This is a highly aquatic freshwater turtle. Adult carapace length 235 mm, maximum size approaches 300 mm; hatchlings 41−48 mm; males do not exceed 170 mm carapace length.
The head relatively wide and flattened; tympanum large; two barbles on chin; carapace depressed in adults, juveniles have a carapace with a medial keel that is lost with age; the bridge is small, equal to the length of three marginal scutes; plastron long and wide with a deep anal notch. All digits have extensive webbing. Crown is olive brown, jaws yellow; carapace lacks a pattern, dark brown or black; plastron dark brown to yellow.
This turtle has a wide, flat head with a large tympanum, granular skin on top of the head, and two small barbles on the chin. Adults have a smooth depressed carapace; juveniles may have a slight median keel lost with age; digits extensively webbed. Head olive-brown or gray, jaws yellow; carapace dark brown or black without a pattern; plastron light to dark brown or yellow. There is a population in Trinidad, Venezuela and Guyana and another in the western Amazon; it is also present in the Parana drainage of the Paraguay River.
Literature records from Trinidad suggest it is a lowland species, but uncommon. Mohammed et al. (2010) surveyed Trinidad freshwater turtles and updated the island distributions. They found M. gibba isolated in the south-western drainages of the South Oropuche catchment based based on anecdotal evidence from local residents (in the Penal district) who gave vivid descriptions of the characteristic side folding neck.
The species prefers stagnant or slow moving waters, including marshes, ponds, and streams in or near primary rainforests, gallery forests, in closed canopy situations; also in permanent ponds near Mauritia palms; ephemeral forest pools; often associated with mud substrates. Dry season aestivation occurs when temporary ponds dry. It is nocturnal.
The diet includes insects, crustaceans, amphibian larvae, rivulid fish; plant material such as palm seeds and fruits; aquatic algae, florescences and seeds of aquatic macrophytes.
Reproduction information is poorly known. Five clutches with 2−4 eggs; incubation ranges from 178−200 days; hatchlings show a preference for terrestrial rather than aquatic habitats. Captive reproduction data: clutch sizes of 2−8 eggs (x = 3.92), incubation periods of 140−272 days. Captive females grew from hatchlings to 190 mm in eight years, first reproduction at six years; males grew to 187 mm in six years, attempted copulation in fourth year.