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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.)

Review questions

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?

Clarkson, E.N.K. 1998. Invertebrate Palaeontology and Evolution, 4th edn. Chapman and Hall, London. (An excellent, more advanced text, clearly written and well illustrated.)

Rigby, J.K. 1987. Phylum Porifera. In Boardman, R.S., Cheetham, A.H. & Rowell, A.J. (eds) Fossil Invertebrates. Blackwell Scientific Publications, Oxford, UK, pp. 116-39. (A comprehensive, more advanced text with emphasis on taxonomy; extravagantly illustrated.)

Rigby, J.K. & Gangloff, R.A. 1987. Phylum Archaeocy-atha. In Boardman, R.S., Cheetham, A.H. and Rowell, A.J. (eds) Fossil Invertebrates. Blackwell Scientific Publications, Oxford, UK, pp. 107-15. (A comprehensive, more advanced text with emphasis on taxonomy; extravagantly illustrated.)

Rigby, J.K. & Scrutton, C.T. 1985. Sponges, chaetetids and stromatoporoids. In Murray, J.W. (ed.) Atlas of Invertebrate Macrofossils. Longman, London, pp. 3-10. (A useful, mainly photographic review of the group.)

Scrutton, C.T. 1997. The Palaeozoic corals, I: origins and relationships. Proceedings of the Yorkshire Geological Society 51, 177-208. (First of two useful review papers.)

Scrutton, C.T. 1998. The Palaeozoic corals, II: structure, variation and palaeoecology. Proceedings of the Yorkshire Geological Society 52, 1-57. (Second of two useful review papers.)

Scrutton, C.T. & Rosen, B.R. 1985. Cnidaria. In Murray, J.W. (ed.) Atlas of Invertebrate Macrofossils. Longman, London, pp. 11-46. (A useful, mainly photographic, review of the group.)

Wood, R. 1999. Reef Evolution. Oxford University Press, Oxford, UK. (Comprehensive overview of reefs through time.)

References

Debrenne, F. 2007. Lower Cambrian archeocyathan bioconstructions. Comptes Rendus Palevol 6, 5-19.

Dewel, R.A. 2000. Colonial origin for Eumetazoa: major morphological transitions and the origin of bilateralian complexity. Journal of Morphology 243, 35-74.

Gill, I.P., Dickson, J.A.D. & Hubbard, D.K. 2006. Daily banding in corals: implications for paleoclimatic reconstruction and skeletalization. Journal of Sedimentary Research 76, 683-8.

Hammer, 0. 1998. Regulation of astogeny in halysitid tabulates. Acta Palaeontologica Polonica 43, 635-51.

Hou Xian-guang, Stanley, G.D. Jr., Zhao Jie & Ma Xiao-ya 2005. Cambrian anemones with preserved soft tissue from the Chengjiang biota, China. Lethaia 38, 193-203.

Kershaw, S. 1990. Stromatoporoid palaeobiology and taphonomy in a Silurian biostrome in Gotland, Sweden. Palaeontology 33, 681-706.

Riley, N.J. 1993. Dinantian (Lower Carboniferous) bio-stratigraphy and chronostratigraphy in the British Isles. Journal of the Geological Society, London 150, 427-46.

Savarese, M. 1992. Functional analysis of archaeocya-than skeletal morphology and its paleobiological implications. Paleobiology 18, 464-80.

Sperling, E.A., Pisani, D. & Peterson, K.J. 2007. Porif-eran paraphyly and its implications for Precambrian paleobiology. Special Paper Geological Society, London 286, 355-68.

Wood, R. 1990. Reef-building sponges. American Scientist 78, 224-35.

Wood, R. 2001. Biodiversity and the history of reefs. Geological Journal 36, 251-63.

Wood, R., Grotzinger, J.P. & Dickson, J.A.D. 2002. Proterozoic modular biomineralized metazoan from the Nama Group, Namibia. Science 296, 2383-6.

Wood, R., Zhuravlev, A.Yu., Debrenne, F. 1992. Functional biology and ecology of Archaeocyatha. Palaios 7, 131-56

Further reading

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