Boundary Working Group in 1984, with Convenors René Mouterde

partly by integrating also other dating and correlation techniques. During the Triassic/Jurassic boundary interval there was a major extinction event (that almost resulted in the demise of the ammonoids) associated with a large global sea-level fall, tectonic and volcanic events. As a result there are very few sections with a continuous sequence of marine strata containing key fossils. From a historical perspective, in the geologically classic areas of NW Europe there was a major palaeogeographic change from predominantly continental non-marine sedimentation in the Triassic to mainly marine environments in the Jurassic. Background The degree of uncertainty about the Triassic/Jurassic boundary can be illustrated by reference to the Rhaetian Stage, variously placed in the Triassic or the Jurassic- compare, for example, the classic and influential works of W.J.Arkell. In his earlier review (Arkell 1933) of the Jurassic of Britain, the Rhaetian (White Lias etc.) was included in the Jurassic because it is marine or marginal-marine. However, with a broader perspective in his later review (Arkell 1956) of the Jurassic of the World the Rhaetian was placed in the Triassic because the faunas are of Triassic character, for example ceratites rather than ammonites (sensu.sticto). Serious investigation and the search for a definition of the base of the Jurassic began with the establishment by the International Subcommission on Jurassic Stratigraphy (ISJS) of a Triassic/Jurassi