Chelidae

The Chelidae are a family of freshwater turtles. They are side-necked 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, and its fossil history goes back to the Cretaceous. 

The family Chelidae has about 60 living species in about twenty genera.Georges, A. et al 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. The family is entirely Gondwanan in origin, with no members found outside Gondwana, either now or as a fossil.Georges A. & Thomson S. 2006. Evolution and zoogeography of Australian freshwater turtles. In: Merrick J.R. et al (eds) Evolution and zoogeography of Australasian vertebrates. Sydney: Australia.

They are called side-necked turtles because they withdraw their heads by moving the head sideways (see Pleurodira). Other turtles, if they withdraw their heads, pull them back in the vertical plane. 

The subfamilies within Chelidae show that most of the South American species and all the Australian species are a clade, that is, evolved from the same ancestor (monophyly.  The far more ancient turtle genus Hydromedusa is their sister taxon.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. et al 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.

Nowadays, all these species live in fresh water, but we know from the fossil record that many of their forebears lived in seas around the world.Gaffney ES; Tong H. & Meylan P.A. 2006. Evolution of the side-necked turtles: the families Bothremydidae, Euraxemydidae, and Araripemydidae. Bulletin of the American Museum of Natural History 300, 1-700.

 References 

Category:Turtles