Updating the phylogenetic dating of New Caledonian biodiversity with a meta-analysis of the available evidence

International audienceFor a long time, New Caledonia was considered a continental island, a fragment of Gondwana harbouring old clades that originated by vicariance and so were thought to be locally ancient. Recent molecular phylogenetic studies dating diversification and geological data indicating important events of submergence during the Paleocene and Eocene (until 37 Ma) brought evidence to dismiss this old hypothesis. In spite of this, some authors still insist on the idea of a local permanence of a Gondwanan biota, justifying this assumption through a complex scenario of survival by hopping to and from nearby and now-vanished islands. Based on a comprehensive review of the literature, we found 40 studies dating regional clades of diverse organisms and we used them to test the hypothesis that New Caledonian and inclusive Pacific island clades are older than 37 Ma. The results of this meta-analysis provide strong evidence for refuting the hypothesis of a Gondwanan refuge with a biota that originated by vicariance. Only a few inclusive Pacific clades (6 out of 40) were older than the oldest existing island. We suggest that these clades could have extinct members either on vanished islands or nearby continents, emphasizing the role of dispersal and extinction in shaping the present-day biota. Island biology has spurred scientific innovation and has been crucial to the formulation of new paradigms since the early days of evolutionary biology 1-6. More recently, it stimulated a methodological discussion on molecular phylogenetic dating and the nature of biogeographical events 7-9. Central to this discussion is the case of large Gondwanan islands, such as Madagascar 10 , and some islands that are traditionally considered to be continental, but which are probably oceanic, such as New Zealand 11 and New Caledonia. The biogeographical paradigm for the origin of the New Caledonian biota has been profoundly revisited over the last ten years e.g. refs 12-15. The traditional perspective considers the main island Grande Terre (hereafter NC) to be a small piece of the Gondwanan continent that separated from Australia 80 million years ago. This view mainly relies on NC's old geological basement and the local occurrence of relicts 16-19. In fact, as summarized in recent reviews 13, 20 , geological studies have concluded that the island has a very complex sedimentary and tectonic history with several long episodes of submergence and a final emergence dated at 37 (±3) Ma 21-26. Despite the progressive accumulation of independent phylogenetic evidence for post-emergence colonization (reviewed in refs 13, 27 and 28), some authors are still reticent to adopt this new view, as epitomized by M. Heads 7. This author argued that i) molecular dating could be corrupted by poor calibrations and ii) lineages could be considered to be locally permanent if they hopped between neighbouring islands. Methodological as well as theoretical discussions in defence of molecular dating have been recently addressed 8, 9. We will not discuss here the ways of testing dating hypotheses, which is a rich and generally well-understood subject. The second statement refers to a less commonly broached topic which needs to be clearly formulated in order to be subjected to a more rigorous hypothesis testing. Island-hopping, there and back, is not equivalent to permanence on the same piece of land. It actually implies multiple short-distance dispersal events between nearby and temporary islands or othe