Obura-Journal_of_Biogeography

Indian Ocean centre of origin
diversity-generating influence around the temperate tip of
South Africa. Here invasion and establishment of Atlantic
species, and diversification of temporarily isolated Indian
Ocean populations, may occur with climatic and sea level
fluctuations affecting connectivity and dispersal around
southern Africa (Teske et al., 2011).
Synthesis
Figure 5 Schematic of the evolutionary changes in scleractinian
corals during the Cenozoic, focused on patterns described in this
paper for the western and northern Indian Ocean (W&NIO).
Tropical marine regions are aligned on the horizontal axis from
west (left) to east (right). Time is on the vertical axis from
about 65 Ma (bottom) to the present (top, not to scale), and
the major geological periods and epochs (Table 1) are labelled.
Major stages described in the text: A – the Tethyan period,
including transition of the biodiversity hotspot from the West
Tethys (WT) to the East African/Arabian Plate (EAAP) in the
Palaeogene; B – closure of the Tethys Sea during the Miocene
(25–15 Ma); C – migration of species from the Tethys/EAAP to
the Indo-Australian Archipelago (IAA) prior to and during the
Miocene (25–15 Ma); D – westward invasion of the W&NIO by
IAA species as east-west connectivity across the equatorial
Indian Ocean increases. E – diversification of Neogene endemics
within tectonically active subregions of the W&NIO. The
percentages at the top give the approximate composition of
extant Indian Ocean Neogene (left/darker, 5%), Indian Ocean
Palaeogene (centre, 5%) and Indo-Pacific (right, 90%) species
(see text and Obura, 2012a). Broad evolutionary divergence is
illustrated by colouration. Figure adapted from Obura (2012b).
Journal of Biogeography
ª 2015 John Wiley & Sons Ltd
The following major steps in origins of W&NIO corals can
be deduced from these results (Fig. 5): (A) The circumequatorial
Tethys Sea promoted a relatively uniform tropical/equatorial
coral fauna. During the Palaeogene the Tethys
was progressively narrowing, with the hotspot for species
diversity shifting from the WT in the Eocene to the EAAP
in the Oligocene. At the same time, as a result of the migration
of India northwards and the formation of the Mascarene
Ridge, the W&NIO remained relatively isolated from
the Eastern Indian Ocean/Central Indo-Pacific, which at this
time was an open oceanic region with very little shallow
habitat in tropical latitudes. (B) During the final stages of
closure of the Tethys Sea in the late Oligocene/early Miocene,
differentiation between Atlantic and Indo-Pacific faunae
intensified. (C) During the late Oligocene, as shallow
habitats in the IAA began to form, invasion of species from
the Tethys/EAAP occurred, seeding later stages of speciation
during the Miocene (Wilson & Rosen, 1998; Harzhauser
et al., 2008). This fauna differentiated from the lineages that
remained in the W&NIO, which can be attributed at least
partially to different radiation pressures in the tectonically
active IAA vs the tectonically inactive W&NIO. (D) During
the Miocene the present configuration of ocean currents
likely became established, conveying species from the IAA
westwards. Younger species often have greater ‘vigour’ than
older ones (Ricklefs, 2011), and certainly the IAA/West Pacific
fauna has come to dominate coral reef communities in
the W&NIO and throughout the Indo-Pacific (Bowen et al.,
2013; Veron et al., 2015). (E) Simultaneously, diversification
within the tectonically active subregions of the W&NIO (the
Red Sea, Arabian Seas and the Mascarene islands/banks), has
led to diversity/endemism hotspots in these two sub-regions
and a recent (Neogene) contribution to the W&NIO regional
fauna. At present, the extant hard coral fauna comprises
three groups: wide-ranging Indo-Pacific species, relict species
with Tethyan (Palaeogene) origins (Fig. 2) and new species
originating within the W&NIO during the Neogene (Fig. 3).
These results suggest a more complex ‘Indian Ocean origins’
hypothesis (Rosen, 1971; Pichon, 1978) with multiple
temporal and spatial dimensions. First, a Tethys Sea centre
of origin during the Palaeogene (with two sequential centres
in the WT then the EAAP), from which tectonic drivers of
speciation (Renema et al., 2008) are recorded in deeper phylogenetic
levels (genus and family, Table 2). Next, Neogene
processes of dispersal, vicariance, accumulation and survival
(e.g. Carpenter et al., 2011; Teske et al., 2011; Bowen et al.,
2013; Pellissier et al., 2014) acted in geologically dynamic
subregions of the Indian Ocean, particularly in the Red and
Arabian Seas, and in the Mascarene Islands, recorded in shallower
phylogenetic levels (species and intra-species, Table 2).
In terms of differentiating an Indian Ocean fauna from the
Central Indo-Pacific, and in reconciling differences among
biogeographical classification schemes (Fig. 4) (Spalding
et al., 2007; Briggs & Bowen, 2012; Obura, 2012a; Kulbicki
et al., 2013; Veron et al., 2015), this analysis suggests these
W&NIO endemic groups (Tethyan relicts and Neogene species)
should be the focus for further analysis, against the
homogenizing background of the majority (90%) of Indo-
Pacific species.
The phylogenetic patterns described here are based on an
incomplete revision of hard coral phylogenetics (Fukami
et al., 2008; Budd et al., 2010) and on increasing research
effort on Indian Ocean locations and their taxa. Further
advances in both areas may provide additional evidence in
support of this hypothesis. Once the ongoing revision of
hard coral phylogenetics is complete the full membership of
species in the two W&NIO groups – Tethyan relicts versus
Neogene endemics – can be determined, and a more
complete assessment of the hypothesis can be done. One
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