164 J. A. CRAME & B. R. ROSENCAI may have had an effect since 15-11 Ma BP
(Cronin & Dowsett 1996).
Even though temperature changes did occur
in the Western Atlantic, they were almost cer-
tainly of secondary importance in the formation
of diversity patterns. Instead it is likely that the
major reorganization of ocean currents brought
about by the full emplacement of the CAI led to
a reduction in the areas of upwelling and thus
primary productivity (Allmon 2001), At first a
decline in productivity may have actually pro-
moted the formation of isolates, and thus local
speciation, but in time it would have led to a
marked decrease in the rate of isolate persis-
tence, decreased origination and increased
extinction (Allmon 2001). The striking turnover
in East Pacific coral faunas since the Pliocene
closure of the CAI can be related to changes in
temperature, salinity and nutrient levels (Budd
et al 1996; Budd & Johnson 1999). Today there
is only a very depauperate coral fauna in this
region but a surprisingly rich molluscan one.
Discussion
Taxonomic diversity gradients can be traced
back in the fossil record to the Late Palaeozoic
Era, and perhaps even farther than that (Crame
2001, and references therein). Despite the fact
that it is often difficult to make meaningful com-
parisons between these gradients, there is a
distinct impression that they were not nearly so
steep as those seen at the present day. Dramatic
steepening appears to have occurred through
the Cenozoic, and we now suspect that this was
especially so during the Neogene (i.e. the last
23 Ma), Indeed it is unlikely that, in the marine
realm, the present-day IWP and ACEP foci of
high taxonomic diversity are any more than
20 Ma old. This was the time of crucial plate
tectonic movements that essentially gave the
tropics their modern form.
It is also striking how the imposition of steep
diversity gradients coincided with marked global
climatic cooling, and tempting to postulate a link
between the two. Whilst some form of large-
scale differentiation diversity undoubtedly
contributed to the increase in global biodiversity
at this time, it is not easy to see how it could have
specifically promoted radiations in two or more
tropical foci. There may have been some form of
longitudinal as well as latitudinal biotic differen-
tiation, but it is unlikely that this would have
been on a scale sufficient to generate the
features seen at the present day.
Within the tropical marine realm, the richest
Paleogene (i.e. 65-23 Ma BP) coral faunas
occurred in Europe and the Caribbean (Rosen
1988, table 2; Wilson & Rosen 1998, fig. 9). In the
Early Miocene, there is evidence to suggest that
at least 12 common IWP genera were formerly
more widespread (Veron 1995). Of particular
interest in this context is the fossil record of the
diverse branching genus, Acropora. The oldest
occurrence is in the Paleocene of Somalia, and
there are also numerous European records from
the Eocene-Miocene. However, Acropora does
not occur in either the Caribbean or the IWP
until the Oligocene (Wilson & Rosen 1998, and
references therein), and a very marked radiation
in the IWP only is dated as latest Miocene or
Pliocene (McCall et al. 1994). Similarly, fungiid
corals, which show their richest diversity at the
present day in the IWP centre, can be traced
back to records of Cvcloseris in the Paleocene of
Somalia and western Pakistan, as well as other
occurrences in the Eocene of SE Asia and
Europe (Wilson & Rosen 1998).
Similar patterns to those of certain coral
genera are shown by mangroves. The fossil
record strongly supports a Tethyan origin both
for mangroves and their associated gastropod
fauna in the Maastrichtian/Paleocene (Ellison et
al. 1999). A pan-tropical distribution of many
key taxa had been achieved by the Middle
Eocene, with, for example, Bruguiera being
recorded from the London Clay. Range retrac-
tions first occurred during a phase of global
cooling at the Eocene-Oligocene boundary and
a cosmopolitan biota was then split by a series of
vicariant events. Subsequent in-situ speciation
within both the IWP and ACEP foci occurred
from the mid- to late Miocene onwards (Ellison
et al. 1999).
If such trends are repeated in other tropical
taxa then it will be extremely difficult to classify
tropical high-diversity foci as either evol-
utionary 'cradles' or 'museums' (Briggs 1992;
Jablonski 1993, and references therein); in a
sense they could be seen as both. A seminal
study by McCoy & Heck (1976) first emphasized
the importance of tectonic and climatic events in
producing disjunct tropical distributions in the
marine realm and this has since been substanti-
ated by a number of other workers (Rosen &
Smith 1988; Pandolfi 1992; Veron 1995; Paulay
1997). In essence, we would appear to be looking
at a Paleogene phase of range retractions and
disjunctions followed by a Neogene phase of
in-situ radiations from various foci.
It is important to determine whether the mid
to late Neogene tropical radiations represent an
acceleration of evolutionary rates, and, if so,
what caused them. Are the various tropical
diversity patterns that we see at the present day
simply a function of province/region size, or is