The phylogeny of softshell turtles and the giant Shanghai softshell turtle, Rafetus swinhoei

The softshell turtles of the family Trionychidae have highly derived morphology evolved to adapt the turtles to an almost entirely aquatic environment. These adaptations include a smooth leathery skin covering with a reduced bony shell, a flattened body shape, and heavily webbed toes. Thirty-one species in 13 genera are distributed in Africa, Asia (including New Guinea), the Mediterranean, and North America. The fossil record supports the family’s former presence in Australia, Europe, and South America. Trionychid phylogenetic relationships are a fairly well resolved and robust molecular phylogeny of trionychids has been established. And, species boundaries within a number of widely distributed species or species complexes have been clarified. However, to date the taxonomic status of the critically endangered Shanghai softshell turtle, Rafetus swinhoei, is still a matter of debate. Ranked as one of the 100 most endangered species (only four live individuals of this species are known, two in Vietnam and two in China). Historically, this species had a large distribution range, including the Yellow River, Yangtze River, and their tributaries in China and the Red River system, as well as Ma River and associated wetlands in Vietnam. Previous molecular and morphological comparisons suggested that this is a single species but more recent work produced radically different results, and described populations in Vietnam as a new species, R. vietnamensis. A subsequent study shed doubt on this view by highlighting sources of potential errors. Despite this, it is likely that populations from Vietnam and China constitute independent evolutionary lineages given the distance and river systems separating them.

To test this hypothesis, Minh Le and colleagues analyzed data from two mitochondrial loci (cytochrome b and ND4) and one nuclear intron (R35) for all trionychid turtle species, except Pelochelys signifera, and for all known populations of Rafetus swinhoei in Vietnam and one from China. Phylogenetic analyses using three methods (maximum parsimony, maximum likelihood, and Bayesian inference) produce a well resolved and strongly supported phylogeny. The results the time-calibration and biogeographic optimization analyses show trionychid dispersed out of Asia between 45 and 49 million years ago in the Eocene. Interestingly, the accelerated rates of diversification and dispersal within the family correspond surprisingly well to global warming periods between the mid Paleocene and the early Oligocene and from the end of the Oligocene to the mid Miocene. The study also indicates that there is no significant genetic divergence among monophyletic populations of Rafetus swinhoei, and that previous taxonomic revision of this species is unwarranted.

Citation

Le, M., Duong, H. T., Dinh, L. D., Nguyen, T. Q., Pritchard, P. C., & McCormack, T. (2014). A phylogeny of softshell turtles (Testudines: Trionychidae) with reference to the taxonomic status of the critically endangered, giant softshell turtle, Rafetus swinhoei. Organisms Diversity & Evolution, 1-15.