The Chelidae are one of the two living families of the turtle suborder Pleurodira and are commonly called the Austro-South American Side Neck turtles^[1]. 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 its origin with no members found outside of Gondwana, either in the present day or as a fossil^[2].

Description

Like all Pleurodirous turtles the Chelid's withdraw their necks sideways into the shell, differing from Cryptodire's 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, Macrochelodina, Macrodiremys 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^[3]. However, species such as the Eastern Long-Neck Turtle (Chelodina longicollis) from Australia spend significant periods of time on land and is considered highly terrestrial.

The smaller members of the family include the Macleay River Turtle (Emydura macquarii) at around 12 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^[4].

Shell Morphology

Members of the Chelidae have unique shell morphology. The carapace often has reduced surface exposure of neural bones, or even none at all[5]. This is due to less requirement for enlarged longissimus dorsi muscles in side necked turtles[6]. 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[6]) 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 compliment 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. 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. Comprising of paired gular's, humeral's, pectorals's, abdominal's and anal's for the scutes. The skeetal elements consist of a single entoplaston, as well as paired epiplastra, entoplastra, hyoplastra, hypoplastra and xiphiplastra.

Scute and Skeletal elements of the Chelid Carapace

Scute and Skeletal elements of the Chelid Plastron

Classification

Phrynops geoffroanus

The family Chelidae contains approximately 60 species within around 20 genera. Following based on Georges et al. 1998^[7].

Suborder Pleurodira

• Family Pelomedusidae
• Family Chelidae Gray, 1825^[8]
  • Subfamily Chelodininae
    • Genus Chelodina Fitzinger 1826, Australian Snake-necked Turtles
    • Genus Macrochelodina Wells & Wellington 1985, Thick-necked Snakeneck Turtles
    • Genus Macrodiremys McCord & Joseph-Uoni 2007 Oblong Turtle
    • Genus Elseya Gray 1867, Australian Snapping Turtles
    • Genus Emydura Bonaparte 1836, Australian Short-necked Turtles
    • Genus Elusor, Cann & Legler, 1994^[9] Mary River Turtle
    • Genus Myuchelys Thomson & Georges 2009^[10] Australian Saw Shelled Turtles
    • Genus Pseudemydura Siebenrock 1901^[11] Western Swamp Tortoise
    • Genus Rheodytes Legler and Cann 1980,^[12] Fitzroy River Turtles
  • Subfamily Chelidinae
    • Genus Chelus Duméril 1806, Matamata turtles
    • Genus Acanthochelys Gray, 1873^[13] , South American Side-necked Swamp Turtles
    • Genus Mesoclemmys, Gibba Turtle
    • Genus Phrynops, Toad-headed Turtles
    • Genus Platemys Wagner 1830, Twisted-necked Turtles
    • Genus Rhinemys, Red-headed Sideneck Turtle
    • Genus †Bonapartemys Lapparent de Broin and de la Fuente 2001^[14]
    • Genus †Lomalatachelys Lapparent de Broin and de la Fuente 2001^[14]
    • Genus †Prochelidella Lapparent de Broin and de la Fuente 2001^[14]
    • Genus †Palaeophrynops Lapparent de Broin and de la Fuente 2001^[14]
    • Genus †Parahydraspis Wieland 1923
    • Genus †Linderochelys de la Fuente et al. 2007
  • Subfamily Hydromedusinae
    • Genus Hydromedusa Wagler 1830, South American Snake-necked Turtles
    • Genus †Yaminuechelys de la Fuente et al. 2001

References