VOLCANISM AND DIVERSIFICATION 109
Fig. 6. Ostracode range chart for North Wales and northern England. See Figure 4 for explanation.
enhanced speciation over the intervals contain-
ing frequent ash beds in each area, and low
diversity outside these episodes. Although no
similar comparison has been made with areas
lacking ash beds, the correlation of diversity
with volcanism appears to be precise in the
current data; this view is reinforced by the
marked differences between adjacent regions.
The question of sampling bias must be
addressed in this situation, since volcanic areas
tend to preserve more resistant lithologies,
which may aid the preservation and exposure of
fossils. However, if this is true, identical patterns
should emerge among taxonomic groups that
shared similar environments and are extracted
by similar processes. The ostracode data derive
largely from silicified specimens extracted by
acid digestion of limestones, and from mouldic
material in siliciclastic sedimentary rocks.
Sutton et al. (1999-2000) monographed Welsh
Basin lingulid brachiopods, using very similar
material from a similarly comprehensive
