Antarctica as an evolutionary incubator: evidence from the cladistic biogeography of the amphipod Family Iphimediidae

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Abstract
The modern Antarctic marine fauna is quite distinct, with levels of endemism reaching 90% at the species level and 70% at the generic level. Since the modern Antarctic waters are very cold and isolated from all of the worlds oceans except the deep-sea, the origin of the modern Antarctic fauna has remained in doubt. The amphipod Family Iphimediidae is represented in Antarctic and Sub-Antarctic waters by 16 of its 21 genera and 490f its species. A phylogenetic analysis of the family indicates that the most primitive genera are distributed primarily outside Antarctic waters. It is suggested that these are relicts of a former global distribution, and that once the Antarctic began to cool (38 Ma ago), a radiation of the iphimediids occurred in the Southern Ocean. This radiation was furthered by the reorientation of the mandible from one where the incisor cut in the horizontal transverse plane to cutting in the vertical frontal plane. With this advance, the family spread outward from the Antarctic, principally through the genus Iphimedia, which is today represented by 35 known species worldwide. The tracking of features in Iphimedia suggests that its initial spread beyond Antarctic waters occurred before the continent was finally thermally isolated 23 Ma ago. It is proposed that the cooling Antarctic waters acted as an incubator for this family, producing an evolutionary advance that allowed it to be a successful colonizer of the thermally changing global ocean.