Taxonomic revision of the critically endangered big-headed turtles (Reptilia: Testudines: Platysternidae Gray, 1869), with description of a new species

The big-headed turtle (Platysternon megacephalum), currently the only extant member of the genus Platysternon and the family Platysternidae, has undergone severe population declines driven by poaching, illegal trade, and habitat loss, leading to its classification as Critically Endangered (CR) by the International Union for Conservation of Nature (IUCN). Despite its conservation status, persistent taxonomic ambiguities and unresolved phylogenetic relationships have hindered effective protection and management. This study integrated evidence from genome-wide single nucleotide polymorphisms (SNPs), mitochondrial DNA sequences (COI, 12S), and morphological data to reconstruct the phylogeny and phylogeography of Platysternon and revise its taxonomy. Results revealed that Platysternon megacephalum megacephalum and Platysternon megacephalum peguense represent deeply divergent evolutionary lineages, which are herein recognized as distinct species, Platysternon megacephalum Gray, 1831 sensu stricto and Platysternon peguense Gray, 1870. The subspecies Platysternon megacephalum shiui Ernst & McCord, 1987 is taxonomically invalid. In addition, a highly divergent lineage endemic to the Baise-Hechi region of Guangxi is described as a third species, Platysternon baiseensis sp. nov. This revised taxonomic framework resolves key phylogenetic relationships and delineates species boundaries, establishing a robust systematic foundation for big-headed turtles and providing a critical taxonomic basis for future conservation efforts targeting this endangered group of turtles.