Background. Phrynops present four valid species, including P. geoffroanus that might represents a complex of cryptic species. Here, we provide a preliminary analysis of the taxonomy and phylogenetic affinities within Phrynops, with special reference to the taxonomic status of populations of P. geoffroanus and P. tuberosus, and the recognition of a new species from Southern Brazil. Methods. We studied populations from ten Brazilian river basins. A linear morphometric analysis was performed in order to define taxonomically distinct populations. Also, a phylogenetic analysis using morphology and molecular data (sequenced for the genes R35, RAG2, c-mos, cytb, ND4, and 12S) were carried out. Three distinct sets of phylogenetic analyses were performed: parsimony to morphological and combined data, and maximum likelihood to molecular data. Results. The combined analysis shows that Phrynops represents a well supported clade. The set of skeletal data supports Mesoclemmys as the sister group of Phrynops, whereas the molecular and combined data sets show Phrynops as the sister group of a clade composed by all the remaining genera of Chelidae, except Hydromedusa. Our morphological analyses suggest that P. hilarii is the sister group of P. geoffroanus, but in both molecular and combined analyses, P. hilarii appears nested within the clade formed by the populations of P. geoffroanus. Futhermore, P. tuberosus and P. geoffroanus are not distinguishable by the set of osteological and morphometric data. On the other hand, both morphometric and osteological data show that the population of P. geoffroanus from the Paraná river basin is a distinct species. Discussion. The sister group relationships of Phrynops could not be clearly defined due to the different topologies achieved. Phrynops hilarii is included within of P. geoffroanus in both molecular and combined data, but this position has little statistical support and therefore does not express a clear position of P. hilarii within the genus Phrynops. Besides, we were not able to distinguish P. geoffroanus and P. tuberosus. However, a sampling of specific locations are still needed to objectively define the taxonomic status of P. tuberosus. Finally, the population of P. geoffroanus from the Paraná basin is clearly distinct from the remaining populations of this species. Qualitative osteological characters and morphometric results seem to demonstrate that this population is a new species of Phrynops.
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