Chelidae

The Chelidae are one of three living families of the turtle suborder Pleurodira and are commonly called the Austro-South American side neck turtles (Obst, 1998). 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 going back to the Cretaceous. The family is entirely Gondwanan in origin, with no members found outside of Gondwana, either in the present day or as a fossil (Georges and Thomson, 2006).

Description

Like all pleurodirous turtles, the chelids withdraw their necks sideways into the shell, 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, several species have 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 (Gordos et al., 2004). 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 (Cann, 2008), 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 (Thomson et al., 2006).

Shell Morphology

Members of Chelidae have unique shell morphology. The carapace often has reduced surface exposure of neural bones, or even none at all (Thomson & Georges, 1996). This is due to less requirement for enlarged longissimus dorsi muscles in side necked turtles (Thomson, 2003). 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) demonstrated it is linked to feeding methods and the prevention of internal torsion of the shell. Chelid's 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 4 costals, 5 vertebrals and 12 marginals (per side). Internally the carapace is made of 8 pleurals (per side), 11 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 (Thomson and Georges, 2006). The plastron of chelids does not contain any hinges as can appear in some cryptodire turtles. The scute pattern is a unique feature of the Pleurodira and can be used to immediately identify a shell as belonging to this suborder. All cryptodire's have 12 plastral scutes whereas pleurodires have 13. 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 entoplaston, as well as paired epiplastra, entoplastra, hyoplastra, hypoplastra and xiphiplastra (Pritchard & Trebbau, 1984).

 

Classification

There have been a number of theories of the relationships within the large chelid family. Using shared derived characters an early attempt in the 1970s used strict [[parsimony]] to determine that the 3 long-necked genera (Chelodina, Chelus and Hydromedusa) were each other's closest relatives (Gaffney, 1977). This was accepted for sometime but brought into scrutiny by Pritchard (1984) who discussed the major differences between the 3 genera showing that 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 the structural differences were different.

A number of additional datasets were developed that used electrophoresis and nuclear and mtDNA analysis these all agreed on the independent evolution of the 3 long-necked clades (Georges et al. 1998, Seddon et al., 1997). This was culminated in a re-analysis of the morphological data which demonstrated the convergence of the 3 clades on a sweep of distinctive features needed for their piscivorous diet (Thomson, 2003, Thomson, 2000. The 3 sub-families 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 approximately 60 species within around 20 genera.
The following is based on Georges et al. 1998.

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Gaffney, E.S. 1977. The side-necked turtle family Chelidae: a theory of relationships using shared derived characters. American Museum Novitates 2620:1-28.
Georges, A.; J. Birrell, K. M. Saint, W. McCord und S. C. Donnellan (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
Gray, J. E. 1831. Synopsis Reptilium or short descriptions of the species of reptiles. Part 1. Cataphracta, tortoises, crocodiles, and enaliosaurians. London. 85 pp.
Gray, J.E. (1867) Description of a new Australian tortoise (''Elseya latisternum''). Ann. Mag. Nat. Hist. (3) 20: 43-45.
Gray, J.E. 1873. Observations on chelonians, with descriptions of new genera and species. Annals and Magazine of Natural History (4)11:289-308.
Broin, F. de. and de la Fuente, M.S. 2001. Oldest world Chelidae (Chelonii, Pleurodira), from the Cretaceous Patagonia, Argentina. Palaeontology 333:463-470.
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.
Megirian, D. and 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.
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.
Seddon, J., Georges, A., Baverstock, P. and 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.
Seibenrock, F. 1901. Beschreibung einer neuen schildkrotengattung aus der familie Chelydidae aus Australien: ''Pseudemydura''. Anz. Akad. Wiss. Wien 38:248-251.
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 of ''Chelodina oblonga'' (Testudinata: Chelidae) with a discussion of the taxonomic implications. Chelonian Conservation and Biology 3:745-749.
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. Wieland, G. R. 1923. A new Parana Pleurodiran. American Journal of Science. 5(25):1-15.