Chelidae is one of three living families of the turtle suborder Pleurodira, and are commonly called Austro-South American side-neck turtles.<ref name=EoR>Template:Cite book</ref> The family is distributed in Australia, New Guinea, parts of Indonesia, and throughout most of South America. It is a large family of turtles with a significant fossil history dating back to the Cretaceous. The family is entirely Gondwanan in origin, with no members found outside Gondwana, either in the present day or as a fossil.<ref name=GT>Georges, A. & Thomson, S. (2006). "Evolution and Zoogeography of Australian freshwater turtles". In: Merrick, J. R.; Archer, M.; Hickey, G. & Lee, M. (eds.) Evolution and Zoogeography of Australasian Vertebrates. Sydney: Australia.</ref>
Like all pleurodirous turtles, the chelids withdraw their necks sideways into their shells, differing from cryptodires that fold their necks in the vertical plane. They are all highly aquatic species with webbed feet and the capacity to stay submerged for long periods of time. The snake-necked species (genera Chelus, Chelodina, and Hydromedusa) are largely strike-and-gape hunters or foragers feeding on fish, invertebrates, and gastropods. The short-necked forms are largely herbivorous or molluscivorous, but are also opportunistic, with several species having specialized to eating fruits.
The highly aquatic nature of the group is typified by the presence of cloacal breathing in some species of the genera Elseya and Rheodytes.<ref name=Gor>Gordos, M. A.; Franklin, C. E. & Limpus, C. J. (2004). "Effect of water depth and water velocity upon the surfacing frequency of the bimodally respiring freshwater turtle, Rheodytes leukops". The Journal of Experimental Biology. 207: 3099-3107.</ref> However, some species, such as the eastern long-neck turtle (Chelodina longicollis) from Australia, spend significant periods of time on land and are considered highly terrestrial.
The smaller members of the family include the Macleay River turtle (Emydura macquarii) at around 16 cm,<ref>Cann, J. (2008). Freshwater Turtles: A Wild Australia Guide. Queensland, Australia: Steve Parish Publishing, p. 46.</ref> twist-necked turtle (Platemys platycephala) at 18 cm and the western swamp turtle (Pseudemydura umbrina) at 15 cm, whereas the larger species such as the mata mata (Chelus fimbriata) and the white-throated snapping turtle (Elseya albagula) both exceed 45 cm in shell length.<ref name=TG>Thomson, S.; Georges, A. & Limpus, C. (2006). "A New Species of Freshwater Turtle in the Genus Elseya (Testudines: Chelidae) from Central Coastal Queensland, Australia". Chelonian Conservation and Biology. 5 (1): 74-86.</ref>
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Chelid turtles have unique shell morphology. The carapace often has reduced surface exposure of neural bones, or even none at all.<ref name="tg96">Thomson, S. & Georges, A. (1996). "Neural bones in chelid turtles". Chelonian Conservation and Biology 2: 82-86.</ref> This is due to less requirement for enlarged longissimus dorsi muscles in side-necked turtles.<ref name="st06">Thomson S. (2003). Template:Usurped. World Chelonian Trust.</ref>
The inside of the carapace is often heavily buttressed. This has sometimes been seen as a defense mechanism, that is it increases the strength of the shell against biting force, however Thomson (2003)<ref name=st06 /> demonstrated it is linked to feeding methods and the prevention of internal torsion of the shell. Chelids also lack mesoplastra, which separates them from the Pelomedusidae.
The cervical scute is usually present, though it is absent in some species of Elseya and Myuchelys. Otherwise, the carapace has the usual complement of four costals, five vertebrals and twelve marginals (per side). Internally, the carapace is made of eight pleurals (per side), eleven peripherals (per side), a nuchal at the front and a suprapygal and pygal at the rear of the shell. As noted earlier, neurals, although always present, often exist as subsurface elements above the vertebral column.<ref name=tg96 />
The plastron of chelids does not contain any hinges as can appear in some cryptodire turtles. The scute pattern is a unique feature of Pleurodira and can be used to immediately identify a shell as belonging to this suborder. All cryptodires have 12 plastral scutes, whereas pleurodires have thirteen. The extra scute is called the intergular. The rest of the scutes and the skeletal structure beneath them are the same as all turtles: paired gulars, humerals, pectorals, abdominals, and anals. The skeletal elements consist of a single entoplastron, as well as paired epiplastra, entoplastra, hyoplastra, hypoplastra and xiphiplastra (Pritchard & Trebbau, 1984).<ref name=pt>Pritchard, Peter C. H. & Trebbau, Pedro (1984). The Turtles of Venezuela. Society for the Studies of Amphibians and Reptiles: 403 pp.</ref>
A number of theories of the relationships within the large chelid family have been posited. Using shared derived characters, an early attempt in the 1970s used strict parsimony to determine the three long-necked genera (Chelodina, Chelus, and Hydromedusa) were each other's closest relatives.<ref name=gaf77>Gaffney, E. S. (1977). "The side-necked turtle family Chelidae: a theory of relationships using shared derived characters". American Museum Novitates. 2620: 1-28.</ref> This was accepted for some time, but brought into scrutiny,<ref name=prit84>Pritchard, P. C. H. (1984). "Piscivory in turtles, and evolution of the long-necked Chelidae". in Ferguson, M. W. (ed) The Structure, Development and Evolution of Reptiles. Zoological Society of London, Symposium. 52: 87-110.</ref> because the major differences between the genera showed they all appeared to have evolved independently of each other, hinging on the fact that although they had long necks, how they used them and their structures were different.
