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Nyasasaurus
''Nyasasaurus'' (meaning "Lake Nyasa lizard") is an extinct genus of dinosauriform reptile from the Middle Triassic Manda Beds of Tanzania that appears to be the earliest known dinosaur. The type species ''N. parringtoni'' was first described in 1956 in the doctoral dissertation of English paleontologist Alan J. Charig, but it was not formally described until 2013. Previously, the oldest record of dinosaurs was from Argentina and dated back to the late Carnian stage, about 231.4 million years ago (Ma). ''Nyasasaurus'' comes from a deposit that dates back to the Anisian, meaning that it predates other early dinosaurs by about 12 million years.
==Description==
The type specimen, NHMUK R6856, is a partial skeleton belonging to an individual estimated to have been two to three metres in length. It consists of a right humerus, three partial presacral vertebrae and three sacral vertebrae (of the back, above the location of the hip bones). A second specimen, SAM-PK-K10654 consisting of three cervical vertebrae (neck vertebrae) and two posterior presacral vertebrae, is also known. It was attributed to the same species as NHMUK R6856 because the dorsal or back vertebrae of the two specimens are nearly identical. However, the vertebral features that link NHMUK R6856 and SAM-PK-K10654, including a connection between two bony projections called the hyposphene and hypantrum, are also found in other Triassic archosaurs. Because these characteristics are not unique to the two specimens (in other words, they are not autapomorphies), they do not by themselves provide sufficient evidence for grouping NHMUK R6856 and SAM-PK-K10654 under the same species. The authors of the 2013 description of ''Nysasaurus'' used a second line of evidence, the similar positions of the two specimens on evolutionary trees, to justify their placement in the same species.〔
Nesbitt ''et al.'' (2013) also mentioned the similarity between the presacral vertebrae of both specimens of ''N. parringtoni'' and those of "Teleocrater rhadinus" (another ''nomen nudum'' from Charigs' dissertation, based on NHMUK R6795 from Lifua Member). Additionally, the anterior cervical vertebra attributed to NHMUK R6795 is extremely elongated relative to that of the middle dorsal vertebrae with a low centrum to neural arch ratio and a significant displacement between the two sides of the articular facet of the centrum. However, it is probable that the limb bones and other elements included in NHMUK R6795 do not belong to the same individual. Therefore, it is possible that the vertebrae of "T. rhadinus" are also referable to ''N. parringtoni''.〔
An analysis of the interior structure of the humerus indicates that bone growth was rapid, with interwoven bone fibers, many channels for blood vessels that radiate in all directions, and few lines of arrested growth. This structure more closely matches that of the early dinosaur ''Coelophysis'' than it does of dinosaur ancestors, suggesting that ''Nyasasaurus'' was closer to the ancestry of dinosaurs than were other archosaurs at the time.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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