The twist-necked turtle (Platemys platycephala), also known as the flat-headed turtle, is distributed widely across northern South America. Twist-necked turtles have extremely flat shells that help them hide from predators under rocks and debris. When threatened, this turtle withdraws by twisting its head into its shell. P. platycephala is the only species of the genus Platemys and occurs in northern and central South America. Platemys platycephala means "flat turtle, flat-head" and accurately describes the structure of the head and shell. This species inhabits shallow creek beds and frequently forages on the floor of the Amazon rainforest for insects, amphibians, and mollusks. Camouflage, head and body shape, and advanced sexual anatomy allow this species of turtle to effectively populate much of South America. Mating occurs during rainy months (March–December) and egg deposit occurs during dry months (January–March). Males have been known to behave aggressively towards females during copulation by squirting water from nostrils and biting. A few genomic studies show mosaicism exists among populations of the twist-necked turtle in Suriname. In other words, diploid and triploid levels exist among individuals at this particular location. No threats have been reported for this species and the International Union for Conservation of Nature does not have a current listing. Lack of human consumption due to the species' small size and its wide range explain why scientists are not concerned about this turtle species.
The twist-necked turtle now represents the only species of Platemys and can be distinguished from close relatives (Acanthochelys) by its doubly flat body plan, both a flattened shell and a flattened head. Also, this turtle displays an unusual dual grooved pattern on the carapace. P. platycephala inhabits a wide range compared to other Acanthochelys turtles. Two subspecies, P. p. platycephala and P. p. melanonota is described based on the color pattern. Camouflage is highly useful for the twist-necked turtle because it inhabits shallow pools and creek beds. The orange and brown head pattern makes it difficult to spot from above because of similarly colored leaf litter along small pools and creek beds. Other distinctions include a hook at the base of the foot for copulation in males and a specialized opening at the base of the plastron for oviposition (egg deposit).
P. platycephala is a medium-sized turtle ranging from 14 to 18 cm in shell length.[3] Females are slightly smaller on average, with shorter tails. The carapace (top portion of the shell) is elliptical and flattened with two raised portions (keels) forming a trough (depression). The carapace is orange to yellow-brown and black in various amounts depending on subspecies.[3] The plastron (lower portion of the shell) is dark brown or black in color while the bridge (side portion of the shell) is yellow with a black bar across. The consistency of these bars also depends on subspecies. Face and neck patterns consist of orange or yellow-brown dorsal stripes and black ventral and lateral stripes. The head is small, triangular, smooth, and undivided. The neck has some conical tubercles- rounded projections and scales that protect against predator attack.P. platycephala is a member of the Pleurodira- a suborder of turtles. These turtles withdraw their heads into their shells by bending their necks sideways instead of straight back like Cryptodira. The snout projects slightly and the irises are brown. Black limbs contain large anterior scales, while the tail is short and black. Intermediate webbing exists on both anterior and posterior limbs because this turtle moves in water and on land.[4]