Chelidae is one of three living families of the turtle suborderPleurodira, and are commonly calledAustro-South American side-neck turtles.[2] The family is distributed inAustralia,New Guinea, parts ofIndonesia, 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 entirelyGondwanan in origin, with no members found outside Gondwana, either in the present day or as a fossil.[3]
Like all pleurodirous turtles, the chelids withdraw their necks sideways into their shells, differing fromcryptodires 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 (generaChelus,Chelodina, andHydromedusa) 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 ofcloacal breathing in some species of the generaElseya andRheodytes.[4] However, some species, such as the eastern long-neck turtle (Chelodina longicollis) from Australia spend significant periods of time on land and are considered highlyterrestrial.
The smaller members of the family include the Macleay River turtle (Emydura macquarii) at around 16 cm,[5] twist-necked turtle (Platemys platycephala) at 18 cm and the western swamp turtle (Pseudemydura umbrina) at 15 cm, whereas the larger species such as themata mata (Chelus fimbriata) and the white-throated snapping turtle (Elseya albagula) both exceed 45 cm in shell length.[6]
Scute and skeletal elements of the chelid carapace (top) and plastron (bottom)
Chelid turtles have unique shell morphology. Thecarapace often has reduced surface exposure of neural bones, or even none at all.[8] This is due to less requirement for enlargedlongissimus dorsi muscles in side-necked turtles.[9]
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)[9] demonstrated it is linked to feeding methods and the prevention of internal torsion of the shell. Chelids also lack mesoplastra, which separates them from thePelomedusidae.
The cervical scute is usually present, though it is absent in some species ofElseya andMyuchelys. 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.[8]
The plastron of chelids does not contain any hinges as can appear in somecryptodire turtles. The scute pattern is a unique feature ofPleurodira 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).[10]
The oldest records of Pan-Chelidae (theclade containing Chelidae and all other species more closely related to Chelidae than other pleurodires) first appear in the mid Cretaceous in South America and Australia, represented byProchelidella cerrobarcinae from theCerro Barcino Formation of Argentina, which dates from 118 to 110 million years ago,[11][12] and indeterminate remains from theGriman Creek Formation, of New South Wales, Australia, dating to around 100 million years ago.[13][14]
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 strictparsimony to determine the three long-necked genera (Chelodina,Chelus, andHydromedusa) were each other's closest relatives.[15] This was accepted for some time, but brought into scrutiny,[16] 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.[17][18] 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,[19] Thomson, 2000.[20] The subfamilies within Chelidae show the monophyly of the majority of the South American species and all the Australian species, with the far more ancientHydromedusa as sister taxon to both these other groups.
The family Chelidae contains about 60 species within around twenty genera:[17]
1 Forelimbs each with five claws; gular scutes separated by the intergular; intergular scute in broad contact with the anterior margin of the plastron2 – Forelimbs each with four claws; gular scutes in contact; intergular scute not in broad contact with the anterior margin of the plastronChelodina
2 Intergular scute not in contact with the pectoral scutes3 – Intergular scute contacts and partly separates the pectoral scutesPseudemydura
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 scute4 – 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 scuteRheodytes
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 absent5 – 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 length6 – 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 malesElusor
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 tympanumElseya –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 tympanumMyuchelys
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.[citation needed]
^abGray, J. E. (1831).Synopsis Reptilium Or Short Descriptions of the Species of Reptiles. Part 1. Cataphracta, Tortoises, Crocodiles, and Enaliosaurians. London. 85 pp.
^Obst, Fritz Jurgen (1998). Cogger, H. G.; Zweifel, R. G. (eds.).Encyclopedia of Reptiles and Amphibians. San Diego: Academic Press. pp. 111–112.ISBN0-12-178560-2.
^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.
^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.
^Cann, J. (2008).Freshwater Turtles: A Wild Australia Guide. Queensland, Australia: Steve Parish Publishing, p. 46.
^Thomson, S.; Georges, A. & Limpus, C. (2006). "A New Species of Freshwater Turtle in the GenusElseya (Testudines: Chelidae) from Central Coastal Queensland, Australia".Chelonian Conservation and Biology.5 (1): 74-86.
^Gaffney, E. S. (1977). "The side-necked turtle family Chelidae: a theory of relationships using shared derived characters".American Museum Novitates. 2620: 1-28.
^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.
^abGeorges, 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.
^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.
^Thomson S. (2003). "Long necks, flat heads and the evolution of piscivory". World Chelonian Trust
^Thomson S. A. (2000). "On the identification of the holotype ofChelodina oblonga (Testudinata: Chelidae) with a discussion of the taxonomic implications".Chelonian Conservation and Biology. 3: 745-749.
^abcdBroin, F. de & de la Fuente, M. S. (2001). "Oldest world Chelidae (Chelonii, Pleurodira), from the Cretaceous Patagonia, Argentina".Palaeontology 333: 463-470.
^Wieland, G. R. (1923). "A new Parana Pleurodiran".American Journal of Science.5 (25): 1-15.
^Baur, Georg (1893). "Notes on the classification of the Cryptodira".American Naturalist. 27 :672–674.
^Megirian, D. & Murray, P. (1999). "Chelid turtles (Pleurodira, Chelidae) from the Miocene Camfield Beds, Northern Territory of Australia, with a description of a new genus and species".The Beagle (Records of the Museums and Art Galleries of the Northern Territory). 15: 75–130.
^Gray, J. E. (1867). "Description of a new Australian tortoise (Elseya latisternum)".Annals and Magazine of Natural History. (3)20: 43-45.
^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.
^Thomson, S. & Georges, A. (2009). "Myuchelys gen. nov. — a new genus forElseya latisternum and related forms of Australian freshwater turtle (Testudines: Pleurodira: Chelidae)".Zootaxa. 2053: 32–42.
^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.
^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.
^Gray, J. E. (1873). "Observations on chelonians, with descriptions of new genera and species".Annals and Magazine of Natural History. (4) 11: 289-308.
^Baur, Georg. (1893). "Notes on the classification and taxonomy of the Testudinata".Proceedings of the American Philosophical Society. 31: 210–225.
^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 TortoisePseudemydura umbrina (Pleurodira: Chelidae) using a complete mitochondrial genome".Molecular Phylogenetics and Evolution.115: 58-61.
^Seibenrock, F. (1901). "Beschreibung einer neuen schildkrotengattung aus der familie Chelydidae aus Australien:Pseudemydura".Anzeiger der Akademie der Wissenschaften in Wien. 38: 248-251.
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