Size and Weight:
For a dinosaur that was discovered so early in the history of paleontology--in 1872, by the famous fossil hunter Othniel C. Marsh--Claosaurus has remained a bit obscure. Originally, Marsh thought he was dealing with a species of Hadrosaurus, the genus that gave its name to the hadrosaurs, or duck-billed dinosaurs; he then assigned his discovery the name Claosaurus ("broken lizard"), to which he later assigned a second species, which turned out to be a specimen of yet another duck-billed dinosaur, Edmontosaurus. Confused yet?
Nomenclature issues aside, Claosaurus is important for having been an unusually "basal" hadrosaur. This dinosaur was relatively small, "only" about 15 feet long and half a ton, and it probably lacked the distinctive crest of later, more ornate hadrosaurs (we can't know for sure, since no one has found a Claosaurus skull). The teeth of Claosaurus were similar to those of a much earlier ornithopod of the Jurassic period, Camptosaurus, and its longer-than-usual tail and unique foot structure also place it on one of the earlier branches of the hadrosaur family tree.