The persistence of Burgess Shale-type faunas: implications for the evolution of deeper-water faunas

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Abstract
Discoveries, most of them recently, in more than thirty Lower and Middle Cambrian horizons with soft-bodied fossils have shown that forty-one of the genera occur also in the celebrated Burgess Shale (Middle Cambrian). Significantly, they tend to have lengthy stratigraphic durations which together encompass an interval from the early Lower Cambrian (Tommotian) to near the end of the Middle Cambrian. At least some genera have also wide geographical ranges, with occurrences around much of the Laurentian (N America) craton, and also in N and S China, Australia, Siberia, Spain and Poland. Although a few genera, e.g.Isoxys, may have been pelagic, for the most part these distributions are explained in terms of a deeper-water biota with an evolutionarily conservative aspect. Both the origins and further recruitment to this biota may have been from shallower water, with more limitedin situdiversification. It is speculated that this distinctive Cambrian biota was gradually driven to extinction with the arrival of Ordovician competitors, although some relics may have survived until at least the Devonian. This history has implications for our understanding of deeper-water faunas throughout the Phanerozoic, and supports the notion that archaic forms may take refuge in this environment.