Platysternon megacephalum has a head so large that it cannot be withdrawn into the shell for protection; it is about half as wide as the carapacial width and completely covered dorsally by an enlarged tough scute. Osteologically, the skull shows a unique combination of features. The temporal region is almost completely roofed over, and the dorsal surface is composed of an enlarged postorbital bone which separates the parietal and squamosal bones. The jugal is prevented from entering the orbit by the meeting of the postorbital and maxilla anterior to it. The frontal bone is also missing from the orbital rim. The jugal does touch both the pterygoid and palatine bones. The quadratojugal and maxilla meet; and the premaxillae medially form a strong hooked beak. Triturating surfaces of the jaw contain at least one well-developed ridge, and the jaw sheaths are large and well-developed. In fact, the horny upper jaw sheath is so extensive that it almost extends to the large dorsal scute so that only a narrow band of skin, extending from the orbit to the nares, separates these. A small splenial bone is present, and the quadrate closes behind the stapes. Its carapace (to 20.1 cm) is flat with a slight medial keel (better developed in juveniles), and is indented anteriorly to the small cervical scute. Posteriorly, the carapace contains a slight notch, and the marginals are slightly serrated. Dorsal rib heads are well-developed. In color, the carapace ranges from yellow brown to olive and may contain some dark radiations which fan out from posterior to anterior on each scute. Growth annuli may also be present on each scute. The hingeless plastron is well-developed, but connected to the carapace only by ligaments at the narrow bridge. It is squared off anteriorly and widely notched posteriorly. Plastron and bridge are yellow and there may be a central pattern of faded brown stipples, or dark-brown blotches, extending along the seams. The plastral formula is: an > hum > fem > pect > abd >< gul. In the pelvic girdle, there is ventral contact between the pubes and ischia from each side. Anteriorly, the two thyroid fenestrae are separated only by a ligament. The head is yellow brown to olive, and dorsally may have some dark-brown or red to orange longitudinal striations or spots. Laterally the head may be darker in color and mottled with yellow or red spots. The chin is yellow to brown and may show either dark or light mottling. The neck is gray brown dorsally and yellowish laterally and ventrally. Its dorsal and lateral surfaces are covered with small scales, and there are large scales ventromedially. The toes are webbed and each digit contains three phalanges. The femoral trochanteric fossa is slightly reduced. Forelimbs are light to dark brown and covered with large scales; hindlimbs are brown with a row of large scales along the inner and outer borders and on the heel. The tail is almost as long as the carapace and is covered with large scales.
Haiduk and Bickham (1982) found the diploid number to be 54 (16 macrochromosomes with median or submedian centromeres, 10 macrochromosomes with terminal or subterminal centromeres, and 28 microchromosomes).
Males have concave plastra and the anal vent situated beyond the carapacial rim.
The big-headed turtle ranges from southern China (including Hainan Island) southwestward through northern Vietnam, Laos, Kampuchea, and northern Thailand to southern Myanmar.
Five subspecies have been described. Platysternon megacephalum megacephalum Gray, 1831c, the Chinese big-headed turtle, from southern mainland China, has an unpatterned yellow plastron, a slightly keeled carapace with poorly developed growth annuli, and a slightly serrated posterior rim, the marginals above the bridge flared, a well-developed cephalic shield that often covers the rear of the orbit, yellow mottling on the jaws, and a pattern of narrow radiating lines on top of the head. P. m. peguense Gray, 1870b, the Burma big-headed turtle, is found from western Vietnam west to southern Myanmar (Burma), and on Hainan Island, China (De Bruin and Artner, in press). Juveniles have a dark seam-following plastral pattern, the carapacial keel pronounced and sometimes indications of lateral keels, well-developed growth annuli, the posterior carapacial rim serrated, unpatterned jaws, a strongly hooked upper jaw, and a black-bordered postorbital stripe; adults resemble the nominate subspecies (De Bruin and Artner, in press). The Thailand big-headed turtle P. m. vogeli Wermuth, 1969 occurs in northwestern Thailand. It is similar to P. m. peguense in possessing a dark plastral figure, but differs in having a short, narrow, less hooked upper jaw, and a smooth, unserrated carapace. P. m. tristernalis Schleich and Gruber, 1984, the Yunnan big-headed turtle, is from Yunnan Province, China, and similar to P. m. megacephalum except it has three additional small scales at the medial junction of the gular and humeral scutes. P. m. shiui Ernst and McCord, 1987, the Vietnam big-headed turtle, is found in northern Vietnam. Its head, shell, limbs, sockets, and ventral surface of the tail are heavily speckled with yellow, orange, or pink spots; the carapacial surface is smooth and the posterior rim unserrated; the cephalic shield is moderately developed, not entering the orbit; and the upper jaw is strongly hooked. De Bruin and Artner (in press) also found big headed turtles strongly resembling P. m. shiui on Hainan Island.
This turtle prefers rocky mountain brooks, usually less than a meter wide and 10 cm deep, and often flowing at a mere trickle (Peter Paul van Dijk, pers comm.).
One to 6 eggs comprise a clutch (Budde, 1991), although 2-3 are more normal. The eggs are white and ellipsoidal to elongate-tapered (33-37 x 22 mm); hatchlings have 38-40 mm carapaces (Weissinger, 1987). The young are more brightly colored with a more serrated posterior carapacial rim, a more pronounced vertebral keel, and a tail longer than the carapace.
Platysternon seems to be nocturnal, spending the daylight hours sheltered in rock crevices, generally at the stream edge or behind waterfalls (Peter Paul van Dijk, pers. comm.). It apparently seldom basks in the wild. When disturbed, P. megacephalum bites viciously and retains its grip for some time; the strongly hooked jaws can produce serious bites. However, this savagery is apparently strictly defensive since it is seldom aggressive toward other turtles confined with it.
Big-headed turtles feed on a variety of meats, fishes, and invertebrates in captivity, and are probably carnivorous in nature. At night they probe about the stream bottom for small animals, and may even leave the water to search along the bank and among low shrubs for food. They are accomplished climbers and in captivity have been known to climb out of aquaria and over wire fences.
IUCN Red List Status (1996)