Obura-Journal_of_Biogeography

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D. O. Obura Work on individual taxonomic groups shows some variation from this structure, recently focusing on the two best-studied taxa on coral reefs – scleractinian corals (Bellwood & Hughes, 2001; Veron et al., 2009, 2015) and reef-associated fish (Briggs & Bowen, 2012; Bowen et al., 2013; Kulbicki et al., 2013). In general, species diversity across the Indo-Pacific declines in all directions from the high-diversity centre in the Indo- Australian Archipelago (IAA), with the evidence for corals and fish being held as proxies for tropical taxa in general (Bellwood & Hughes, 2001). Primary attention has remained squarely on the global diversity hotspot in the IAA and subregional centres of endemism (Briggs & Bowen, 2012). Comparatively little attention has been paid to the Indian Ocean (Wafar et al., 2011), in spite of Rosen (1971) and Pichon (1978) noting evidence for a ‘second centre origin’ for coral species in it. This gap is now being addressed; for example, in differentiating Indian versus Pacific Ocean species within what were previously thought to be Indo-Pacific-wide species of corals (Y. Chuang, unpublished data; Stefani et al., 2011) and fish (Gaither et al., 2011; Eble et al., 2011), as well as further splitting within the Indian Ocean of iconic taxa such as the coral Stylophora pistillata (Keshavmurthy et al., 2013), and the crown of thorns seastar Acanthaster planci (V€ogler et al., 2008). Further, a peak of species diversity has now been recognized in the western part of the Ocean, in particular in the northern Mozambique Channel, shown by scleractinian corals (Obura, 2012a; Veron et al., 2015) and stomatopods (Reaka et al., 2008), with some corroboration in phylogenetic analysis in ophiuroids (Hoareau et al., 2013) and fish (Muths et al., 2014). In addition, phylogenetic work on corals focused in the Red Sea and Arabian Sea is building increasing evidence for the evolutionary distinctiveness of Indian Ocean coral taxa (Stefani et al., 2011; Arrigoni et al., 2012, 2014; Benzoni et al., 2012). This paper reviews major aspects of the tectonic and palaeoceanographic history of the Indian Ocean in order to propose mechanisms underlying phylogenetic and biogeographical relationships in ‘Indian Ocean corals’, in a region from Sri Lanka and India westwards that is referred to here as the western and northern Indian Ocean (W&NIO), and loosely described as ‘the Indian Ocean’. Analysis focuses on two major periods of earth history during the Cenozoic era: the Eocene-Oligocene (in the Palaeogene), when shallow marine diversity was in full recovery following the Pg-T (K-T) extinction that marked the beginning of the Cenozoic, and the Miocene-Pliocene (the Neogene), when the marine diversity peak in the IAA (Renema et al., 2008) came to dominate global marine biodiversity patterns. THE EVIDENCE The Palaeogene (Eocene – Oligocene) The Eocene (56–33.9 Ma) and Oligocene (33.9–23 Ma), the last two epochs of the Palaeogene, mark a time of tectonic 2 change in what is now the northern and central Indian Ocean (Table 1, Fig. 1a). At this time Africa was drifting northwards and colliding with the Eurasian land mass, resulting in narrowing of the Tethys Sea and bringing to a close its dominance of global oceanographic patterns and shallow marine biodiversity that started with its formation about 200 Ma (Stow, 2010). Coincident with the transition from the Eocene to the Oligocene (33.9 Ma), the Mascarene- Reunion hotspot was active, creating an island chain across the centre of the Indian Ocean that now comprises the submerged carbonate banks of Saya de Malha, Nazareth and Cargados Carajos, capping the volcanic Mascarene Ridge. In contrast, the passive plate margins of the East African coast, produced by the initial rift of Gondwana some 140–180 Ma (Parson & Evans, 2004; Yoder & Nowak, 2006), formed what is now the Mozambique Channel, and the east coast of Madagascar, produced by the rifting of Australia, Antarctica and India from Madagascar, formed between 120 and 90 Ma. The palaeoceanography of the Indian Ocean has received little attention other than as a sidebar to other interests, such as general ocean circulation and climate interactions (Brass et al., 1982; Bickert et al., 2008), or exchanges between Madagascan, African and Asian terrestrial faunae during the Cenozoic (Masters et al., 2006; Ali & Huber, 2010). During the early Eocene it is likely that the west-flowing circumequatorial current of the Tethys Sea passed between India and the Asian landmass, and as this narrowed, the current was deflected southwards by the Indian land mass (Brass et al., 1982). Following collision of India with Asia, the remnants of the Tethys Sea closed during the Oligocene, slowly obliterating the once-abundant shallow habitats found there (R€ogl, 1998). It is also likely that the then emergent Mascarene Ridge islands (Fig. 1a) formed an intermittent north– south barrier that would have further isolated the western Indian Ocean (WIO). These have been considered in island stepping-stone models for both terrestrial (e.g. Masters et al., 2006) and marine fauna (e.g. Postaire et al., 