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.
Literature
Bonin, F., Devaux, B., and A. Dupré. 2006. Turtles of the World. Translated by P.C.H. Pritchard. Johns Hopkins University Press, Baltimore MD.
Brinkman, D.B. and X.-C. Wu. 1999. The skull of Ordosemys, an Early Cretaceous turtle from Inner Mongolia, People’s Republic of China, and the interrelationships of Eucryptodira (Chelonia, Cryptodira). Paludicola 2:134-147.
Druzisky, K.A. and E.L. Brainerd. 2001. Buccal oscillation and lung ventilation in a semi-aquatic turtle, Platysternon megacephalum. Zoology – Analysis of Complex Systems 104:143-152.
Ernst, C.H. and R.W. Barbour. 1989. Turtles of the World. Smithsonian Institution Press, Washington DC.
Ernst, C.H. and A.F. Laemmerzahl. 2002. Geographic variation in the Asian big-headed turtle, Platysternon megacephalum (Reptilia : Testudines : Platysternidae). Proceedings of the Biological Society of Washington 115:18-24.
Ferri, V. 2002. Turtles and Tortoises. Firefly Books, Willowdale, ON.
Gaffney, E.S. 1972. An illustrated glossary of turtle skull nomenclature. American Museum Novitates 2486:1-33.
Gaffney, E.S. 1979. Comparative cranial morphology of recent and fossil turtles. Bulletin of the American Museum of Natural History 164:1-376.
Gaffney, E.S. and P.A. Meylan. 1988. A phylogeny of turtles. In: Benton, M.J., editor. The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Clarendon Press, Oxford, pp. 157-219.
Haiduk, M.W. and J.W. Bickham. 1982. Chromosomal homologies and evolution of testudinoid turtles with emphasis on the systematic placement of Platysternon. Copeia 1982:60-66.
IUCN 2006. 2006 IUCN Red List of Threatened Species (On-line). Accessed 28 March 2007 at http://www.iucnredlist.org.
Joyce, W.G. 2007. A phylogeny of Mesozoic turtles. Bulletin of the Peabody Museum of Natural History 48:3-102.
Orenstein, R. 2001. Turtles, Tortoises, and Terrapins: Survivors in Armor. Firefly Books, Buffalo, NY.
Parham, J.F., Feldman, C.R. and J.L. Boore. 2006. The complete mitochondrial genome of the enigmatic bigheaded turtle (Platysternon): description of unusual genomic features and the reconciliation of phylogenetic hypotheses based on mitochondrial and nuclear DNA. BMC Evolutionary Biology 6:Art. No. 11.
Peng, Q.L., Nie, L.W. and Y.G. Pu. 2006. Complete mitochondrial genome of Chinese big-headed turtle, Platysternon megacephalum, with a novel gene organization in vertebrate mtDNA. Gene 380:14-20.
Pritchard, P.C.H. 1979. Encyclopedia of Turtles. TFH Publishing, Neptune FL.
Shaffer, H.B., Meylan, P., and M.L. McKnight. 1997. Tests of turtle phylogeny: molecular, morphological, and paleontological approaches. Systematic Biology 46:235-268.
Links
Platysternon megacephalum page on the Animal Diversity Web (University of Michigan Museum of Zoology)
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