chelid

Chelidae is one of three living families of the turtle suborder Pleurodira, and are commonly called Austro-South American side-neck turtles. 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.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.

Description

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''.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. 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, 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, 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.

Chelids exhibit XX/XY genetic sex determination, in contrast to most other turtles, which have temperature-dependent sex determination.

Shell morphology

Chelid turtles have unique shell morphology. The carapace often has reduced surface exposure of neural bones, or even none at all.Thomson, S. & Georges, A. (1996). "Neural bones in chelid turtles". ''Chelonian Conservation and Biology'' 2: 82-86. This is due to less requirement for enlarged longissimus dorsi muscles in side-necked turtles.Thomson S. (2003). . World Chelonian Trust.

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. 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.

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).Pritchard, Peter C. H. & Trebbau, Pedro (1984). '' The Turtles of Venezuela''. Society for the Studies of Amphibians and Reptiles: 403 pp.

Evolutionary history

The oldest records of Pan-Chelidae (the clade 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 by '' Prochelidella cerrobarcinae'' from the Cerro Barcino Formation of Argentina, which dates from 118 to 110 million years ago, and indeterminate remains from the Griman Creek Formation, of New South Wales, Australia, dating to around 100 million years ago.

Classification

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.Gaffney, E. S. (1977). "The side-necked turtle family Chelidae: a theory of relationships using shared derived characters". ''American Museum Novitates''. 2620: 1-28. This was accepted for some time, but brought into scrutiny,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. 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.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. 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,Thomson S. (2003). "Long necks, flat heads and the evolution of piscivory". World Chelonian Trust Thomson, 2000.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. 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: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.

Taxonomy after TTWG 2021
*Stem-group taxa
**Genus †'' Bonapartemys'' Lapparent de Broin & de la Fuente, 2001Broin, F. de & de la Fuente, M. S. (2001). "Oldest world Chelidae (Chelonii, Pleurodira), from the Cretaceous Patagonia, Argentina". ''Palaeontology'' 333: 463-470. Bajo Barreal Formation, Argentina, Late Cretaceous (Cenomanian-Turonian)
**Genus †'' Calvarichelys'' Oriozabala 2025 La Colonia Formation, Argentina, Late Cretaceous (Campanian-Maastrichtian)
**Genus †'' Linderochelys'' de la Fuente et al., 2007 Río Neuquén Subgroup, Argentina, Late Cretaceous (Turonian- Coniacian)
**Genus †'' Lomalatachelys'' Lapparent de Broin & de la Fuente 2001 Bajo de la Carpa Formation, Argentina, Late Cretaceous (Santonian)
**Genus †'' Palaeophrynops'' Lapparent de Broin & de la Fuente 2001 Los Alamitos Formation, Argentina, Late Cretaceous (late Campanian -lower Maastrichtian)
**Genus †'' Prochelidella'' Lapparent de Broin & de la Fuente 2001 Cerro Barcino Formation, Argentina, Early Cretaceous (Aptian-Albian) Candeleros Formation, Argentina, Cenomanian Bajo Barreal Formation, Argentina, Cenomanian-Turonian Portezuelo Formation, late Turonian – early Coniacian
**Genus †'' Salamanchelys'' Bona, 2006 Salamanca Formation, Argentina, Paleocene (Danian)
**Genus †'' Parahydraspis'' Wieland 1923Wieland, G. R. (1923). "A new Parana Pleurodiran". ''American Journal of Science''. 5 (25): 1-15. Ituzaingó Formation, Argentina, Miocene (Huayquerian)
** Family Chelidae Gray, 1831Gray, J. E. (1831). ''Synopsis Reptilium Or Short Descriptions of the Species of Reptiles. Part 1. Cataphracta, Tortoises, Crocodiles, and Enaliosaurians''. London. 85 pp. (Crown group)
*** Subfamily Chelodininae Baur 1893Baur, Georg (1893). "Notes on the classification of the Cryptodira". ''American Naturalist''. 27 :672–674.
**** Genus †'' Birlimarr'' Megirian & Murray 1999Megirian, 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. Camfield Beds, Northern Territory, Australia, Middle Miocene
**** Genus ''Chelodina'' Fitzinger 1826 – Australian snake-necked turtles
**** Genus '' Elseya'' Gray 1867 – Australian snapping turtlesGray, J. E. (1867). "Description of a new Australian tortoise (''Elseya latisternum'')". ''Annals and Magazine of Natural History''. (3) 20: 43-45.
**** Genus '' Emydura'' Bonaparte 1836 – Australian short-necked turtles
**** Genus ''Elusor'', Cann & Legler, 1994Cann, 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. – Mary River turtle
**** Genus '' Myuchelys'' Thomson & Georges 2009Thomson, 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. – Australian saw-shelled turtles
**** Genus ''Rheodytes'' Legler & Cann, 1980Legler, 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. – Fitzroy River turtles
*** Subfamily Chelinae Gray, 1825Gray, 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.
**** Genus '' Chelus'' Duméril 1806 – matamata turtles
**** Genus ''Acanthochelys'' Gray, 1873Gray, J. E. (1873). "Observations on chelonians, with descriptions of new genera and species". ''Annals and Magazine of Natural History''. (4) 11: 289-308. – South American side-necked swamp turtles
**** Genus ''Mesoclemmys'' – including gibba turtle
**** Genus '' Phrynops'' – toad-headed turtles
**** Genus ''Platemys'' Wagner 1830 – twist-necked turtles
**** Genus ''Ranacephala'' McCord, Joseph-Ouni & Lamar 2001 – Hoge's side-necked turtle
**** Genus '' Rhinemys'' – red-headed side-necked turtle
*** Subfamily Hydromedusinae Baur, 1893Baur, Georg. (1893). "Notes on the classification and taxonomy of the Testudinata". ''Proceedings of the American Philosophical Society''. 31: 210–225.
**** Genus '' Hydromedusa'' Wagler 1830 – South American snake-necked turtles
**** Genus †'' Yaminuechelys'' de la Fuente et al. 2001 Anacleto Formation, Argentina, Santonian-Campanian La Colonia Formation, Allen Formation, Los Alamitos Formation, Argentina, Campanian-Maastrichtian, Salamanca Formation, Roca Formation, Argentina, Paleocene (Danian)
*** Subfamily Pseudemydurinae Zhang et al., 2017Zhang, 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.
**** Genus '' Pseudemydura'' Siebenrock 1901Seibenrock, F. (1901). "Beschreibung einer neuen schildkrotengattung aus der familie Chelydidae aus Australien: ''Pseudemydura''". ''Anzeiger der Akademie der Wissenschaften in Wien''. 38: 248-251. – western swamp tortoise

Phylogeny

Relationships of the living forms based on Georges et al., 2014.

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.

References

External links and further reading

Chelidae

Gondwanan turtle site

{{Authority control

Taxa named by John Edward Gray
Turtle families
Extant Albian first appearances