This is a medium-sized (to 18 cm), side-necked turtle with a flattened, elliptical carapace bearing a pronounced medial groove between the posterior part of the 1st vertebral and the anterior part of the 5th. In juveniles, all vertebrals are broader than long, but in adults the 3rd is usually longer than broad; the 4th and 5th are smallest. Posterior marginals are flared (slightly serrated in juveniles); those lateral are upturned. The surface of the scutes of turtles < 90 mm is usually roughened with numerous rounded rugosities. The carapace is highest just anterior to the 2nd intervertebral seam and broadest at the level of the 7th or 8th marginals. It is yellow with dark-brown or black pigment covering varying amounts of the surface (see the section on geographic variation for a more concise description of the two distinct patterns). The plastron is dark brown or black with a yellow border; the bridge is yellow with a dark transverse bar. The forelobe is slightly upturned and longer and slightly broader than the hindlobe, which contains a wide posterior notch. The intergular scute is about half as long as the length of the plastral forelobe. The plastral formula is usually: intergul > abd > fem > an > gul > hum >< pect. The head is orange to yellow brown dorsally, but dark brown to black laterally and ventrally; the light dorsal pigment extends downward on the sides to the midpoint of the orbit and tympanum. The unnotched jaws are dark brown. Dorsally, the head is covered with smooth, undivided skin; one to three rows of large scales occur laterally. The snout is short and only slightly protruding. Two small brown chin barbels are present, and the iris is brown. The neck is colored similarly to the head, and its dorsolateral surface contains numerous blunt tubercles. Anterior surfaces of the black limbs are covered with large scales, and small blunt tubercles are present on the thighs. The black tail is short.
The normal 2n karyotype is probably 64 (Barros et al., 1976); however, Kiester and Childress (in Gorman, 1973) reported it to be 68, and Bull and Legler (1980) discovered that two specimens they examined had an unusual triploid karyotype of 96. Bickham et al. (1993) found that viable diploids, triploids, diploids diploid-triploid mosaics and triploid-tetraploid mosaics are found in natural populations. Polyploids were of both sexes. but in spite of somatic ploidy content males only have diploid spermatocytes, produce only balanced gametes, and polyploid males appear fertile.
Males are slightly larger, have concave plastra and longer tails with the vent extending beyond the carapacial rim.
Platemys platycephala is restricted to northern South America where it occurs in the Caribbean drainages of Venezuela and the Guianas and the Amazon drainages from northeastern Bolivia, eastern Ecuador and Peru and southeastern Colombia eastward to the vicinity of Belem, Brazil.
Two subspecies are recognized. The common twist-necked turtle Platemys platycephala platycephala (Schneider, 1792) occurs in the Caribbean drainages of Venezuela and the Guianas and the Amazon drainages as far upstream as the Rios Purus, Juruá, Yavari, and Putumayo. The dark pigment on the yellow carapace is restricted to the border of the seam separating the vertebrals and pleurals and to an incomplete band extending on each side of the medial groove downward through the 2nd and 3rd pleurals to the lateral carapacial rim (the medial groove remains yellow); the dark bar crosses less than 80% of the bridge. Platemys p. melanonota Ernst, 1984, the black-backed twist-necked turtle, is restricted to the upper Amazon drainages of the Rios Santiago and Cenepa in Peru and the Rios Napo and Curaray in Ecuador. It intergrades with P. p. platycephala in the Rio Mamoré drainage, Bolivia, and the Rios Madre de Dios, Purus, and Ucayali of Peru. It is a dark subspecies with the yellow pigment on the brown carapace restricted to the vertebral groove, in some on the extreme anterior part of the 1st pleurals, and on the posterior part of the 4th pleurals; the dark bar crosses more than 90% (usually 100%) of the bridge.
Platemys platycephala, a poor swimmer, is an inhabitant of shallow rainforest streams, pools, and marshes; it frequently wanders about the forest floor, but does not enter large rivers.
Courtship and mating take place primarily during the rainy season (late March to early December) in or out of water (Medem, 1983a). During courtship, the male pursues the female and mounts from behind. He bends his head over hers, touches his barbels to the top of her head, and swings his head back and forth from side to side, occasionally expelling a stream of water from his nostrils over her face (Harding, 1983).
Oviposition occurs early in the dry season (primarily from August to February). The female does not dig a nest cavity but instead makes a shallow groove or lays her egg directly on the ground. The egg is always laid under rotten leaves and may then be partially covered by sand or dirt (Medem, 1983a).
Usually only one oblong to elliptically tapered, brittle-shelled egg (51-61 x 26-29 mm) is laid at one time (Medem, 1983a). Hatchlings are 43-57 mm in carapace length and have numerous small rounded rugosities on each carapacial scute; coloration is like that of adults.
Food preferences of captives include worms, snails, slugs, insects, amphibians, fish, and some vegetation.
IUCN Red List Status (1996)