Shortly before or during Tremadoc time the majority of pre-existing trilobite families died and were replaced by new, short-lived groups and early members of dominant Ordovician families. Endemic forms define six faunas separated into northern and southern provinces, without regard for enclosing lithic type. Constancy of distribution from Arenig to Llandeilo implies physical stability. Reduced endemism in the Caradoc and Ashgill probably reflects reduced barriers and broad oceanic circulation.

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