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Asterotrygon
''Asterotrygon'' is an extinct genus of stingray from the Eocene Green River Formation in Wyoming. Several complete skeletons representing juveniles, adults, males and females have been uncovered from the late early Eocene Fossil Butte Member of the formation. The type and only species, ''A. maloneyi'', was named in 2004 on the basis of these fossils. Another stingray, ''Heliobatis'', is also known from the formation. ''Asterotrygon'' is a primitive stingray closely related to the living family Urolophidae whose ancestors likely originated in the Indo-Pacific. It lived in Fossil Lake, a body of water that existed in a subtropical mountainous region for only about 2 million years.
==Description==
''Asterotrygon'' had a typical stingray shape with a flat, rounded disk formed from the head and pectoral fins and a long, narrow tail with sharp stingers. It ranged in size from in the smallest juvenile (the specimen SMMP 83.25) to in the largest adult (FMNH 15166). The shape of the disk is rounder than those of other extinct stingrays such as ''Heliobatis'', which are more diamond-shaped. The upper surface of the disk is covered in small dermal denticles, each with a small hook. Unlike ''Heliobatis'' and modern stingrays, it has a small dorsal fin in front of its stingers. While most stingrays have a cartilaginous rod extending from the stinger to the tip of the tail, ''Asterotrygon'' retains separate vertebrae throughout the tail's entire length. The tail is also somewhat thicker at its base than those of other stingrays. Small fin-like tail folds are present at the tip of the tail. The puboischiadic girdle, an element to which the pelvic fins attach, is primitively narrow and arch-shaped. Like other stingrays, ''Asterotrygon'' lacks thoracic ribs but possesses cartilage around the spine called thoracolumbar synarcual cartilage.
Several details of the neurocranium link ''Asterotrygon'' with modern stingrays. Like other stingrays, the hyomandibulae, which allow the jaw to extend outward, are completely separated from the lower jaw. In other cartilaginous fish, the hyomandibulae and lower jaw are fused by the hyomandibular-Meckelian ligament. Despite this loss, ''Asterotrygon'' still retains some calcified cartilage in the space where the ligament once was. The postorbital processes behind the eye sockets are broad, flat, and shelf-like.〔
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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