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100 JOSEPH P. BOTTING
some extent, origination of novel life-habits
provides additional subsidiary niches, most
obviously in the development of epifaunal
tiering. Only through understanding detailed
diversity patterns can these influences possibly
be separated, a procedure that has been advo-
cated strongly by Miller (1997a, 1998, 2000).
The approach of Sepkoski (e.g. 1993) and
most others to global diversity changes has been
that of large-scale datasets, at intermediate
taxonomic levels (usually family or genus), but
also primarily in undivided continental or
global format. Unless specifically utilized in
subsets, global databases will generally provide
only broad patterns, where the influence of each
factor is disguised by its combination with
others. Complex patterns of local and regional
migration also confuse data on precise geo-
graphic origins; it has not yet been shown
whether high-diversity, stable late Ordovician
communities occupied the same environments
as the centres of diversification, or whether the
development of long-term global diversity is
independent of areas where rapid speciation
and overturn take place. Swain (1996), for
example, described exceptionally rapid ostra-
code turnover in a bentonite-rich succession,
where diversity at the top of the section did not
greatly exceed the initial, but standing diversity
at intermediate levels was much higher. Thus, in
order to recognize meaningful diversification
patterns reliably, overall diversity changes must
be examined on regional or even intra-basinal
scales (Miller 2000). Then, to identify the
significance of any given process, a precise
ecological-evolutionary signature must be
established, where possible, and then tested.
This is best examined by testing hypotheses in
limited regions, under the specific conditions
required for the testing. If a meaningful effect is
observed, locally or regionally, then large-scale
databases (e.g. Owen & McCormick 1999) can
allow assessment of the wider significance of
established local effects. Although many factors
may have acted simultaneously in promoting
the Ordovician Radiation, specific predictions
of major regional processes should be recogniz-
able on larger scales. Indeed, the ultimate
challenge in understanding the Ordovician
Radiation will be the extrapolation of known
small-scale patterns into a global situation,
where interactions between regions and pro-
cesses will require complex palaeogeography-
based analyses.
Among the many possible influences on
Ordovician diversity, the apparent correlation
with global pyroclastic volcanism has retained a
perennial but uncertain significance. Miller
(1997a) provided a correlation with global
tectonic/volcanic intensity, in which Palaeozoic
standing diversity appeared approximately to
match levels of orogenic activity. However,
correlation of this nature may represent a
relationship between standing diversity and
instantaneous tectonism, or between rapid
speciation and tectonism plus another factor,
which was restricted to the Ordovician.
Extreme fluctuations in tectonism during the
Permian, in a relatively stable part of the diver-
sity curve, and the lack of correlation in the
Mesozoic and Tertiary, lend some support to the
latter; the correlation is much more precise for
the Ordovician than for subsequent periods.
Similarly, Miller & Mao (1995) presented a geo-
graphical correlation of diversification centres
with forearc basins. Assuming some causal
relationship, it is unclear whether the volcanism
itself or associated tectonic activity was the
more significant influence on diversity. Miller
(1997a) suggested habitat partitioning as a
primary cause of diversification, in addition to
considering substrate transitions and nutrients,
while Swain (1996) preferred an increase in
nutrient supply to explain ostracode diversifica-
tion following major ash-falls. Increased nutri-
ent supply from run-off in uplifted areas is
perhaps inevitable, but the possible link to
diversification is unclear; in modern oceans,
abundant nutrients generally encourage low
diversity and very high dominance (e.g. Boyd et
al. 2000). In particular, a regional increase in
nutrient availability would tend to homogenize
subfacies variations for which scarcity of food
supply was critical for their definition. Para-
patric or sympatric speciation based largely on
behavioural food-acquisition procedures is.
intuitively, more likely to arise in situations
where food sources are relatively rare and
competition most intense.
This chapter introduces a mechanism by
which pyroclastic volcanism could have directly
influenced the speciation rate on regional scales,
with predictions for the evolutionary patterns to
be expected. The available global data sub-
divided by taxonomy (e.g. Sepkoski 1995)
appear to be consistent with these predictions
regarding the differences in diversity changes
between ecological groups. The hypothesis
potentially allows explanation of the differential
evolutionary rates between ecological/taxo-
nomic groups, particularly the decoupled
planktic-mobile benthic-sessile benthic diver-
sity curves. A preliminary test of the hypothesis
is included, using Welsh Basin ostracodes, and
similar methods are encouraged for those
investigating other hypotheses.