Cenozoic palaeogeography and the rise of modern biodiversity
patterns
J.A. CRAME
1& B.R. ROSEN
2
1British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK
(e-mail: [email protected],)
2Department of Palaeontology, The Natural History Museum, Cromwell Road,
London SW75BD, UK (e-mail: [email protected])
Abstract: The steepest latitudinal and longitudinal gradients in taxonomic diversity at the
present day are those associated with tropical high diversity foci. Although there has been
a tendency in the past to regard these features as either evolutionary 'cradles' or 'museums'
of considerable antiquity, this may not be the case. Within the marine realm, a uniform,
pan-tropical fauna was progressively disrupted by a series of plate tectonic events, the most
important of which were the Early Miocene (c, 20 Ma) collisions of Africa/Arabia with
Europe and Australia/New Guinea with Indonesia, and the Middle Miocene-latest
Pliocene rise of the Central American Isthmus. This had the net effect of establishing two
main tropical high diversity foci: the Indo-West Pacific and the Atlantic-Caribbean-East
Pacific. Similar foci were also established at the same time in the terrestrial realm.
Together with the physical isolation of Antarctica, these same tectonic events con-
tributed significantly to global cooling throughout the Cenozoic Era. This in turn led to the
imposition of a series of thermally defined provinces, and thus a considerable degree of
biotic differentiation on a regional scale. However, something else seems to have been
involved in the creation of very steep tropical diversity peaks. This could in part be a
coincidental radiation of a series of unrelated taxa, or some sort of evolutionary feedback
mechanism between interacting clades. Alternatively, Late Cenozoic rates of origination
may have been enhanced by an external forcing mechanism such as changes in Orbital
Range Dynamics.Despite the enormous increase in biodiversity diversity are equally impressive contemporary
research in recent years we are still a long way biogeographical phenomena (Crame 2000a,b,
from understanding the true nature of the and references therein). They are of particular
larger-scale patterns of life on Earth. There is a interest as they are far less likely to co-vary with
general appreciation that, for many groups of any known environmental parameter than lati-
plants and animals, there are simply more of tudinal gradients (but see Bellwood & Hughes
them in the tropical and low-latitude regions 2001).
than in the high-latitude and polar ones, and We also lack a convincing integrated theory or
latitudinal gradients in taxonomic diversity model to account for the origins of large-scale
constitute one of the most striking macroeco- biodiversity patterns in both time and space
logical patterns at the present day (e.g. Gaston (Gaston 2000). Are they, as some have argued.,
& Blackburn 2000, and references therein). In the product of the 'ecological moment', repre-
addition we are also beginning to understand senting no more than the adjustment of regional
that tropical high diversity is distinctly hetero- biotas to climates and habitats that have devel-
geneous. Two tropical hotspots or high diversity oped since the Last Glacial Maximum (i.e. some
foci are often distinguished in the marine realm, 18 ka ago)? Or are there longer-term processes
one in the Indo-West Pacific (IWP) and the involved too, stretching back over a geological
other in the Atlantic, Caribbean and East Pacific timescale of millions or even tens of millions of
region (ACEP) (sensu Ellison et al. 1999). Anal- years? There has recently been a considerable
ogous patterns have now been established in development in our understanding of the ways
the terrestrial realm too, and in a global study of in which geological processes could influence the
the family richness of seed plants, amphibians, formation of large-scale biodiversity patterns,
reptiles and mammals, Williams et al. (1997) and this has led to an intense debate on the
found hotspots of maximum richness in Colom- relative roles of contemporary versus historical
bia, Nicaragua, Oaxaca (Mexico) and Malaysia, factors in the creation of taxonomic diversity
Tropical longitudinal gradients in taxonomic gradients (e.g. Turner et al 1996; Francis &
From: CRAME, J.A. & OWEN, A. W. (eds) 2002. Palaeobiogeography and Biodiversity Change: the Ordovician
and Mesozoic-Cenozoic Radiations, Geological Society, London, Special Publications, 194,153-168.
0305-8719/02/$15.00 © The Geological Society of London 2002.