2014), but without consideration of their potential effect on currents (e.g. Ali & Huber, 2010). Modelling of global circulation during the Oligocene suggests a strong westward current akin to the present South Equatorial Current (SEC) but farther south, and a clockwise gyre in the northern Indian Ocean (von der Heydt and Dijkstra, 2006), and during the Miocene, weak circulation between the Tethys Sea and WIO (Bickert et al., 2008). These currents would have provided a mechanism for transport from the Tethys Sea into the WIO and towards the incipient IAA. Overall, it appears that what is now the WIO may have been relatively isolated in a semi-enclosed sea during the Oligocene (Fig. 1b), with the greatest degree of connectivity, in terms of both currents and larval sources, with the Tethys Sea. The Tethys Sea between North Africa/the Arabian Peninsula and the Eurasian land mass became the hotspot of shallow marine diversity, initially in the West Tethys (WT; Journal of Biogeography ª 2015 John Wiley & Sons Ltd
Indian Ocean centre of origin Table 1 Major geological, oceanographic and climatic features affecting the coral reef and shallow marine biota of the Indian Ocean during the Cenozoic. Time units are in millions of years before present (Ma), following Cohen et al. (2013). Adapted from Obura, 2012b. Periods Epochs Time (Ma) Plate Tectonics Tethys Sea Mascarene Hotspot Currents and dispersal Diversity Quarternary Holocene Present Current configuration reached Dominance of I-P diversity patterns by IAA and West Pacific species Pleistocene 0.01–2.58 Reunion (2 Ma) Full establishment of E–W connectiveity and dispersal Mauritius (7–8 Ma) Diversity hotpot developing in IAA Tertiary Neogene Pliocene 2.58–5.33 Miocene 5.33–23.0 Full development of IAA Closure of Tethys Sea, by 15 Ma Diversity hotspot in Arabian region (EAAP) Palaeogene Oligocene 23.0–33.9 Collision of Asian and Australian plates (25 Ma) Collision of India with Asia (35 Ma) Diversity hotspot in West Isolation of WIO by India & Mascarene plateau, likely flow from Tethys Narrowing of Tethys Sea Mascarene plateau/central IO banks formed (30–45 Ma) Tethys. Eocene 33.9–56.0 Rapid northward migration of India ‘Palaeogene gap’ in fossil record western Europe/north Africa) in the Eocene, and subsequently in the East Africa-Arabian Province (EAAP) in the Oligocene (Harzhauser et al., 2007; Renema et al., 2008). As the primary region of tectonic collision shifted from this region to the Indo-Australian region in the Miocene, due to collision of the Australian and Asian plates, this latter region became the new hotspot of shallow marine diversity (Wilson & Rosen, 1998). At the same time, further collision in the Tethyan region resulted in uplift of the active crusts and obliteration of the Tethys Sea (Renema et al., 2008). The Neogene (Miocene-Pliocene) The Neogene, comprising the Miocene (23–5.33 Ma) and Pliocene (5.33–2.58 Ma), is defined by collision of the Australian and Asian plates, forming a new biodiversity hotspot in the IAA (Wilson & Rosen, 1998; Renema et al., 2008; Fig. 1c), now dubbed the Coral Triangle (Roberts et al., 2002; Hoeksma, 2007). While the gap between Australia and Asia has narrowed during this time, the configuration of the Indian Ocean has remained relatively stable. The Tethys Sea finally closed about 15 Ma, and the Mascarene-Reunion hotspot produced just the two relatively small islands of Mauritius (7–8 Ma) and Reunion (2 Ma). Westward flow of the SEC (Schott & McCreary, 2001) has likely remained consistent during the Neogene, the main differences with the Palaeogene being less obstruction across the Mascarene Ridge (Fig. 1c) and a growing source fauna of newly diversifying species in the IAA. Coral phylogenetics and biogeography Recent advances in coral phylogenetics and systematics using genetic techniques and microstructural characters are revealing the true phylogeny of corals (Fukami et al., 2008; Budd et al., 2010; Budd & Stolarski, 2011). The main macromorphological features historically used in coral taxonomy (e.g. Wells, 1956) experience considerable convergence, resulting in incorrect phylogenetic reconstructions. As a result, the accepted phylogeny of corals was strongly biased by the locations and primary material studied by taxonomists, focusing on the Atlantic and IAA, and Tethyan fossil sites. Poorly studied regions such as the Indian Ocean were therefore poorly treated in evolutionary interpretations. This section outlines emerging phylogenetic relationships of corals (Table 2) that show concordance with tectonic and oceanographic patterns (Table 1). The genus Acropora, known mostly for its extreme species radiation associated with the IAA in the Neogene and Quaternary (Wallace, 1999) shows its first fossil appearance in Somalia, in the Eocene East Africa/Arabian Province (Carbone et al., 1994). This was followed by radiation of 9 of 20 currently recognized species groups in the WT (Wallace & Rosen, 2006). Subsequent eastward migration of the diversity hotspot during the late Oligocene and early Miocene is associated with the proliferation of Acropora in Journal of Biogeography ª 2015 John Wiley & Sons Ltd 3
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