Cambrian fauna

Figure 2.8 Reconstruction of a Middle Cambrian sea floor about 600,000,000 years ago. The fauna includes siliceous sponges (the upright cones), jellyfish, and two genera of trilobites (Paradoxides, the large form, and Eliipsocephalus, the small form). Bacteria had probably already been in existence for millions of years.

In the last twenty years, in conclusion, paleontology has discovered that in Cambrian times there have been not one but three different explosions ofanimallife one documented by trace fossils, a second which left behind small shelly fossils, and finally the classical explosion that was dominated by trilobites. It must also be added that Cambrian life was preceded by the so-called Ediacara fauna, a vast assembly of soft-bodied animals (almost all with radial symmetry). Many scholars now regard them as a failed evolutionary experiment, while others believe that they may have left modified descendants (Figure 7.1). 

Brasier M., Green O., and Shields G. (1997) Ediacarian sponge spicule clusters from southwestern MongoUa and the origins of the Cambrian fauna. Geology 28, 303—306. 

Fig. 1.13 Diversity trends in (a) marine fauna with preserved skeletal hard parts (C = Cambrian fauna, P = Palaeozoic fauna, M = Modern fauna, = major extinctions after Sepkoski 1984) (b) all organisms (variation in line thickness reflects uncertainty in taxonomic differentiation after Benton 1995).

Clarkson PD, Hughes CP, Thomson MRA (1979) Geological significance of a Middle Cambrian fauna from Antarctica. Nature 279 791-792... 

Figure 31.3 Graphic summary of latitudinal brachiopod distributions through the Cambrian and early Ordovician and relationships with Cambrian and Palaeozoic evolutionary faunas. Mode of life of shallow-water carbonate platform morphotypes after Sanchez and Toffolo(l996).

Popov, L.E. and Holmer, L.E. (1994) Cambrian-Ordovician lingulate brachiopods from Scandinavia, Kazakhstan, and South Ural Mountains. Fossils and Strata, 35, 1-156. Sepkoski, J.J. and Miller, A.I. (1985) Evolutionary faunas and the distribution of Paleozoic marine communities in space and time. In Valentine (ed.) Phanerozoic Diversity Patterns (Princeton, New Jersey Princeton University Press), pp. 153-190. 

Cambrian chordates have been documented from two Konservat Lagerstdtten (cases of exceptional soft-tissue preservation)—the Early Cambrian Chengjiang Lagerstdtte of Kunming, South China, and the Middle Cambrian Burgess Shale of British Columbia, Canada. In both cases, the animals interpreted as chordates constitute a very minor part of the faunas, which are dominated by arthropods and a number of worm phyla. 

The first major diversification of vertebrates, represented by both Anatolepis and conodonts, was underway by Late Cambrian time. This evolutionary radiation event continued across the Cambro-Ordovician boundary in the case of conodonts, with faunas reaching diversities of 30-40 species by the Arenig (480 Ma). 

Chen, J.-Y. and Zhou, G.-Q. (1997) Biology of the Chengiang fauna , in Chen, J.-Y, Cheng, Y.-N. and Iten, H.V. (eds) The Cambrian Explosion and the fossil record. Bulletin of the National Museum of Natural Science, Taichung, Taiwan National Museum of Natural Science, pp. 11-105. 

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