Hadrosaurid

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iHadrosaurids
Fossil range: Cretaceous

Corythosaurus and trackway by Frederik Spindler.

Scientific classification

Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Superorder:	Dinosauria
Order:	Ornithischia
Suborder:	Ornithopoda
Infraorder:	Iguanodontia
Superfamily:	Hadrosauroidea
Family:	Hadrosauridae Cope, 1869

Genera

See text.

Hadrosaurids or duck-billed dinosaurs are members of the superfamily Hadrosauroidea, and include ornithopods such as Edmontosaurus and Parasaurolophus. They were common herbivores in the Upper Cretaceous Period of what are now Asia, Europe and North America. They are descendants of the Upper Jurassic/Lower Cretaceous iguanodontian dinosaurs and had similar body layout. They were ornithischians.

Hadrosaurids are divided into two subfamilies. The lambeosaurines (Lambeosaurinae) have large cranial crests or tubes, and are less bulky. The hadrosaurines (Hadrosaurinae) lack the cranial crests or tubes and are larger.

1 Discoveries
2 Characteristics
3 Taxonomy
4 External links

[edit] Discoveries

An old drawing of the defunct hadrosaur genus "Trachodon"

Hadrosaurids were the first dinosaur family to be identified in North America, the first traces being found in 1855-1856 with the discovery of fossil teeth. Joseph Leidy examined the teeth, and erected the genera Trachodon and Thespesius (others included Troodon, Deinodon and Palaeoscincus). One species was named Trachodon mirabilis. Now it seems that the teeth genus Trachodon is a mixture of all sorts of cerapod dinosaurs, including ceratopsids. In 1858 the teeth were associated with Leidy's eponymous Hadrosaurus foulkii, named after the fossil hobbyist William Parker Foulke. More and more teeth were found, resulting in even more (now obsolete) genera.

A second duck-bill skeleton was unearthed, and was named Diclonius mirabilis in 1883 by Edward Drinker Cope, which he incorrectly used in favor of Trachodon mirabilis. But Trachodon, together with other poorly typed genera, was used more widely and, when Cope's famous "Diclonius mirabilis" skeleton was mounted at the American Museum of Natural History, it was labeled as "Trachodont dinosaur". The duck-billed dinosaur family was then named Trachodontidae.

A very well-preserved complete hadrosaurid specimen (Edmontosaurus annectens) was recovered in 1908 by the fossil collector Charles Hazelius Sternberg and his three sons, in Converse County, Wyoming. It was known as the "Trachodon mummy". This specimen's skin was almost completely preserved, together with some muscles and was analysed by Henry Osborn in 1912. Sternberg was in Cope's camp during his famous competition to name new species with Othniel Charles Marsh. This discovery was a victory for Cope in the Bone Wars.

A well preserved specimen of Edmontosaurus.

Lawrence Lambe erected the genus Edmontosaurus ("lizard from Edmonton") in 1917 from a find in the Edmonton Rock Formation, Alberta. Hadrosaurid systematics were a mess until 1942, when Richard Swann Lull and Nelda Wright proposed the genus Anatosaurus. Cope's famous mount at the AMNH became Anatosaurus copei. In 1975, Anatosaurus was moved to Edmontosaurus, because the species were just too similar to the Edmontosaurus type species, E. regalis and because Edmontosaurus was older, it had precedence. The original sample was probably a young Edmontosaurus. One former Anatosaurus species was distinct enough from Edmontosaurus to be placed in a separate genus, named Anatotitan, so in 1990 the AMNH mount was re-labelled Anatotitan copei.

Some paleontologists found a hadrosaurid leg bone in the Cenozoic rocks.

Fossilized skeletion (partial) of a young hadrosaur excavated and displayed at the Escalante-Grand Staircase National Mounument in Southern Utah. Click image for display placard text.

[edit] Characteristics

Skull of Edmontosaurus, showing duck-bill and dentition Oxford University Museum of Natural History.

The hadrosaurs are known as the duck-billed dinosaurs due to the similarity of their head to that of modern ducks. In some species, most notably Anatotitan, the whole front of the skull was flat and broadened out to form a beak, ideal for clipping leaves and twigs from the forests of Asia, Europe and North America. However, the back of the mouth contained literally thousands of teeth suitable for grinding food before it was swallowed. Hadrosaurs, like their iguanodontian cousins, had a rudimentary dental specialisation analogous to incisors and molars. This has been hypothesized to have been a crucial factor in the success of this group in the Cretaceous, compared to the sauropods which were still largely dependent on gastroliths for grinding their food.

[edit] Taxonomy

Skin impressions of Edmontosaurus.

Sereno (2005) defines Hadrosauridae as the most inclusive possible group containing Saurolophus (a well-known hadrosaurine), Parasaurolophus (a well-known lambeosaurine), and Hadrosaurus (the type geus of the family, which ICZN rules state must be included, despite its status as a nomen dubium). According to some studies, this would place a few other well-known hadrosaurs (such as Telmatosaurus and Bactrosaurus) outside the family, which has led some authors (Horner, 2004) to define the family to include Telmatosaurus by default. Probable hadrosaurids that cannot be assigned to either sub-family are listed under Hadrosauridae and preceded by a question mark.

Some of the hadrosaurid "tribes" commonly recognized online are not yet formally defined in the literature.