Leidy. That same year, Leidy described them as *Hadrosaurus foulkii*, giving a clearer picture of what a hadrosaur looked like. He added more description in an 1865 paper. In his 1858 work, Leidy briefly suggested the animal was probably amphibious—an idea about hadrosaurs that would dominate for over a century.

Further finds, such as "Hadrosaurus minor" and "Ornithotarsus immanis," came from the East, while Edward Drinker Cope led an expedition to the Judith River Formation where *Trachodon* had been found. From the fragments he discovered, Cope named seven new species in two genera and also assigned some material to *Hadrosaurus*. After studying hadrosaur jaws, Cope concluded their teeth were fragile and could easily be knocked out. He therefore thought the animals must have fed mostly on soft water plants, presenting this idea to the Philadelphia Academy in 1883. This view strongly influenced later research. Work continued in the Judith River area for years, but the formation never produced much more than fragments, and Cope's species—along with *Trachodon* itself—eventually came to be seen as doubtful. The Eastern states also never yielded particularly informative specimens. Instead, other sites in the American West provided many complete fossils that became the foundation of hadrosaur research. One key specimen was the very complete AMNH 5060 (belonging to *Edmontosaurus annectens*), found in 1908 by fossil collector Charles Hazelius Sternberg and his three sons in Converse County, Wyoming. Henry Osborn described it in 1912, calling it the "Dinosaur mummy." Its skin was almost entirely preserved as impressions. The skin around its hands, thought to show webbing, was taken as more evidence that hadrosaurs were highly aquatic.

Cope had planned to write a monograph on the group Ornithopoda but made little progress before his death. This unfinished project later inspired Richard Swann Lull and Nelda Wright to take on a similar task decades afterward. They eventually found the whole of Ornithopoda too broad and narrowed their focus to North American hadrosaurs. Their monograph, *Hadrosaurian Dinosaurs of North America*, came out in 1942 and reviewed everything known about the family at the time. Meant as a definitive work, it covered all aspects of hadrosaur biology and evolution, re-evaluating every known species and redescribing many. The authors agreed with earlier researchers about the semi-aquatic nature of hadrosaurs, but they re-examined Cope's idea of weak jaws and found the opposite. The teeth were rooted in strong batteries and were continuously replaced to prevent wear. Such a system seemed far too robust for eating soft Mesozoic plants, which confused the authors. Still, they proposed a diet of water plants.