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Elasmosaurus
''Elasmosaurus'' (; from Greek ελασμος ''elasmos'' 'thin plate' (referring to thin plates in its pelvic girdle) + σαυρος ''sauros'' 'lizard') is a genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous period (Campanian stage), 80.5 million years ago.〔Carpenter 2003〕
==Description==
''Elasmosaurus'' was about in length and weighed , making it among the largest plesiosaurs.〔(【引用サイトリンク】title=Elasmosaurus - paleofiles.com )〕 It differs from all other plesiosaurs by having six teeth per premaxilla (the bones at the tip of the snout) and 71 neck (cervical) vertebrae.〔Sachs 2005〕 The skull was relatively flat, with a number of long pointed teeth. The lower jaws were joined at the tip to a point between the fourth and fifth teeth.〔Sachs 2005〕 The neck vertebrae immediately following the skull were long and low, and had longitudinal lateral crests.〔Brown 1993〕〔Carpenter 1999〕 Like most elasmosaurids, ''Elasmosaurus'' had around three pectoral vertebrae. The tail included at least 18 vertebrae.〔〔
The pectoral girdle featured a long bar,〔〔 not present in juveniles. The scapula had margins of approximately equal length for the joint with the coracoid and the articular surface for the upper arm.〔 The anterior edge of the pelvic girdle was made up of three almost straight edges directed to the front and sides of the animal. The ischia, a pair of bones that formed the posterior part of the pelvis, were joined along their medial surfaces.〔 The limbs of ''Elasmosaurus'', like those of other plesiosaurs, were modified into approximately equally sized rigid paddles.〔Everhart 2005〕
Certain aspects of the anatomy of ''Elasmosaurus'' were fairly derived among elasmosaurids, and plesiosaurs in general.〔 As noted, ''Elasmosaurus'' can be distinguished by its six premaxillary teeth and 71 cervical vertebrae.〔 Primitively, plesiosaurs and most elasmosaurids had five teeth per premaxilla. Some elasmosaurids had more: ''Terminonatator'' had nine〔Sato 2003〕 and ''Aristonectes'' had 10 to 13.〔Gasparini 2003〕 In addition, most plesiosaurs had fewer than 60 cervical vertebrae.〔 Aside from ''Elasmosaurus'', plesiosaurs that exceeded 60 cervicals include ''Styxosaurus'', ''Hydralmosaurus'', and ''Thalassomedon''. ''Elasmosaurus'' is the only known plesiosaur with more than 70 cervicals. However, it had roughly the same neck length as ''Thalassomedon''〔Welles 1943〕 because the latter has proportionally longer vertebrae.〔 ''Elasmosaurus'' had more vertebrae than any known animal. The presence of the pectoral bar is also considered an advanced feature.〔 The long, low axis centrum differs from the condition seen in most other plesiosaurs, which have centra that are either shorter in length than height, or about equidimensional.〔 ''Styxosaurus'' and ''Hydralmosaurus'' also have the condition present in ''Elasmosaurus''.〔Welles and Bump 1949〕〔Sachs 2004〕 Another unusual feature of ''Elasmosaurus'' is the relatively equal lengths of the margins of the scapula, as mentioned above. Most plesiosaurs had longer margins for articulation with the coracoid than for articulation with the upper arm.〔Sach 2004〕
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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