VOLCANISM AND DIVERSIFICATION 111
number of sites. If the patterns observed for
ostracodes reflect sampling bias resulting from
areal exposure and preservation, the same
patterns should be seen among 'inarticulate'
brachiopods. Figure 7 shows the pre-Caradoc
'inarticulate' data for Mid-Wales and Shrop-
shire. This reveals a more gradual increase in
diversity, with a significant number of Tremadoc
and Arenig species, although the rate of species
appearance also increased at the time of
Llanvirn volcanism, including similar absolute
origination. However, it is likely that many more
ostracode species remain undiscovered than
brachiopods, due to their inconspicuousness. A
significant proportion of the ostracodes encoun-
tered during fieldwork were undescribed,
whereas brachiopods were almost always
identifiable. Although at an early stage, this
initial analysis suggests a local influence of
volcanism on diversification, which affected
ecological or taxonomic groups in different
ways, these differences being consistent with
ecological studies of the local effects of volcanic
ash deposition.
Conclusions
The local effects of volcanic ash deposition on
planktic and benthic communities in dysaerobic
shelf/basin settings have been summarized. The
resultant elimination of benthos, recolonization
from lateral areas, and bloom events in both
plankton and benthos, have been analysed in
terms of population genetics, and shown to
comprise a conceptual method for encouraging
speciation. Physical conditions necessary for this
process were maximized during the Ordovician,
leading to comparisons with the Ordovician
Radiation. If the process were a significant
factor in this diversification event, predictable
differences would be expected between taxo-
nomic and ecological groups, which may be
assessed by comparison with observed palaeo-
ecological responses, and reasoning from
modern ecological models. A series of such pre-
dictions is provided, with caveats in the case of
ambiguous groups. This is illustrated by a sub-
basin-scale analysis of ostracode diversity, cor-
related with volcanism, and compared with the
less dramatic diversification of 'inarticulate'
brachiopods to reduce concerns of sampling
bias. A possible distinction between those facies
showing very high speciation and extinction
rates, and others able to adopt more modest
long-term diversity increases is also suggested.
Speciation is viewed as occurring largely in the
mid to outer shelf regions, but within an eco-
system insufficiently complex to support this
diversity following environmental stabilization.
However, onshore expansion of new species
may have contributed to the increasing diversity
of more complex shallow-water communities
that typify the Palaeozoic Evolutionary Fauna,
resulting in a gradual offshore expansion of
these ecosystems.
This chapter presents a mechanism that
potentially explains large parts of the Ordo-
vician diversification, with predictions. Readers
are encouraged to present additional data as a
test of the hypothesis, in order to establish its
validity. Even if the hypothesis is disproved, the
use of fine-scale diversity and palaeobiogeogra-
phy databases is suggested as an essential pro-
cedure for understanding processes involved in
the diversity changes. Extrapolation and averag-
ing over larger scales will hopefully then allow
elucidation of global database results.
Development of the population genetics section was
greatly aided by discussion with W. Amos (Zoology,
University of Cambridge), while D. J. Siveter (Uni-
versity of Leicester) and A. W. Owen generously
provided 'in press' data regarding ostracodes and
trilobites respectively. The palaeoecological study was
performed in partial fulfilment of a PhD at the
University of Birmingham, during which discussions
with A. T. Thomas and M. P. Smith are particularly
acknowledged. The manuscript was improved by
constructive reviews from A. I. Miller and D. J.
Siveter. Research is currently funded by Christ's
College, Cambridge. This paper is a contribution to
IGCP 410 The Great Ordovician Biodiversification
Event'.
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