The ammonite biostratigraphy of the Lower Lias ‘Armatum Bed’ (Upper Sinemurian—Lower Pliensbachian) at St Peterʹs Field, Radstock, Somerset

A temporary section exposed at Westfield, Radstock (Somerset) is described in detail. Here, the Lower Lias ‘Armatum Bed’ is divisible into five layers, each containing a discrete ammonite assemblage. A highly condensed basal conglomerate with a fauna representing much of the Echioceras raricostatum Zone is overlain by a thin limestone of Paltechioceras aplanatum Subzone age -possibly a faunal horizon. These are here grouped as the ‘Gleviceras Limestoneʹ. Above, three beds grouped as the ‘Apoderoceras Limestone’ show a succession of Apoderoceras sub-triangulare, A. ferox and A. sparsinodum. Hence, three ammonite horizons can be recognized, at least locally, within the Phricodoceras taylori Subzone, Uptonia jamesoni Zone. The systematics of Apoderoceras are discussed. The stratigraphical position of Fastigiceras clausum within the raricostatum Zone is determined, and its phylogenetic affinities considered. The frequent occurrence of Epideroceras with an acme in the aplanatum Subzone is noted and the stratigraphical position of Vicininodiceras, close to the Sinemurian-Pliensbachian boundary, confirmed. The occurrence of Tetraspidoceras in Britain is reported, and its stratigraphical and systematic positions considered, with the description of a new species, T. westfieldense.