Platysternidae

(vertebrate zoology) The big-headed turtles, a family of Asiatic fresh-water Chelonia with a single species (Platysternon megacephalum), characterized by a large head, hooked mandibles, and a long tail.

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(Platysternidae)

Class: Reptilia

Order: Testudines

Suborder: Cryptodira

Family: Platysternidae

Thumbnail description
A turtle of moderate size with a huge head that cannot be drawn inside its shell, a long scaly tail, and a large plastron

Size
Up to 8 in (20 cm)

Number of genera, species
1 genus; 1 species

Habitat
Mountain streams

Conservation status
Endangered

Distribution
Southeast Asia

Evolution and systematics

Although big-headed turtles (Platysternon megacephalum) were once considered closely related to pond turtles (family Emydidae), most recent studies suggest that they are the sister group to snapping turtles (Chelydridae). Some authorities even consider big-headed turtles to be a subfamily within the Chelydridae. However, differences in skeletons, chromosomes, and DNA sequences argue for separate family recognition. One genus of fossil turtles from the Cretaceous of Southeast Asia is tentatively assigned to this family. No subfamilies are recognized.

The taxonomy of this species is Platysternon megacephalum Gray, 1831, China. Three subspecies are recognized.

Physical characteristics

The most distinctive characteristic of this turtle is its huge head (half as wide as the shell) which cannot be withdrawn into its shell. The top of the skull is completely roofed over in bone and covered with a large epidermal scale. The upper jaw has a strongly hooked beak. The carapace is very low and sometimes bears a single medial keel. The plastron is relatively large, but it is only narrowly connected to the carapace by ligaments. The tail is nearly as long as the shell and is covered by large epidermal scales. Well-developed musk glands are present in front of and behind the bridge.

Distribution

Southern China south to Thailand and southern Myanmar.

Habitat

Known only from small, cool, rocky streams in mountainous areas up to 6,600 ft (2,000 m).

Behavior

Although big-headed turtles have not been well studied in the wild, they appear to be primarily nocturnal and crepuscular (active at twilight), spending the day under rocks or logs. When disturbed, they bite viciously. Their climbing ability is incredible, no doubt an advantage when clambering over rocks in fast-flowing streams. In captivity they have even been known to climb over wire fences and up window curtains to the ceiling. When molested in captivity these turtles are known to produce squeal-like noises. They apparently hibernate in the northern portion of their range, and there is speculation that they might hibernate terrestrially.

Feeding ecology and diet

This turtle is probably a strict carnivore in the wild, although its feeding habits are known only from captive studies. It will eat fish, frogs, and invertebrates. Although it surely feeds primarily on stream bottoms, it may also forage terrestrially along stream banks.

Reproductive biology

Big-headed turtles lay one to four large elongate eggs (1.5–1.7 by 0.9 in [37–44 by 22 mm]) per clutch, although one- or two-egg clutches are most common. Nesting is speculated to occur from May to August; a single captive egg hatched in September. Whether the turtles produce more than one clutch per year is unknown.

Conservation status

This species is listed as Endangered on the IUCN Red List.

Significance to humans

Big-headed turtles are considered to be a delicacy in many parts of their range; the flesh is thought to have the same aphrodisiac properties as rhinoceros horns. These turtles are also exploited for the pet trade.

Resources

Ernst, C. H., and A. F. Laemmerzahl. "Geographic Variation in the Asian Big-Headed Turtle, Platysternon megacephalum (Reptilia: Testudines: Platysternidae)." Proceedings of the Biological Society of Washington 115 (2002): 18–24.

Haiduk, M. W., and J. W. Bickham. "Chromosome Homologies and Evolution of Testudinoid Turtles with Emphasis on the Systematic Placement of Platysternon." Copeia (1982): 60–66.

Schleich, H. H., and U. Gruber. "Eine neue Großkopfschildkröte, Platysternon megacephalum tristernalis nov. ssp., aus Yünnan, China." Spixiana 7, no. 1 (1984): 67–73.

Walsh, T., and M. Russell. "The Natural History and Captive Husbandry of the Big-Headed Turtle (Platysternon megacephalum)." Reptiles 6, no. 2 (1998): 66–75.

Whetstone, K. N. "Platysternon and the Evolution of Chelydrid Turtles." Herpetological Review 8 Suppl. (1997): 20.

[Article by: John B. Iverson, PhD]

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