. Bulletin of the British Museum (Natural History), Geology. . SHELL ISLANDS high turbidity soupy substrates Fig. 1 Schematic cross section of the Western Interior Cretaceous Seaway showing regional palaeoenvironmental factors interacting to produce local habitat effects severely limiting bryozoan development, except where indicated by 'BRYOZ'. Western shoreline bryozoan occurrence exemplified by this paper and Cuffey et al. (1981); central occurrence by Cuffey (1990) and Gill & Cobban (1966); eastern occurrence by Kauffman (1986). present-day reef corals in the Indo-Pacific; Jokiel (1989)
. Bulletin of the British Museum (Natural History), Geology. . SHELL ISLANDS high turbidity soupy substrates Fig. 1 Schematic cross section of the Western Interior Cretaceous Seaway showing regional palaeoenvironmental factors interacting to produce local habitat effects severely limiting bryozoan development, except where indicated by 'BRYOZ'. Western shoreline bryozoan occurrence exemplified by this paper and Cuffey et al. (1981); central occurrence by Cuffey (1990) and Gill & Cobban (1966); eastern occurrence by Kauffman (1986). present-day reef corals in the Indo-Pacific; Jokiel (1989) has suggested that movement of rafted corals and drifted larvae occurs predominantly away from peripheral areas of low coral diversity (cf. Western Interior seaway) and towards centres of high diversity (cf. Europe) which act as areas of species accumulation. However, there is as yet no good evidence from temporal and geographical distributions to suggest that regions like the Western Interior seaway were source areas for the large numbers of bryozoan species present in Cretaceous rocks of Europe. Another observation which may have some bearing on the low diversity of bryozoan species in the Western Interior is that most (or all) of the cheilostome species described lack ovicells and can therefore be inferred to have possessed long-lived, planktotrophic, cyphonautes larvae (see Taylor, 1987, 1988). This contrasts with the European Upper Creta- ceous where the great bulk of species have ovicells and consequently possessed short-lived, non-planktotrophic, cor- onate larvae. Rates of speciation are predicted to have been higher in non-planktotrophic groups than in planktotrophic groups (Taylor, 1988), and it may also be significant that planktotrophic groups at the present day are subordinate to non-planktotrophic groups in most environments except nearshore and estuarine environments—as noted below (p. 16), the Drumheller bryozoans come from a brackish environment. GEOLOGICAL
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