Podocnemididae is afamily ofpleurodire (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 toSouth America: the genusPeltocephalus, with two species, only one of which is extant (P. dumerilianus, the Big-headed Amazon River turtle); and the genusPodocnemis, with six living species of South American side-necked river turtles and four extinct. There is also one genus native toMadagascar:Erymnochelys, the Madagascan big-headed turtle, whose single speciesE. 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.[2][3] Podocnemididae turtles live in aquatic environments and have shells streamlined to aid in swimming.[4]
The family notably contains the largest freshwater turtle to have ever lived,Stupendemys, which lived in South America during theMiocene epoch.
Podocnemis skulls, ventral and side view, with pterygoid and basisphenoid bones labeled "pt" and "bs" respectively.
According to Ferreira et al. (2015), the family name derives from two Greek words: "podos" (foot) and "cnemis" (leg armor worn by Roman soldiers.)[5]
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).[5]
EpifamilyPodocnemidinura: 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.[6]
SuperfamilyPodocnemidoidea: At a higher level yet, the epifamily Podocnemidinura is grouped with the familyBothremydidae to form the superfamily Podocnemidoidea.[6]
Podocnemidinae: An earlier classification, rejected by Gaffney, treated Podocnemididae as a subfamily (Podocnemidinae) within the closely related familyPelomedusidae.[7])
According toGaffney et al. (2011), the family Podocnemididae can be diagnosed from itscranial traits including "the unique possession of a cavum pterygoidei formed by the basisphenoid,pterygoid, prootic, andquadrate [bones], underlain by the pterygoid and basisphenoid."[8]
The pocnemid family dates to the late Cretaceous; it includes 20 genera and 30 species. Only three genera (and eight species) survive.[8]
The three livinggenera of Podocnemididae (one of which ismonotypic) are:
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 (7.9 ft), the largest of any known turtle and is the largest pleurodire known.[10] WhilePeltocephalus andErymnochelys have often been recovered as more closely related to each other than toPodocnemis in morphological analyses,[11][12][9] genetic studies have foundErymochelys to be more closely related toPodocnemis than toPeltocephalus.[13][14]
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 theBothremydidae, another extinct group of side-necked turtles that are also known to have inhabited marine environments.[15]
^Wise, Taylor B.; Stayton, C. Tristan (2017-03-01). "Side-necked Versus Hidden-necked: A Comparison of Shell Morphology Between Pleurodiran and Cryptodiran Turtles".Herpetologica.73 (1): 18.doi:10.1655/HERPETOLOGICA-D-15-00038.ISSN0018-0831.S2CID90226667.
^Fritz Jürgen Obst (1998). "Pelomedusinae". In H. G. Cogger; R. G. Zweifel (eds.).Encyclopedia of Reptiles and Amphibians. San Diego:Academic Press. pp. 112–113.ISBN0-12-178560-2.
^abcEugene S. Gaffney; Peter A. Meylan; Roger C. Wood; Elwyn Simons; Diogenes De Almeida Campos (2011). "Evolution of the Side-Necked Turtles: The Family Podocnemididae".Bulletin of the American Museum of Natural History.350:1–237.doi:10.1206/350.1.hdl:2246/6110.S2CID83775718.The family Podocnemididae consists of 20 genera and 30 species considered here as valid and diagnosable by cranial characters. Three of these genera and eight species persist into the Recent fauna, barely reflecting the evolutionary diversity and distribution of the group. The family extends from the late Cretaceous to the Recent and occurs in North and South America, Europe, Asia, and Africa.
^De Lapparent, France."The oldest pre-Podocnemidid turtle (Chelonii, Pleurodira), from the Early Cretaceous, Ceara State, Brasil, and its environment".Treballs del Museu de Geologia de Barcelona.9:43–95. RetrievedDecember 4, 2021.In the Podocnemidoidea, Brasilemys n.g. represents the earliest known specimen of the Podocnemidoidae, immediately after the divergence of the Bothremydidae. It is part of the formidable radiation in the Pelomedusoides which occurs during the early Cretaceous when South America separated from Africa.
Media related toPodocnemididae at Wikimedia Commons
Data related toPodocnemididae at Wikispecies