Five living genera and six species of hard-shelled sea turtles belong to the family Cheloniidae. Most of their feeding and nesting range is in warmer marine waters, and they occur in all tropical oceans with several species ranging well into the temperate zones. Cheloniids are among the oldest fossil turtles. The extinct genera Allopleuron, Catapleura, Glaucochelone, Glyptochelone, Peritresius and Tomochelone and the living genera Caretta and Chelonia are all known from Upper Cretaceous deposits of Europe or North America. Cheloniids are most closely related to the extinct families Osteopygidae and Desmatochelyidae, and the living family Dermochelyidae and its extinct sister family Toxochelyidae (Gaffney and Meylan, 1988). The extinct family Plesiochelyidae is only distantly related.
The temporal region of the skull is completely roofed. The premaxillae are not fused, meet the vomer, and separate the internal nares and palatines. There is parietal-squamosal contact. A secondary palate is present, but palatine fenestra are lacking. The pterygoids separate the basisphenoid from the palatines but do not touch the maxillae. Bony trabeculae of the basisphenoidal rostrum lie close together or are fused. The quadrate never encloses the stapes, and the maxilla does not contact the quadratojugal. The dentary is confined to the anterior half of the lower jaw. The shell is covered with horny scutes. Neurals are hexagonal, shortest anteriorly, and variable in number. The nuchal bone lacks a costiform process but has a ventral area for attachment to the neural arch of the 8th cervical. Carapaceandplastron are connected by ligaments, and neither is hinged. Reduction of the plastron is indicated by the small entoplastron. A small intergular scute and small postanal scutes may be present. Inframarginal scutes are usually present. The forelimbs are paddlelike with elongated digits. The deltopectoral crest of the humerus is far down the shaft; the trochanteric fossa of the femur is greatly restricted, and the trochanters are united. The neck is short, and ability to retract the head has been lost. The 4th vertebra is biconvex.
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