Gone with Gondwana: Amphipod Diversification in Freshwaters Followed the Breakup of the Supercontinent.

The evolutionary origins of modern taxa are best understood as arising from the interplay of vicariance and dispersal. Vicariant events have long been considered responsible for Gondwanan distributions; such species are relics of the eponymic supercontinent on which they were thought to have originated. One such set of taxa are the freshwater members of the amphipod superfamily Hyaloidea, which due to their marine relatives and current distributions serve as an excellent model for testing vicariance and dispersal hypotheses. We investigated the evolutionary and biogeographic histories of the Hyaloidea using a molecular phylogenetic approach. Maximum likelihood analyses and Bayesian inference, using two nuclear genes and one mitochondrial gene, reveal the freshwater amphipods within the superfamily (Hyalellidae/Chiltoniidae) as a monophyletic group diverging from their extant marine ancestors during the Mesozoic. This is suggestive of the group entering freshwater relatively early, instead of geologically recent marine invasions as have been previously hypothesized. Despite the group's apparent monophyly, it is likely that marine hyaloids exploited shallow water marine/brackish habitats created following the breakup of Gondwana to invade continental freshwaters. Given the divergence times recorded and shallow cladogenetic events observed, it is possible that this occurred through multiple invasions by closely related taxa. Mesozoic invasions by the Hyaloidea suggest that freshwater members represent a much older lineage than previously considered, occupying continental freshwaters prior to the gammarids in the Cenozoic and contemporaneouslywith the crangonyctids in the Mesozoic. Our results highlight the Gondwanan origin of taxa with enigmatic distributions and the utility of amphipods for testing biogeographic hypotheses that infer the origin of freshwater taxa.