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protostegidae
Protostegidae is a family of extinct marine turtles that lived during the Cretaceous period. The family includes some of the largest sea turtles that ever existed. The largest, ''Archelon'', had a head a meter long. Like most sea turtles, they had flattened bodies and flippers for front appendages; protostegids had minimal shells like leatherback turtles of modern times.
==Anatomy==
As some of the first marine turtles, the protostegids set the general body plan for future species of sea turtles. They had a generally depressed turtle body plan, complete with four limbs, a short tail, and a large head at the end of a relatively short neck. Like other sea turtles, they possessed oar-like front appendages especially evolved for swimming in the open ocean. Similar to the still-extant, possibly closely related Dermochelyidae, protostegids possessed extremely reduced carapaces. Some specimens had skeletal protrusions from their ribs almost wrapping around their bodies in place of a complete shell. Like modern sea turtles, protostegids had sharp beaks. One of the defining characteristics of the members of the family was their almost-disproportionately large heads. Specifically, some specimens of ''Archelon'' have been found with heads a meter long. In addition, the members of the family had somewhat reduced plastrons, as well.
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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