162 J. A. CRAME & B. R. ROSEN
many modern genera and species evolved
(Veron 1995; Crame 2001, and references
therein). At the present day some 55% (by area)
of the world's coral reefs occur in the SE
Asia-New Guinea-Australia region (Wilson &
Rosen 1998).
There can be no doubt that this dramatic rise
in the numbers of both reef and reef-associated
taxa in the IWP region through the Neogene was
linked to a huge increase in the availability of
tropical shallow-water habitats. Besides the
northward movement of Australia and New
Guinea into the coral reef belt, collision-related
uplift led to the provision of more islands and
carbonate shelves in SE Asia (Fig. 4). This in
turn must have greatly increased the amount of
local habitat heterogeneity and the potential for
allopatric speciation between fragmented
shallow-water areas. As a result of the syn-
chronous closure of Tethys in the Middle East
and progressive westward movement of the
Pacific archipelagos associated with the Darwin
Rise/Superswell, SE Asia also became a 'cross-
roads* for tropical shallow-marine organisms
(Rosen 1988; Pandolfi 1992).
Finally, it should be emphasized that the
Australian-New Guinea block continued to
move northwards throughout the Neogene, It
has been suggested that at approximately 4 Ma
BP (Early Pliocene) it reached a critical point
when it came into close contact with the rapidly
growing island of Halmahera. This had the effect
of deflecting warm south Pacific waters east-
wards at the Halmahera eddy to form the
Northern Equatorial Countercurrent (Cane &
Molnar 2001). Thus warm waters in the Indo-
nesian throughflow were replaced by relatively
cold ones from the north Pacific, leading to a
drop in sea surface temperatures in the Indian
Ocean and the aridification of East Africa.
These changes were the catalyst for a shift in the
relative heat balance between the east and west
Pacific, which in turn may have helped trigger
the onset of northern hemisphere glaciation
(Cane & Molnar 2001).
Neogene climate change and
biodiversification
Following the lines of evidence presented above
it could be maintained that Cenozoic palaeo-
geographic changes were very largely respons-
ible for the evolution of some of the major
patterns of life on Earth. A once-homogeneous
tropical biota was disrupted by vicariant events
such as the closure of Tethys in the Middle East,
the collision of Australia-New Guinea with SE
Asia, and the rise of the CAI, The net effect, in
the marine realm, was to isolate an ACEP centre
of high tropical diversity from an IWP one. Even
in the terrestrial realm, which is complicated to
some extent by pockets of high diversity in both
central and southern Africa, the effects were to
produce not dissimilar Palaeotropical and
Neotropical realms (e.g. Barthlott etal. 1997). In
both the marine and terrestrial realms the
steepest latitudinal gradients at the present day
are associated with the western margins of the
Neotropics/ACEP and eastern margins of
the Palaeotropics/IWP, respectively (Crame
2000a,b).
Perhaps the marked heterogeneity observed
in the tropical biota at the present day can be
attributed simply to the range in sizes of the
various subregions imposed by Cenozoic
tectonics. For example, in the shallow marine
realm the Indo-West Pacific province is approxi-
mately four times the area of the Western
Atlantic and Eastern Pacific provinces com-
bined (Briggs 1996). If we assume that the
greater species richness of the tropics is a time-
invariant feature, caused perhaps by the greater
size of the tropics in comparison with all other
biomes (e.g. Rosenzweig 1995), or some form of
species-energy hypothesis (e.g. Wright et al.
1993), then what we see at the present day may
be due as much to tectonic as to biological
factors.
Nevertheless, important as these processes
undoubtedly are to the generation and mainten-
ance of large-scale diversity patterns, there is a
distinct impression that something else must
have been involved too. As our knowledge of
the tropical fossil record slowly improves it is
becoming apparent that much of the very pro-
nounced tropical Cenozoic diversification event
actually occurred in the mid- to late Neogene
(i.e. last 10-15 Ma) In the marine realm this is
certainly the case for zooxanthellate corals
(Veron 1995; Wilson & Rosen 1998), as well as
certain reef-associated molluscan taxa (Crame
2001, appendix 2). There is also some palaeon-
tological evidence to demonstrate that certain
major eudicot angiosperm clades are of essen-
tially Neogene origin (Magallon et al 1999).
Some of this Neogene rise could well be
attributed to differentiation diversity, with com-
munities and provinces being distinguished as
much in a longitudinal sense as a latitudinal one.
Nevertheless, there has long been a suspicion
that global climate change was an important
driver of diversification too, for this was a time
of marked intensification of Milankovitch
cyclicity (Bennett 1997). These cycles, which are
based on the obliquity of the Earth's axis and
