154 J. A. CRAME & B. R. ROSEN
Currie 1998; Huston 1999; Ricklefs 1999; Qian &
Ricklefs 2000).
It is the intention of this study to further this
line of enquiry by focusing on a series of major
tectonic and climatic events that occurred
throughout the Cenozoic Era (i.e. the last 65 Ma
of Earth history). It will be suggested that this
was a time of major expansion in the size and
scale of taxonomic diversity gradients and the
question asked whether this can be attributed
directly to the relative isolation of the conti-
nents, fundamental reorganization of oceanic
circulation systems and progressive imposition
of a steep latitudinal temperature gradient. The
potential role of biotic factors in promoting the
massive Cenozoic radiation of selected tropical
taxa will not be ignored, but the emphasis here
will essentially be on evolving Cenozoic palaeo-
geographies and how they affected such key
processes as climate change, the establishment
of new colonization and dispersal routes, and,
possibly, pronounced changes in origination
(speciation) and extinction rates.
Cenozoic global diversification
There is general agreement that there was a very
substantial rise in global biodiversity through
the Cenozoic Era. This was particularly so at the
lower taxonomic levels of species and genus
where the rise was the steepest and most pro-
longed of the entire Phanerozoic interval
(Signer 1990; Benton 1999; Foote 2000). We
could perhaps be looking at an order of magni-
tude increase in species diversity at this time
(Signer 1990; but see also Alroy et al 2001).
Equally striking is the fact that this radiation is
recorded across such a broad range of organisms
in both the marine and terrestrial realms. This
was the time of widespread proliferation of
groups such as the neogastropods, heteroconch
bivalves, cheilostome bryozoans, decapod crus-
taceans and teleost fish in shallow seas, and
amphibians, reptiles, birds, mammals, flowering
plants and insects on land (e.g. Hallam 1994).
Parallel temporal trends in so many different
groups has been taken as a strong indication that
there may be a single, basic mechanism under-
pinning the process of Cenozoic biodiversifica-
tion (Signer 1990; Benton 1999).
The presence of very clear latitudinal gradi-
ents at the present day strongly suggests that this
pulse of Cenozoic diversification was concen-
trated in low-latitude and tropical regions.
Unfortunately, we cannot yet demonstrate this
by reconstructing diversity gradients directly
from the fossil record, but a number of indirect
lines of evidence suggest that this was indeed the
case. For example, in a recent study of latest
Mesozoic-Cenozoic bivalve diversification,
Crame (2000a) demonstrated that the steepest
latitudinal gradients occurred in the youngest
clades (and especially in the very large hetero-
conch clade). He went on to suggest that the
same phenomenon may occur within the marine
gastropods, where the youngest major clade, the
neogastropods, shows particularly strong latitu-
dinal gradients (Crame 2001). In the terrestrial
realm the three largest core eudicot angiosperm
clades are amongst the youngest, and very
similar trends are shown by various insects,
birds, mammals, fish and reptiles. There is
growing evidence to suggest that the age of
certain major clades and the strength of their
latitudinal gradients are consistently negatively
correlated (Crame 2001).
Cenozoic global cooling
One of the most striking and puzzling aspects of
the Cenozoic biodiversification event is that it
took place against a backdrop of global climatic
deterioration. From an Early Eocene climatic
optimum (55 Ma BP), global climates deterio-
rated in a series of pronounced steps to their
present-day values. Although the Eocene seems
to have been an epoch of relative global warmth,
a 5°C drop in oceanic surface water tempera-
tures at the Eocene-Oligocene boundary (33
Ma BP) is widely interpreted to mark the onset
of permanent Antarctic glaciation. Further
abrupt cooling phases occurred in the Middle-
Late Miocene (approx. 15-13 Ma BP), latest
Miocene-earliest Pliocene (approx. 6.2-4.8
Ma BP), Middle-Late Pliocene (3.6-2.4 Ma BP),
and Late Pliocene-Recent. Even during the
Neogene climatic optimum (early Middle
Miocene, 17-15 Ma BP) average marine global
temperature values only briefly returned to
those of the Late Eocene (Clarke & Crame 1992;
Frakes et al 1992; Zachos et al 2001).
The principal effect of these changes within
the tropical and subtropical regions was not so
much a reduction in temperature as one in
area. It has been estimated that, since the
Early-Middle Eocene (i.e. the last 45 Ma), the
tropical biome has contracted some 40-50% in
size (Adams et al 1990). In other words, the
marked Cenozoic diversification event took
place as the tropics actually contracted in size
and the temperate and polar regions expanded.
The principal steps in the physical control of this
process are outlined below.
One of the crucial early Cenozoic palaeogeo-
graphic events to affect global oceanographic
circulation patterns and climates was the
