The ellipsoidal, slightly bowed carapace (to 23.3 cm) has a medial keel, is somewhat serrated, has a shallow supracaudal notch, and is usually broadest at the level of the 8th marginals and highest on the 3rd vertebral. Its surface is smooth or slightly roughened. Vertebrals are broader than long and the 3rd to 5th may bear a small posterior projection on a low keel. Neural bones vary from none to five, but, if present, they are rudimentary and never contact the nuchal (Pritchard and Trebbau, 1984). The carapace is chestnut brown to dark gray or black. The plastron is well-developed and slightly upturned anteriorly. Its intergular scute completely separates the gulars, but not the humerals, and is shorter than its distance from the abdominals. The posterior plastral lobe curves inward and the anals are deeply notched posteriorly. The plastral formula is variable, but usually is: fem > intergul > abd > hum > an > pect > gul. The plastron is red brown to yellow with a brown blotch on each scute, and a narrow yellow border may occur anteriorly and posteriorly. Bridge and undersides of the marginals are brown to yellow. Together, the head and neck are considerably shorter than the carapace. Dorsally and laterally, the head is covered with numerous convex scales; those between the orbit and tympanum are smaller than those on the top and sides of the head. The snout protrudes and the upper jaw is not notched or serrated. Head and neck are red brown to dark gray dorsolaterally and grayish to pale yellow ventrally; the jaw may contain dark spots and the two small chin barbels are yellow. The upper jaw is often yellow to white with black bars. The toes are heavily webbed and there is a fringe of large scales on the outer border of the forelimbs and hindlimbs. Limbs and tail are gray black, limb sockets yellow.
According to Killebrew (1976), the diploid chromosomes total 50; 26 macrochromosomes (16 metacentric, 6 submetacentric, 4 acrocentric) and 24 microchromosomes. However, Bull and Legler (1980) reported a karyotype of 60 chromosomes, and Barros et al. (1976) reported the karyotype of Phrynops gibbus to be 2n = 60 (but listed 4 metacentric, 2 submetacentric, and 16 acrocentric macrochromosomes, and 40 microchromosomes, which adds up to 62). Since other Phrynops have karyotypes with 58 chromosomes, the total number of chromosomes is probably close to 60.
Males have slightly longer, thicker tails with the vent nearer the tip and a deep posterior plastral notch; females have a wide, shallow posterior plastral notch.
Phrynops gibbus ranges from central and northeastern Peru, eastern Ecuador, and southeastern Colombia northward through eastern Colombia to the Rio Negro in southwestern Venezuela. It also occurs east of the Sierra Nevada de Mérida in northeastern Venezuela and on Trinidad, eastward through Guyana, Surinam, and French Guiana to northeastern Brazil; recently, specimens have been taken in Paraguay.
No variation has been described, but Phrynops vanderhaegei may prove to be a subspecies. It differs in having no dark bars on its jaws, the dark plastral pattern ending on the pectorals and femorals, and a slightly narrower parietal width on the skull.
This turtle lives in shallow marshes, pools and ponds, streams, and blackwater rivers usually located under the tree canopy of primary forests.
The nesting period extends from July to November. Usually two to four elongated (41-50 x 25-32 mm), white, hard-shelled eggs are laid in a cavity about 10 cm deep. Some females, however, lay their eggs on the surface of the ground in leaf litter or among roots. Incubation may last up to 200 days. The 45-48 mm hatchlings range in carapacial coloration from totally black to cream with black flecks.
Phrynops gibbus is an omnivore, eating plant materials, aquatic insect larvae, and tadpoles. In captivity these turtles accept a wide variety of meats.
They are shy and basically nocturnal, although some bask in the daytime. When first captured they emit a foul-smelling musk and often bite.
IUCN Red List Status (1996)