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''Atychodracon'' is an extinct genus of rhomaleosaurid plesiosaurian known from the Late Triassic - Early Jurassic boundary (probably early Hettangian stage) of England. It contains a single species, ''Atychodracon megacephalus'', named in 1846 originally as a species of ''Plesiosaurus''. The holotype of ''"P." megacephalus'' was destroyed during a WWII air raid in 1940 and was later replaced with a neotype. The species had a very unstable taxonomic history, being referred to four different genera by various authors until a new genus name was created for it in 2015. Apart from the destroyed holotype and its three partial casts (that survived), a neotype and two additional individuals are currently referred to ''Atychodracon megacephalus'', making it a relatively well represented rhomaleosaurid. ==Taxonomic history and naming== The type species of ''Atychodracon'' was first described and named by Samuel Stutchbury in January 1846, as a species of the wastebasket taxon ''Plesiosaurus''. The specific name means "large-headed" in Greek in reference to the very large skull compared to the rest of the skeletal elements ''"Plesiosaurus" megacephalus'' had, relatively to other / actual species of ''Plesiosaurus''. The pliosauroid nature of ''"Plesiosaurus" megacephalus'' remained unnoted until a revision by Richard Lydekker in 1889. Lydekker recognized the rhomaleosaurid affinities of ''"P." megacephalus'', but because he and Harry G. Seeley "refused steadfastly to recognize the generic and specific names proposed by one another", he moved ''"P." megacephalus'' to the genus ''Thaumatosaurus'' which was regarded by him as a replacement to Seeley's ''Rhomaleosaurus'' - creating the new combination ''T. megacephalus''.〔 In a revision of many pliosauroid taxa, Andrews (1922) was the first to recognized that ''"P." megacephalus'' is morphologically more closely related to ''"Plesiosaurus" arcuatus'' than to species of the ''Rhomaleosaurus''/''Thaumatosaurus'' complex. He concluded that the two species belong to the same genus, which he erected as ''Eurycleidus'', with the type species being ''Eurycleidus arcuatus'', and ''E. megacephalus'' as a referred species. Nevertheless, this was not followed by all authors, such Swinton (1930) who used ''T. megacephalus'' and later in 1948 ''P. megacephalus'', to avoid confusion soon after the holotype was destroyed. This conservative name, ''"P." megacephalus'', was followed by Taylor and Cruickshank (1989) and Taylor (1994). However in 1994 Cruickshank designated a neotype for the species, and due to the ''Rhomaleosaurus''/''Thaumatosaurus'' issue being resolved in favor of the former (while ''Thaumatosaurus'' is a ''nomen dubium'') he referred to it by the new combination, ''Rhomaleosaurus megacephalus''.〔〔 Adam S. Smith in his 2007 thesis on the anatomy and classification of the family Rhomaleosauridae, found the genus ''Rhomaleosaurus'' that became a wastebasket taxon itself to have only three valid species, and considered ''"P." megacephalus'' to be rather a second species of ''Eurycleidus'' as suggested by Andrews (1922). Smith and Dyke (2008) recognized a fourth valid species in ''Rhomaleosaurus'', and tentatively referred to ''"P." megacephalus'' as ''"Rhomaleosaurus" megacephalus''. However, they recognized the need for a new genus name for ''"P." megacephalus'', as was supported by their phylogenetic analysis of all valid ''Rhomaleosaurus'' species, and most valid rhomaleosaurids. The cladogram below follows the preliminary phylogenetic analysis of Smith & Dyke (2008), with the asterisk noting species recently removed from ''Rhomaleosaurus'' to their own genera. }} |2= }} }} }} }} }} }} Cladistic analyses by Ketchum & Benson, 2010, Benson ''et al.'', 2011, Ketchum & Benson, 2011, and various later studies found ''"R." megacephalus'' to be basal to the clade containing ''Rhomaleosaurus'' and ''Eurycleidus'', thus it should be in its own genus as suggested by Smith and Dyke (2008). Following this, the new genus name ''Atychodracon'' was erected by Smith in 2015 for ''"R." megacephalus'', with the type species being ''Atychodracon megacephalus''. The generic name is derived from Greek ατυχής, ''atychis'', meaning "unfortunate" in reference to the unfortunate destruction of the holotype during a WWII air bombing in 1940, plus δράκωνe, ''drakon'', meaning "dragon" - a common suffix in genus names of various mesozoic reptile groups.〔 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「Atychodracon」の詳細全文を読む スポンサード リンク
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