Astrodon (aster: star, odon: tooth) is a genus of large herbivorous sauropod dinosaur, measuring in length, in height and in body mass. It lived in what is now the eastern United States during the Early Cretaceous period, and fossils have been found in the Arundel Formation, which has been dated through palynomorphs to the Albian about 112 to 110 million years ago. Discovery and species Two dinosaur teeth were received in late November 1858 by chemist Philip Thomas Tyson from John D. Latchford. They had been found in Latchford's open iron ore pit in the Arundel Formation at Swampoodle near Muirkirk in Prince George's County, Maryland. Tyson let them be studied by the dentist Christopher Johnston, professor at the Baltimore Dental College, who cut one tooth in half and thereby discovered a characteristic star-formed cross-section. Johnston named Astrodon in 1859. However, he did not attach a specific epithet, so Joseph Leidy is credited with naming Astrodon johnstoni (the type species) in 1865, with as holotype specimen YPM 798. If Johnston had attached a specific epithet, it would have been the second dinosaur species identified in the United States. Johnston incorrectly stated that the site of the discovery had been an iron mine near the town of Bladensburg. In 1888, O. C. Marsh named some bones from the Arundel found near Muirkirk, Maryland Pleurocoelus nanus and P. altus. However, in 1903 John Bell Hatcher, taking into account the similarity of the teeth of Astrodon johnstoni and the teeth from the Arundel Formation referred to Pleurocoelus nanus, argued that the latter represents the same species as the former and that the name Astrodon therefore had priority. In 1921 Charles W. Gilmore agreed that the genus Pleurocoelus is a junior synonym of Astrodon, but at the same time kept P. nanus and P. altus as separate species of Astrodon. Other species at one time assigned to the genus include Astrodon valdensis and Astrodon pussilus. In 1962 R. F. Kingham assigned Brachiosaurus, including all its species, to Astrodon as a subgenus. Carpenter and Tidwell (2005) accepted Hatcher's argument that there is only one species of sauropod dinosaur known from the Arundel Formation and that Astrodon johnstoni is the senior synonym of Pleurocoelus nanus (as well as P. altus) in the first in-depth description of this dinosaur. The majority of the bones of Astrodon are of juveniles, and Carpenter and Tidwell considered the two species