THE BASE OF THE JURASSIC SYSTEM - NEW DATA ON THE FIRST APPEARANCE OFPSILOCERAS-PLANORBIS IN SOUTHWEST BRITAIN

Recent collecting in the Lower Lias of localities at St Audrie's Bay, north Somerset, Sedbury Cliff, Gloucester and Lavernock Point, South G lamorgan has yielded Psiloceras planorbis (J. de C. Sowerby) in beds c onsidered by some to be of Rhaetian, Triassic age. These are the earli est records of this ammonite to date in southwest Britain. Evidence is provided of a major facies change at the top of the pre-planorbis bed s followed by the diachronous first occurrence of Psilocelas planorbis . These new data have implications for the use of earliest occurrence of Psiloceras planorbis as a chronostratigraphic marker in defining th e base of the Jurassic System, and for the definition of the base of t he Hettangian Stage in the recently proposed candidate Global Stratoty pe Section at St Audrie's Bay.