A number of additional data sets were developed that used electrophoresis and nuclear and mtDNA analysis; these all agreed on the independent evolution of the three long-necked clades.<ref name=GBS /><ref name=sedd97>Seddon, J.; Georges, A.; Baverstock, P. & McCord, W. (1997). "Phylogenetic relationships of chelid turtles (Pleurodira: Chelidae) based on mitochondrial 12S rRNA gene sequence variation". Molecular Phylogenetics and Evolution. 7: 55-61.</ref> This was culminated in a reanalysis of the morphological data which demonstrated the convergence of the clades on a sweep of distinctive features needed for their piscivorous diets,<ref name=thom03>Thomson S. (2003). "Long necks, flat heads and the evolution of piscivory". World Chelonian Trust</ref> Thomson, 2000.<ref name=thom00>Thomson S. A. (2000). "On the identification of the holotype of Chelodina oblonga (Testudinata: Chelidae) with a discussion of the taxonomic implications". Chelonian Conservation and Biology. 3: 745-749.</ref> The subfamilies within Chelidae show the monophyly of the majority of the South American species and all the Australian species, with the far more ancient Hydromedusa as sister taxon to both these other groups.
The family Chelidae contains about 60 species within around twenty genera:<ref name=GBS>Georges, A.; Birrell, J.; Saint, K. M.; McCord, W. & Donnellan, S. C. (1998). "A phylogeny for side-necked turtles (Chelonia: Pleurodira) based on mitochondrial and nuclear gene sequence variation". Biological Journal of the Linnean Society. 67: 213-246.</ref>
Genus †Bonapartemys Lapparent de Broin & de la Fuente, 2001<ref name="BF1">Broin, F. de & de la Fuente, M. S. (2001). "Oldest world Chelidae (Chelonii, Pleurodira), from the Cretaceous Patagonia, Argentina". Palaeontology 333: 463-470.</ref> Bajo Barreal Formation, Argentina, Late Cretaceous (Cenomanian-Turonian)
Genus †Palaeophrynops Lapparent de Broin & de la Fuente 2001<ref name="BF1" /> Los Alamitos Formation, Argentina, Late Cretaceous (late Campanian -lower Maastrichtian)
Genus †Parahydraspis Wieland 1923<ref name="wei23">Wieland, G. R. (1923). "A new Parana Pleurodiran". American Journal of Science. 5 (25): 1-15.</ref> Ituzaingó Formation, Argentina, Miocene (Huayquerian)
Family Chelidae Gray, 1831<ref name="gray31">Gray, J. E. (1831). Synopsis Reptilium Or Short Descriptions of the Species of Reptiles. Part 1. Cataphracta, Tortoises, Crocodiles, and Enaliosaurians. London. 85 pp.</ref> (Crown group)
Subfamily Chelodininae Baur 1893<ref name="baur1893">Baur, Georg (1893). "Notes on the classification of the Cryptodira". American Naturalist. 27 :672–674.</ref>
Genus Chelodina Fitzinger 1826 – Australian snake-necked turtles
Genus Elseya Gray 1867 – Australian snapping turtles<ref name="JEG67">Gray, J. E. (1867). "Description of a new Australian tortoise (Elseya latisternum)". Annals and Magazine of Natural History. (3) 20: 43-45.</ref>
Genus Emydura Bonaparte 1836 – Australian short-necked turtles
Genus Elusor, Cann & Legler, 1994<ref name="Can">Cann, J. & Legler, J. M. (1994). "The Mary River Tortoise: a new genus and species of short-necked chelid from Queensland, Australia (Testudines; Pleurodira)". Chelonian Conservation and Biology. 1 (2): 81-96.</ref> – Mary River turtle
Genus Myuchelys Thomson & Georges 2009<ref name="Th1">Thomson, S. & Georges, A. (2009). "Myuchelys gen. nov. — a new genus for Elseya latisternum and related forms of Australian freshwater turtle (Testudines: Pleurodira: Chelidae)". Zootaxa. 2053: 32–42.</ref> – Australian saw-shelled turtles
Genus Rheodytes Legler & Cann, 1980<ref name="Leg">Legler, J. M. & Cann, J. (1980). "A new species of chelid turtle from Queensland, Australia". Contributions to Science, Natural History Museum of Los Angeles County. 324: 1-18.</ref> – Fitzroy River turtles
Subfamily Chelinae Gray, 1825<ref name="gray1825">Gray, John Edward (1825). "A synopsis of the genera of reptiles and amphibia, with a description of some new species". Annals of Philosophy. (2) 10: 193–217.</ref>
