Time of re-emergence of Christmas Island and its biogeographical significance

Abstract

Christmas Island in the eastern Indian Ocean forms the pinnacle of a once-drowned, coral atoll. Exactly when the seamount re-emerged above the seas is, unfortunately, imprecisely understood. One of the consequences of this uncertainty is that it can hamper studies dealing with its land-locked faunal assemblage. We attempt to address this issue using geological and geophysical information. A first constraint is provided by sub-aerially erupted volcanic rocks that accumulated on a karstified land-surface. Radiometric ages for five associated basalt samples yield a mean of 4.39 ± 0.08 Ma (2σ). These data, together with the geological observations, indicate that the island existed in the mid-Early Pliocene. A second constraint is provided by the seamount's postulated uplift path as it ascended the outer-trench high on its journey towards the subduction zone south of Java. Here, the edifice is estimated to have become sub-aerial between 5.66 and 4.49 Ma (5.03 + 0.63/−0.54 Ma). In terms of biogeography, landmass emergence in the latest Miocene-Early Pliocene is largely concordant with the island's native land-locked reptile and mammal species (eight of the nine extant and recently extirpated forms are endemic) as most show only small to negligible levels of genetic differentiation with respect to their nearest off-island relatives. Significantly, though, molecular-clock data for at least two, possibly three, of the lizard species indicate that they split from their nearest living congeners (incidentally, all from lands to the east of the Wallace Line >1100 km away) prior to Christmas Island's emergence, that is in the Late Miocene or earlier (specifically at c. 7 Ma, c. 13 Ma and c. 26 Ma). Assuming the ages are reliable, these dichotomies could be explained if there are/were intermediate relatives that exist elsewhere/are now-extinct as this would facilitate the ancestors of the Christmas Island species colonizing the landmass sometime in the last five million years or so.