The southeastern Asiatic family Platysternidae is represented by a single living species, Platysternon megacephalum, the big-headed turtle. A fossil genus, Planiplastron, is also assigned to this family.
Most authorities recognize Platysternidae as a separate family, but Romer (1956) regarded it as only a subfamily (Platysterninae) of the large family Testudinidae, in which he also included tortoises and batagurids. More recently, Gaffney (1975a) regarded it a tribe (Platysternini) of Chelydridae on the basis of the decreasing jugal exposure and lateral skull emargination, and the increase in temporal roofing. However, Whetstone (1978) pointed out that Platysternon has two biconvex cervical vertebrae, as do only the batagurids and testudinids. He also failed to find any derived characters unique to fossil and recent chelydrids which are also shared with Platysternon. Additional evidence has been supplied by Haiduk and Bickham (1982), who found the karyotype most similar to the primitive condition of the Bataguridae. The batagurid karyotype can be derived from that of Platysternon (or the reverse) by one change, whereas several changes are required to derive Platysternon from Chelydridae. Some derived chromosomal characters are also shared between the Bataguridae and Platysternon. Haiduk and Bickham thought Platysternidae a valid family most closely related to the Bataguridae and Testudinidae. Shaffer et al. (1997) produced several clades based on an extensive study of both molecular and morphological characters. Some of these that received strong bootstrap support indicated the families Chelydridae and Platysternidae form a monophyletic unit, and based on that, Shaffer et al. included Platysternon in the family Chelydridae. This was, however, supported only to some extent by the individual molecular and morphological data. Even if the two families have a common ancestry, as Shaffer et al. (1997) indicate, the current data do not exclude the two from continuing to be classified as separate families. Therefore, for the present, it is still best to consider P. megacephalum representing a separate monotypic family.