160 J. A. CRAME & B. R. ROSEN
Extrinsic mechanisms of diversification
Closure of the Tethyan Ocean in the Middle East.
At the end of the Cretaceous period a vast,
circum-equatorial tropical ocean (Tethys) could
be traced westwards from the Indonesian
region, through the Middle East and southern
Europe, to the proto-Atlantic Ocean (Fig. 3).
There is thought to have been a predominantly
westward current flow in the ocean and this con-
tinued through the Central American Seaway to
the eastern Pacific Ocean (Panthalassa).
Although there are some signs of faunal differ-
entiation within the Late Cretaceous Tethyan
Ocean, this does not appear to have been such as
to create high-diversity foci in any way equival-
ent to those seen at the present day (Kauffman
1973; Hallam 1994).
The same picture is essentially true for the
early part of the Cenozoic Era (i.e. the Paleo-
gene). There was one very large, homogeneous
tropical marine realm, with only minor evidence
of physical differentiation in areas such as the
Caribbean, Middle East and deep-water eastern
Pacific (Newell 1971; Kay 1984; Rosen 1988)
(Fig. 3). A major pulse in coral reef development
occurred at the end of the Oligocene epoch, but
even then there was marked similarity, at the
generic level, between western Tethys (i.e.
Mediterranean) and Caribbean/Gulf of Mexico
corals (Frost 1981; Veron 1995). It was not in fact
until the Early Miocene (c. 20 Ma BP) that the
Tethyan Ocean was closed in its central
region by the northward movement of the
Africa/Arabia landmass. This dramatically
curtailed the westward-flowing tropical current
and led, eventually, to the Mediterranean region
being excluded from the coral reef belt (Fig. 3).
At the same time the eastern Atlantic became
significantly cooler and this accentuated the
physical isolation of the Caribbean-eastern
Pacific region (Rosen 1988; Veron 1995). It is to
this time that we can trace the origins of the
distinct IWP and ACEP foci of high tropical
marine diversity.
The Early Miocene fossil record suggests that
there was a reduced coral fauna in the
Caribbean region that was transitional in com-
positional terms between a Late Oligocene
cosmopolitan one and a later Miocene one con-
taining a number of endemics (Veron 1995). It
would appear that the overall trend through the
Neogene (i.e. the last 23 Ma) was one of relative
impoverishment of ACEP coral reef faunas and
relative enrichment of IWP ones. For example,
whereas there are some 24 coral genera in the
ACEP region at the present day, there are 87 in
the IWP; a similar comparison at the species
level gives a ratio of approximately 62: 450
(Rosen 1988; Veron 1995). Briggs (1995) esti-
mated that the vast IWP region contains more
than 6000 species of molluscs, 800 echinoderms,
500 hermatypic corals (probably an overesti-
mate; Veron 1995) and 4000 species of fishes.
This equates to a total species richness that is
approximately 2.5 times that of the western
Atlantic, 3.5 times that of the eastern Pacific, and
7.3 times that of the eastern Atlantic (Briggs
1996). Ellison et al (1999) estimated that The
species richness of trees, shrubs and ferns in
mangrove forests is an order of magnitude
higher in the IWP than the ACEP.
Collision of Australia-New Guinea with SE
Asia. At the beginning of the Cenozoic Era the
Australia-New Guinea block was separated
from mainland SE Asia by a deep-water Indo-
Pacific gateway measuring some 3000 km across.
This feature was progressively reduced in width
through the Paleogene (i.e. 65-23 Ma BP),
chiefly by the northward subduction of Indian-
Australian lithosphere beneath the Sunda-
Java-Sulawesi arcs (Hall 1998). By the mid-
Oligocene (30 Ma BP) the gap had narrowed
substantially but there was still a clear deep-
water passage floored by oceanic crust of the
Indian and Pacific plates (Fig. 4). Major changes
in plate boundaries occurred at approximately
25 Ma BP when the New Guinea passive
margin collided with the leading edge of the
east Philippines-Halmahera-New Guinea arc
system and the Australian margin, in the Bird's
Head region, was very close to collision with the
Eurasian margin in West Sulawesi (Hall 1998).
By 20 Ma BP the continent-arc collision had
closed the deep-water passage between the
Pacific and Indian oceans, resulting in a major
reorganization of tropical oceanic current
systems (Kennett et al 1985; Grigg 1988). Con-
tinued northward movement of Australia from
20 to 10 Ma BP caused the rotation of several
plate boundaries and the formation of tectonic
provinces that are recognizable at the present
day (Fig. 4).
In comparison with both the Mediterranean
and Caribbean regions. Eocene and Lower
Oligocene hermatypic coral faunas appear to
have been genuinely sparse in the central IWP.
However, this situation was dramatically
reversed in the Early Miocene when there was a
four-fold increase in the number of coral genera
in the IWP focus (Rosen 1988; Wilson & Rosen
1998). The Early-Middle Miocene was a time of
widespread deposition of coral-dominated
carbonates in northern Australia, New Guinea
and throughout SE Asia; reefs proliferated and
