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158 J. A. CRAME & B. R. ROSEN

Causes of Cenozoic biodiversification

It would appear then that if we are to find a satis-

factory explanation for the global Cenozoic bio-

diversification event, it must be one that involves

a synchronous increase at all four basic levels

(i.e. alpha, beta, gamma and delta diversity). In

other words this must be a process that allows

both an increase in the numbers of taxa within

individual habitats as well as increased differen-

tiation between these habitats on a variety of

geographical scales. For example, at one of the

very simplest levels it has been postulated that

the Cenozoic rise in biodiversity could be linked

to a global rise in trophic resources provided by

the radiation of the angiosperms (Bambach

1977). Even in the marine realm the increased

flux of plant detritus through river systems may

have raised benthic productivity. Similarly, it has

been suggested that oceanic nutrient cycling

may have been influenced by changes in the

global thermohaline circulation system brought

about by the formation of Antarctic deep

bottom waters at the Eocene-Oligocene bound-

ary (Bambach 1977).

Such ideas are indeed appealing, and we

should not lose sight of the fact that there may

well have been times of abrupt increase in both

terrestrial and marine productivity through

Earth history. Nevertheless it is by now well

established that there is not necessarily a direct

link between increased productivity and

increased biodiversity. An increase in produc-

tivity can trigger an increase in biomass, but

there is no reason why this should not, in

turn, just be within one or a small number of

species. Something else is required to generate

a large number of new taxa from a rise in

productivity (Blackburn & Gaston 1996). We

also have to bear in mind that any potential

mechanism for Cenozoic global biodiversifica-

tion must explain the greater relative production

of new taxa in the low- than in the high-latitude

regions. In their discussion of the origin and

diversification of major taxonomic groups,

Jablonski & Bottjer (1990) identified five basic

explanations for global radiations. We might

rationalize these into two main types: intrinsic

and extrinsic.

Intrinsic mechanisms of diversification

One of the simplest explanations for global

Cenozoic diversification would be that it rep-

resents the coincidental expansion of a series of

unrelated clades. Although key adaptive break-

throughs, such as mantle fusion and the develop-

ment of posterior siphons in bivalves, or the

development of plants with flowers, undoubtedly

occurred well before the Cenozoic Era, it may

have taken periods of tens of millions of years for

such clades to build up substantial numbers of

taxa (Jablonski & Bottjer 1990). Once certain

groups became established in the latest Meso-

zoic/earliest Cenozoic they may in turn have

triggered the automatic radiation of others.

Angiosperm assemblages would have provided a

variety of novel terrestrial habitats, and the rapid

development of coral reefs, sea-grass beds and

mangroves at this time could have promoted

diversification in shallow, tropical seas (Verrneij

1977).

It is also possible that intense biological inter-

actions between various taxonomic groups were

a driving force behind diversification. In particu-

lar, if biological hazards due to competitors and

predators have increased through time, then so

may the responses of various prey taxa to them.

The net result is a sort of 'evolutionary arms

race', with first one group gaining a numerical

ascendancy, and then the other (Vermeij 1987,

1994). This special form of co-evolutionary

relationship, known as escalation, emphasizes

the importance of enemies as agents of natural

selection: over periods of geological time

enemy-related adaptations bring about long-

term evolutionary trends in the morphology,

ecology or behaviour of other organisms. It is

essentially a tropical phenomenon and is

particularly well represented in species-rich

groups, such as angiosperms, arthropods, verte-

brates and molluscs, that have highly developed

competitive and defensive capabilities (Vermeij

1987). Striking examples would appear to be the

co-radiation of angiosperms and pollinating

insects in the terrestrial realm, and the link

between durophagous predators and both

deeper-burrowing infauna and more heavily

ornamented epifauna in the marine one.

Although escalation seems to be an intuitively

satisfying explanation for certain tropical radia-

tions, some doubts have been raised as to its

widespread efficacy. For example, Vermeij

(1987) himself drew attention to the low inci-

dence of damage repair within many infaunal

bivalve groups, and the high-resolution fossil

record in general has comparatively few

examples of gradual morphological change

induced within a prey lineage (Hansen et al. 1999;

Dietl et al 2000; Miller 2000). Many biotic tran-

sitions within the fossil record could equally

well be interpreted as the replacement of -

incumbent taxa by some form of fundamental

environmental change (Jablonski 2000). Non-

competitive expansions may be just as common

through geological time (Benton 1999).
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