Genus Acanthochelys

Gray, 1873c
South American side-necked swamp turtles

The carapace is oval and flattened with no medial groove or only a shallow one extending along vertebrals 2-4. It is usually highest and widest behind the center, and the posterior rim is smooth to only slightly serrate. The nuchal bone extends to the anterior carapacial rim. Behind this there usually are no neural bones, so the costals meet at the carapacial midline. The cervical scute is relatively small. The hingeless plastron is large with the forelobe broader than the hindlobe, which bears a posterior notch. The forelobe may be slightly upturned. The intergular scute completely separates the gulars, but not the humerals. The buttresses are strong, and the inguinal buttress is ankylosed to the 5th costals. The skull is broad to moderately broad with an unnotched upper jaw. The frontal bones do not meet at the midline, and the temporal arch is moderate and formed by the squamosal and parietal bones. The dorsal part of the parietal covers the central area of the adductor fossa but not the lateral parts; the parietal does not contact the supraoccipital. The dorsal horizontal part of the supraoccipital is slightly expanded. There is vomer-palatine contact, but no quadrate-basisphenoid contact occurs. Triturating surfaces are smooth. Chin barbels are present, and the dorsal surface of the neck contains numerous tubercles. The toes are fully webbed and each forefoot has five claws. Each thigh has a series of tubercles.

Until recently the four species of South American Acanthochelys were considered members of the genus Platemys, but major biochemical, karyological, and morphological differences exist between them and Platemys platycephala (Derr et al., 1987).

Species identification
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