Genus Acanthochelys Gray, 1873<ref name="GR1">Gray, J. E. (1873). "Observations on chelonians, with descriptions of new genera and species". Annals and Magazine of Natural History. (4) 11: 289-308.</ref> – South American side-necked swamp turtles
Subfamily Hydromedusinae Baur, 1893<ref name="baur1893a">Baur, Georg. (1893). "Notes on the classification and taxonomy of the Testudinata". Proceedings of the American Philosophical Society. 31: 210–225.</ref>
Genus Hydromedusa Wagler 1830 – South American snake-necked turtles
Subfamily Pseudemydurinae Zhang et al., 2017<ref name="zhang2017">Zhang, X.; Unmack, P. J.; Kuchling, G.; Wang, Y. & Georges, A. (October 2017). "Resolution of the enigmatic phylogenetic relationship of the critically endangered Western Swamp Tortoise Pseudemydura umbrina (Pleurodira: Chelidae) using a complete mitochondrial genome". Molecular Phylogenetics and Evolution. 115: 58-61.</ref>
Genus Pseudemydura Siebenrock 1901<ref name="SE1">Seibenrock, F. (1901). "Beschreibung einer neuen schildkrotengattung aus der familie Chelydidae aus Australien: Pseudemydura". Anzeiger der Akademie der Wissenschaften in Wien. 38: 248-251.</ref> – western swamp tortoise
Key to Australasian Chelidae
1 Forelimbs each with five claws; gular scutes separated by the intergular; intergular scute in broad contact with the anterior margin of the plastron 2
– Forelimbs each with four claws; gular scutes in contact; intergular scute not in broad contact with the anterior margin of the plastron
Chelodina
2 Intergular scute not in contact with the pectoral scutes 3
– Intergular scute contacts and partly separates the pectoral scutes Pseudemydura
3 Suture between the second and third costal scutes contacting the seventh marginal scute; suture between the third and fourth costal scutes contacting the ninth marginal scute 4
– Suture between the second and third costal scutes contacting the sixth marginal scute; suture between the third and fourth costal scutes contacting the eighth marginal scute Rheodytes
4 Surface of the temporal region covered with distinct regular scales or low tubercles; dorsal surface of the head with a prominent head shield which may be entire or fragmented; cervical scute present or absent 5
– Skin of the temporal region smooth, sometimes broken into regular scales of low relief; dorsal surface of head without a prominent head shield; cervical scute present (except as a rare variant) Emydura
5 Precloacal tail length greater than postcloacal length only in adult males; tail round in cross section; cloacal orifice round; tail always shorter than half of carapace length 6
– Tail distinctive and large; precloacal length greater than postcloacal length at all ages in both sexes; tail laterally compressed; cloacal orifice a longitudinal slit; tail up to 53% of carapace length in adult males Elusor
6 Prominent alveolar ridge on the triturating surfaces of the mouth; cervical scute absent (except as a rare variant); no prominent process of the head shield extending down the parietal ridge toward the tympanum Elseya
–Alveolar ridge absent; cervical scute absent in Australian species (except as a rare variant), present in New Guinea species (except as a rare variant); posterior process of the head shield extends laterally down the parietal ridge toward the tympanum Myuchelys
The species in the family Chelidae are distributed across Australia, New Guinea, and South America. Over time they were required to disperse out of concerns of food shortage, habitat destruction, and weather disruptions. In all of these warmer climates, they can be found in turbid waters covered by muck and the root-mats of underwater vegetation. The waters that they are found in often lack large species of fish that would put them at risk of predation.Template:Citation needed
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