Hadrosaurus foulkii, the only species in this genus, is known from a single specimen consisting of much of the skeleton and parts of the skull. The specimen was collected in 1858 from the Woodbury Formation in New Jersey, US, representing the first dinosaur species known from more than isolated teeth to be identified in North America. Using radiometric dating of bivalve shells from the same formation, the sedimentary rocks where the "Hadrosaurus" fossil was found have been dated at some time between 80.5 and 78.5 million years ago. In 1858 the only known specimen became the first ever mounted dinosaur skeleton and since 1991 the species "H. foulkii" has become the official state dinosaur of New Jersey.