Platysternon megacephalum, the big-headed turtle, is the sole extant member of the Platysternidae within Cryptodira. A mosaic of morphological features have resulted in conflicting hypotheses of relationship, with some authors suggesting a close relationship to Chelydridae (e.g., Shaffer et al., 1997; Brinkman and Wu, 1999) and other authors suggesting a sister-group relationship to a clade containing Testudinoidea, Trionychia, and Kinosternoidea (e.g., Joyce, 2007).
The most striking feature of Platysternon megacephalum is its extremely large head, which cannot be retracted due to its size (Ernst and Barbour, 1989; Bonin et al., 2006). The dorsal surface of the head is covered by an enlarged scute. The temporal region of the skull shows little emargination, and an enlarged postorbital separates the parietal and squamosals (Ernst and Barbour, 1989). The sheath covering the upper jaw is large and extends almost to the edge of the dorsal scute. The carapace may reach up to 18.5 cm in length (Bonin et al., 2006). The digits are webbed, and each consists of three phalanges. A long tail is present, and the vertebral column contains two biconvex cervical vertebrae. The carapace, head, and limbs are yellowish-brown to olive-colored and may exhibit some mottling (Ernst and Barbour, 1989; Bonin et al., 2006).
Platysternon megacephalum is found in Southeast Asia, including southern and eastern China, Vietnam, Laos, Cambodia, Thailand, and Myanmar (Ernst and Barbour, 1989; Bonin et al., 2006). It inhabits mountain streams with rocky beds. It does not swim well, and moves about by walking on the bottom. It is also able to climb extremely well. It preys on small fish and invertebrates. Females lay 1-2 eggs once per year (Ernst and Barbour, 1989; Bonin et al., 2006). Platysternon megacephalum is listed by IUCN as endangered.
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Platysternon megacephalum page on the Animal Diversity Web (University of Michigan Museum of Zoology)