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''Astrodon'' (aster: star, odon: tooth) is a genus of large herbivorous sauropod dinosaur, related to ''Brachiosaurus'', that lived in what is now the eastern United States during the Early Cretaceous period. Its fossils have been found in the Arundel Formation, which has been dated through palynomorphs to the Aptian-Albian boundary, about 112 million years ago.〔 Adults are estimated to have been more than 9 m (30 ft) high and 15 to 18 m (50 to 60 ft) long. ==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''〔W.E. Swinton, 1936, "The dinosaurs of the Isle of Wight", ''Proceedings of the Geologists' Association'' 47(3): 204-220〕 and ''Astrodon pussilus''.〔Lapparent, A.F. de, & Zbyszewski, G., 1957, ''Les Dinosauriens du Portugal'', Mémoires des Services Geologiques du Portugal. Nouvelle série, numéro 2, 63 pp〕 In 1962 R.F. Kingham assigned ''Brachiosaurus'', including all its species, to ''Astrodon'' as a subgenus.〔Kingham, R.F., 1962, "Studies of the sauropod dinosaur ''Astrodon'' Leidy", ''Proceedings of the Washington Junior Academy of Sciences'', 1: 38-44〕 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. Interestingly, the majority of the bones of ''Astrodon'' are of juveniles, and Carpenter and Tidwell considered the two species named by Marsh, ''P. nanus'' and ''P. altus'', as different growth stages of ''Astrodon johnstoni''. However, other authors did not find the argument in favor of the synonymization of ''Astrodon'' and ''Pleurocoelus'' so convincing. According to Peter Rose (2007) it has not been demonstrated that either the teeth of ''Astrodon johnstoni'' or those attributed to ''Pleurocoelus'' are "morphologically diagnostic among titanosauriforms", which limits their utility when it comes to distinguishing them from the teeth of other taxa. The type series of ''Pleurocoelus nanus'' and ''P. altus'' (four vertebrae and two hindlimbs bones, respectively) cannot be directly compared to the teeth from the type series of ''Astrodon'', so any comparison has to be conducted based on the referred specimens of ''Pleurocoelus''. These, however, are all isolated bones from the Arundel Formation, which themselves were referred to ''Pleurocoelus'' only based "on proximity of the localities and the size of the bones". Rose concludes that, as ''Astrodon'' is not based on the diagnostic material, "new discoveries should not be aligned with that genus" and that "the argument to synonymize the two taxa, ''Astrodon'' and ''Pleurocoelus'', seems unfounded". The type material of ''Pleurocoelus'' may not be diagnostic as well, according to the author. A similar argument was made by Michael D. D’Emic (2013). The author did not find any diagnostic features of the type material of ''Astrodon johnstoni'', ''Pleurocoelus nanus'' and ''P. altus'' and considered the three taxa to be ''nomina dubia''; according to the author there is no direct evidence that any sauropod bones from the Arundel Formation other than their type series can be referred to these taxa. D’Emic also stated that the exact provenance of the bones from the type series of ''Pleurocoelus nanus'' is uncertain and thus "these bones could represent a chimera of individuals or taxa". The author also commented on the diagnosis of ''Astrodon johnstoni'' proposed by Carpenter and Tidwell (which was based on all of the sauropod material from the Arundel Formation, not only on the teeth from the type series); he claimed that most of the supposed autapomorphies of this taxon "are indistinguishable compared to other sauropods such as ''Camarasaurus'' (...) and/or are related to the juvenile nature of the material". 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Astrodon」の詳細全文を読む スポンサード リンク
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