Some problems concerning the Precambrian-Cambrian transition and the Cambrian faunal radiation

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Abstract
In Siberia, the so-called Khailakh and Manykay Stages of ‘pre-Tommotian’ age are inferred to be partly coeval with the type Tommotian, and to be based on a misunderstanding about the control of facies on faunal distributions. Evolutionary patterns are being studied through this interval using a variety of approaches; synoptic reviews are presented on diversity changes, on changes in skeletal composition and on archaeocyathan morphology. The latter group appear to have originated on the Siberian Platform just prior to the Tommotian and migrated globally during the Atdabanian and Botomian to form secondary centres of diversification. Olenellid trilobites may have dispersed in a similar pattern but other groups (such as Volborthella and relatives) appear to have had a separate, siliciclastic centre of origin. The decline in diversity of small shelly fossils after the Tommotian, and the coeval expansion in archaeocyathans and trilobites, may be related to basinal changes connected with the break up of Palaeopangea.