South America, Indonesia, New Guinea, and Australia
South America, Indonesia, New Guinea, and Australia
Fossils of these turtles are known from the Miocene of Australia and South America. They are most closely related to the other pleurodiran families: the Pelomedusidae of Africa and the Podocnemididae of South America and Africa. Their subfamilial relationships remain poorly understood. Molecular evidence suggests that the Chelidae can be further divided into three subfamilies: the Hydromedusinae, containing only the genus Hydromedusa; the Chelidinae, containing all other South American chelids; and the Chelodininae, containing both the short- and long-neck species of Australia and New Guinea. This taxonomic relationship has yet to be diagnosed in terms of morphological characters. Genetic and morphological data support the revision of the genus Phrynops, which resulted in the erection of two new genera (Bufocephala and Ranacephala) and the resurrection of three previously recognized genera (Mesoclemmys, Batrachemys, and Rhinemys). The status of several morphologically distinct Australian "species" is debated among taxonomists because the species are virtually indistinguishable using genetic analysis.
Sideneck turtles are extremely diverse in size, shape, and coloration. Most species have four claws on the hind feet and five claws on the forelimbs, zero to six neural bones present, the pleural bones almost always meeting at the mid-
line behind the neurals, mesoplastral bones absent, and the pelvis fused to the plastron. Neck elongation is extreme; in some species the length of the neck may exceed that of the carapace. While the carapace tends to be dark and cryptic, there are a few spectacularly colored chelids that may have bright red, pink, orange, and yellow on the plastron and/or soft parts.
With the exception of Batrachemys dahli, all of the South American chelids are found east of the Andes and as far south as northern Argentina. The remaining species are found in tropical to temperate regions of Australia and New Guinea. This curious distribution may be the result of a common ancestor that dispersed across Antarctica before the southern continents separated and drifted apart. No fossils attributable to this family have been found outside the present-day range.
Primarily aquatic, most species inhabit permanent freshwater rivers, streams, lakes, and ponds. Some species are found in seasonal wetlands that are dry for most of the year, while others expand their range into flooded forests during the rainy season. The New Guinea snakeneck turtle (Chelodina sieben-rocki) may frequent estuaries and coastal waters.
Several species estivate by burying themselves in the mud during periods of extreme drought. Steindachner's turtle (Chelodina steindachneri), an aquatic species that resides in the deserts of Western Australia, is well adapted to the extremely high temperatures and desiccating conditions prevalent for most of the year. This species is resistant to evaporative water loss and stores fluids in accessory bladders in order to survive while buried for the year or two until the rains return.
Most species are omnivorous to totally carnivorous; however, the adult Elseya dentata may be completely herbivorous. Many diverse adaptations for specialized feeding are found in this family. The snakeneck turtles of Australia and South America have developed very similar modes of prey capture. Both groups have independently evolved a long neck that is adapted for striking at prey while suspended in the water column. The negative pressure generated by the rapid expansion of the mouth and neck pulls the head toward the prey while simultaneously pulling the prey item into the turtle's open jaws. Matamatas lie in wait on the murky bottom of their aquatic habitat and use a gape-and-suck method similar to that of the snakeneck turtles to capture their prey. Alternatively, some Australian chelids are specialized for feeding on mollusks. As they age, macrocephalic females of the genus Emydura develop enormous skulls that provide the force required for crushing larger prey.
Females are generally larger than males; however, the reverse is true in Pseudemydura and Elusor. Males of the latter genus possess an extremely long tail with a unique bony structure that may play a role in copulation. Breeding has been observed in the early spring for many species, but may be year-round in the Tropics. Nests are typically constructed in spring and early summer; however, some tropical species
nest during the winter as well. Clutch size ranges from one to 28 and two or more clutches may be produced per year. The brittle-shelled eggs may be elongate or spherical, 1.2-2.4 in (3-6 cm) in greatest diameter. Incubation may last more than 200 days in species that require a period of diapause (early developmental arrest) for proper development.
A variety of nest chambers are constructed. The gibba turtle may lay its elongate eggs in shallow nests beneath vegetation, whereas the western swamp turtle uses her forelimbs to dig a chamber, which she then enters to deposit her eggs. Researchers in Australia have discovered that, while completely submerged, the female northern snakeneck (Chelodina rugosa) lays her eggs in the muddy bottom of temporary ponds. Development is arrested until the water subsides during the dry season, but resumes in time for incubation to be completed before the pond fills again. Hatchlings may remain in the nest until rain softens the hardened soil plug with which most species seal the nest chamber. In extreme cases emergence may occur nearly two years after the eggs were laid. All species analyzed thus far exhibit genetic sex determination.
Thirteen species are listed as Threatened on the IUCN Red List: six are Vulnerable, four Endangered, and three Critically Endangered. Eight species are listed as Lower Risk/Near Threatened. The western swamp turtle (Pseude-mydura umbrina), with known populations of fewer than 400 individuals, may be one of the world's most endangered turtles. Similarly, Hoge's sideneck (Ranacephala hogei) is known from just a few locales in Brazil, and is thought to be Criti cally Endangered. The major threat to most species is habitat destruction or degradation. Many of the threatened species are being bred in captivity, offering hope for future repatriations. However, active conservation efforts in nature are lacking for most species.
Significance to humans
Many species are consumed locally.
1. Common snakeneck turtle (Chelodina longicollis); 2. Victoria river snapper (Elseya dentata); 3. Gibba turtle (Mesoclemmys gibba); 4. Western swamp turtle (Pseudemydura umbrina); 5. Matamata (Chelus fimbriatus). (Illustration by Barbara Duperron)
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