The Aulacostephanidae (Ammonoidea) of the Oxfordian/Kimmeridgian boundary beds (Upper Jurassic) of southern England

Abstract:  Four areas in southern England centred on Swindon and Westbury (Wiltshire), Bourton (north Dorset) and the Dorset coast near Weymouth (south Dorset) have yielded well-preserved late Oxfordian and early Kimmeridgian (Upper Jurassic) ammonites in abundance. These ammonites belong principally to the aulacostephanid genera Ringsteadia and Pictonia , and their microconch equivalents Microbiplices and Prorasenia . Systematic descriptions of these genera are included herein. Within the zonal and subzonal sequence of the English Oxfordian/Kimmeridgian Stage boundary beds, the established subdivision of the Late Oxfordian Pseudocordata Zone into Pseudoyo, Pseudocordata and Evoluta Subzones is confirmed. In the Early Kimmeridgian Baylei Zone, however, the evidence is that the sequence throughout much of southern England is incomplete compared with more complete sequences such as that at Staffin in the Isle of Skye, with the presence of only one faunal biohorizon, the densicostata horizon.