Abstract: Continuously fossiliferous sections in south Wales and Shropshire spanning the base of the Upper Ordovician permit precise correlation of the Nemagraptus gracilis Biozone, a horizon currently under scrutiny for the selection of a Global Stratotype Section and Point (GSSP). Using graphic correlation we have compiled a new composite global standard for the Middle–Upper Ordovician, which shows that the first occurrence of the key graptolite N. gracilis is not everywhere of the same age, appearing earlier in North American successions than it does in Britain. Species of Dicellograptus, by contrast, appear to be almost synchronous in their first appearance. The status and correlation of standard British biozones through the Llandeilian–Caradoc interval, based on graptolites and trilobites, are revised. Suitable sections for the definition of bases of biozones are proposed. If the base of the N. gracilis Biozone is accepted internationally as the base of the Upper Ordovician, this biozone being coincident with the base of the Caradoc Series, there are sections suitable for its precise definition in the British succession.

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