Since the 19th century geologists working on Cambrian rocks world-wide have relied\ud largely on trilobites as biostratigraphical guides, and these remain important,\ud especially in Series 3 and the Furongian Series, where they enable refined\ud correlations. In Series 2, especially in its lower part where trilobite biostratigraphy\ud becomes increasingly difficult, other criteria are employed, for example the\ud distribution of small shelly fossils, bradoriid arthropods and, where available,\ud archaeocyaths. In Avalonia there are no archaeocyaths, but the bradoriids have been\ud revised and exploited stratigraphically (Siveter & Williams 1997, Williams & Siveter\ud 1998). The recent development of acritarch biostratigraphy, which has made a vital\ud contribution, is discussed below. Towards the base of the Cambrian, body fossils may\ud be very scarce and trace fossils have been used biostratigraphically (Narbonne &\ud Myrow 1988; Bland & Goldring 1995; McIlroy & Horák 2006), though they may be\ud difficult to work with. All the zones referred to are biozones (Rawson et al. 2002),\ud most of those in the Terreneuvian and Series 2 and 3 being assemblage zones, whilst\ud those in the Furongian are local range-zones of selected species
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