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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
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