VOLCANISM AND DIVERSIFICATION 105
with diversification centres on a regional to
basin scale. If the proposed process was signifi-
cant, this should be supported by the global
database results; for example, areas of diversifi-
cation should be dominated by volcanically
active regions showing stratification. The diver-
sity increases in other areas should then be
shown to occur largely through immigration of
previously existing taxa.
A further set of predictions relates to the
differential rates of evolution between different
ecological and taxonomic groups. In these cases,
the evolutionary response of each group can be
predicted from the established ecological
response, in that those groups showing the
highest abundance fluctuations and most limited
individual geographic ranges should respond
most strongly to the variations in population
genetics outlined above. Temporary local eradi-
cation followed by high-amplitude blooms from
rapid recolonization should have maximized
the evolutionary response. These ecological
responses were presumably dependent on
several factors, including larval and adult life
habits, trophic group, size, reproductive strategy
and developmental time-scales, most of which
can only be estimated. Observations of palaeo-
ecological response remove the complications of
reasoning about factors that may be difficult to
assess in extinct taxa. The predicted evol-
utionary responses are described according to
broad ecological divisions.
(1) Small sessile benthos and essentially immo-
bile epibenthos: extensive responses, appar-
ently completely eradicated by substantial
ash deposition, but with rapid recoloniza-
tion in a high-amplitude bloom. Includes
bryozoans, small brachiopods ('articulate*
and 'inarticulate'), ostracodes, sphenothal-
lids. (Ostracodes are included here because
their small size allows very restricted move-
ment. However, they were apparently able
to escape minor ash deposition, while small
sessile taxa were not, and recolonized a
region more rapidly.)
(2) Intermediate to large sessile benthos: signifi-
cant responses, with local eradication and
high-magnitude blooms. May escape slight
ash deposition. Includes crinoids, sponges,
bryozoans, larger brachiopods, conulariids.
Although generally rare, some examples of
this group (sponges, brachiopods, bryo-
zoans) gave rise to brief, intense blooms
(Botting 2000).
(3) Weakly mobile epi- and endobenthos
(excluding ostracodes): often able to escape
deposition of small ash volumes, but bloom
Fig. 3. Summary of differing diversification patterns
by phylum based on Sepkoski (1995), reproduced by
kind permission of the Pacific Section - SEPM. (a)
Taxa in groups 1, 2 and 3 (part) (see text), exhibiting
strong ecological response to ash-fall; (b) taxa in
groups 3 (part), 4 and 5 (see text), exhibiting lesser
ecological response to ash-fall, or existing as
plankton only.
events not obvious due to small population
size. Includes bivalves, gastropods, palaeo-
scolecids, larger lingulids. Their response
may be underestimated here because of
lower preservation potential of aragonitic
and multi-element skeletons.
(4) Mobile epibenthos/nekton: limited
response; little noticeable effect on abun-
dance across ash beds. Includes trilobites,
nautiloids, although aragonitic nautiloids
may be underestimated because of reduced
preservation.
(5) Plankton and pelagic nekton: affected by
extensive blooms, but little or no prior
destruction; rapid migration prevents seg-
regation into genetically distinct communi-
ties, and isolated populations implausible.
Includes graptolites, pelagic trilobites,
acritarchs (and some chitinozoans?).
Further complications include the tolerance
of some taxa to low oxygenation, and ambiguous
interpretations of life habits; 'inarticulate'
brachiopods often appear in dysaerobic settings,
and some were almost certainly pseudoplanktic
