Two ammonites have been found in porcelaneous limestones of the White Lias Formation traditionally considered to be of Rhaetian (Late Triassic) age at Lavernock Point, south Wales (ST 181 681). These ammonites, named here Neophyllites lavernockensis. sp. nov, are the earliest recorded from the UK. This horizon is located directly above a major negative δ13C isotope anomaly (CIE1) in the Upper Cotham Member that marks the top of the Triassic System and below another negative δ13C isotope anomaly (CIE3). Both correlate with negative δ13C isotope anomalies in the Triassic–Jurassic global boundary stratotype section and point (GSSP) at Kuhjoch, Austria. This establishes the base of the tilmanni Chronozone, Hettangian Stage and Jurassic System in SW Britain within the White Lias Formation at Lavernock Point.