modular sponge clonal sponge grade modular sponge
individualized colony (bilaterian)
colonial cnidarian (two branches)
colonial cnidarian (two branches)
Figure 11.38 A possible origin for bilaterians in the colonies? The process involves the development of multicellularity, followed by multifunctional modules (short arrows) and finally a shift in their functional morphology within the cnidarians and the bilaterians. (From Dewel 2000.)
1 Superficially sponges seem to be a compact morphological group but modern molecular data indicate that they are not mono-phyletic. Are there in fact morphological differences between the main sponge groups that back this up?
2 The archaeocyaths were some of the first metazoan reef builders, dominating the Early to Mid Cambrian tropics. How did their reef communities differ from the previous buildups of the Late Proterozoic Namapoikea and those later dominated by the corals and the stromatoporoids?
3 Tabulate corals were important framebuilding organisms during intervals in the Paleozoic. Is there any evidence to suggest that they were associated with zooanthellae?
4 What do aberrant cnidarian taxa such as Archisaccophyllia and Kilbuchophyllum tell us about the possible track of coral evolution?
5 Metazoan reefs have been an important part of the marine ecosystem since the Early Cambrian. But during intervals of extreme stress, for example just after severe extinction events, such reefs disappear and the planet momentarily returns to a "stromatolite world". How can such an ecosystem, most characteristic of the Proterozoic, re-establish itself?
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