Family of turtles
Podocnemididae is a family of pleurodire (side-necked) turtles, once widely distributed. Most of its 41 genera and 57 species are now extinct. Seven of its eight surviving species are native to South America: the genus "Peltocephalus", with two species, only one of which is extant ("P. dumerilianus", the Big-headed Amazon River turtle); and the genus "Podocnemis", with six living species of South American side-necked river turtles and four extinct. There is also one genus native to Madagascar: "Erymnochelys", the Madagascan big-headed turtle, whose single species "E. madagascariensis".
Like other pleurodire turtles, podocs have a "side-necked" defensive posture, turning the head sideways to hide it under the shell. Another characteristic of pleurodires is that the pelvis is fused to the shell which prevents pelvic motion, making it difficult to walk on land. Podocnemididae turtles live in aquatic environments and have shells streamlined to aid in swimming.
The family notably contains the largest freshwater turtle to have ever lived, "Stupendemys", which lived in South America during the Miocene epoch.
Taxonomy and systematics.
According to Ferreira et al. (2015), the family name derives from two Greek words: "podos" (foot) and "cnemis" (leg armor worn by Roman soldiers.)
To clarify some closely related names: 
According to Gaffney et al. (2011), the family Podocnemididae can be diagnosed from its cranial traits including "the unique possession of a cavum pterygoidei formed by the basisphenoid, pterygoid, prootic, and quadrate [bones], underlain by the pterygoid and basisphenoid."
The pocnemid family dates to the late Cretaceous; it includes 20 genera and 30 species. Only three genera (and eight species) survive.
The three living genera of Podocnemididae (one of which is monotypic) are:
Taxonomy.
Fossils show that podocnemidids were once found in Europe, Asia, North America, and Africa. "Stupendemys" lived around 5.5 million years ago in northern South America, and was the largest freshwater turtle with a carapace length of , the largest of any known turtle and is the largest pleurodire known. While "Peltocephalus" and "Erymnochelys" have often been recovered as more closely related to each other than to "Podocnemis" in morphological analyses, genetic studies have found "Erymochelys" to be more closely related to "Podocnemis" than to "Peltocephalus."
All extant podocnemidids inhabit freshwater environments, as did most fossil members, but the extinct tribe Stereogenyini are though to have inhabited marine environments, although they appear to have been restricted to shallow coastal habitats. This is comparable to the Bothremydidae, another extinct group of side-necked turtles that are also known to have inhabited marine environments.
Genera:
Morphology based cladogram after Ferreira et al. 2024
References.
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