American sideneck river turtle

Podocnemididae is a family of pleurodire (side-necked) turtles, once widely distributed. Most of its 20 genera and 30 species are now extinct. Seven of its eight surviving species are native to South America: the genus ''Peltocephalus'', with six living species of South American side-necked river turtles. 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.

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:
* Podocnemidae (Baur, 1893) is an alternate but less commonly-used name for the same biological group as family Podocnemididae (Cope, 1868).
* Epifamily Podocnemidinura: The family Podocnemididae has two sister families (Hamadachelys and Brasilemys); the relationship of these three families is sometimes recognized by grouping them as the epifamily Podocnemidinura.
* Superfamily Podocnemidoidea: At a higher level yet, the epifamily Podocnemidinura is grouped with the family Bothremydidae to form the superfamily Podocnemidoidea.

* Podocnemidinae: An earlier classification, rejected by Gaffney, treated Podocnemididae as a subfamily (Podocnemidinae) within the closely related family Pelomedusidae.)

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 ones/nowiki>, 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 (two of them monotypic) are:
* ''Erymnochelys'' – Madagascan big-headed turtle
* ''Peltocephalus'' – South American side-necked river turtles

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 2.4 metres, the largest of any known turtle and is the largest pleurodire known.

Genera:

* Stem group taxa (Podocnemidoidae)
**†'' Brasilemys'' Romualdo Formation, Brazil, Early Cretaceous (Albian)
**†''Amabilis'' São José do Rio Preto Formation, Brazil, Late Cretaceous (Santonian)
**†'' Hamadachelys'' Kem Kem Group, Morocco, Late Cretaceous ( Cenomanian)
**†'' Portezueloemys'' Portezuelo Formation)
**†'' Cambaremys'' Marília Formation, Brazil, Late Cretaceous (Maastrichtian)
**†'' Bauruemys'' Presidente Prudente Formation, Brazil, Late Cretaceous (Campanian)
**†'' Roxochelys'' Adamantina Formation, Brazil, Campanian
**Family †Peiropemydidae
***†''Peiropemys'' Marília Formation, Brazil, Maastrichtian
*** †'' Pricemys'' Marília Formation, Brazil, Maastrichtian
***†'' Lapparentemys'' Santa Lucía Formation, Bolivia, Paleocene
***†'' Yuraramirim'' Adamantina Formation, Brazil, Campanian
** Family Podocnemididae (crown group)
**†'' Caninemys'' Northern South America, Miocene
***Subfamily Podocnemidinae
**** †'' Cerrejonemys'' Cerrejón Formation, Colombia, Paleocene
****''Podocnemis'' South America, Miocene-Recent
***Subfamily Erymnochelyinae
****†'' Gestemys'' Geste Formation, Argentina, Eocene
****†''Stupendemys'' Northern South America, Miocene
****†''Carbonemys'' Cerrejón Formation, Colombia, Paleocene
****''Peltocephalus