Multilocus phylogeny and statistical biogeography clarify the evolutionary history of major lineages of turtles

Abstract

Despite their complex evolutionary history and the rich fossil record, the higher level phylogeny and historical biogeography of living turtles have not been investigated in a comprehensive and statistical framework. To tackle these issues, we assembled a large molecular dataset, maximizing both taxonomic and gene sampling. As different models provide alternative biogeographical scenarios, we have explicitly tested such hypotheses in order to reconstruct a robust biogeographical history of Testudines. We scanned publicly available databases for nucleotide sequences and composed a dataset comprising 13 loci for 294 living species of Testudines, which accounts for all living genera and 85% of their extant species diversity. Phylogenetic relationships and species divergence times were estimated using a thorough evaluation of fossil information as calibration priors. We then carried out the analysis of historical biogeography of Testudines in a fully statistical framework. Our study recovered the first large-scale phylogeny of turtles with well-supported relationships following the topology proposed by phylogenomic works. Our dating result consistently indicated that the origin of the main clades, Pleurodira and Cryptodira, occurred in the early Jurassic. The phylogenetic and historical biogeographical inferences permitted us to clarify how geological events affected the evolutionary dynamics of crown turtles. For instance, our analyses support the hypothesis that the breakup of Pangaea would have driven the divergence between the cryptodiran and pleurodiran lineages. The reticulated pattern in the ancestral distribution of the cryptodiran lineage suggests a complex biogeographic history for the clade, which was supposedly related to the complex paleogeographic history of Laurasia. On the other hand, the biogeographical history of Pleurodira indicated a tight correlation with the paleogeography of the Gondwanan landmasses.