New data on the Lower Bajocian (Middle Jurassic) ammonite genus Mollistephanus Buckman, 1922 from southern England

Abstract

The dimorphic ammonite genus Mollistephanus Buckman, 1922 (type species: Mollistephanus mollis Buckman, 1922 [M]) is shown to range in southern England from the Discites to the Sauzei Zones of the Lower Bajocian (Middle Jurassic). The microconch counterpart is thought to be Albarracinites Fernandez-Lopez, 1985, previously described from Spain. The earliest species is taken to be ‘Docidoceras’ planulatum Buckman from the Discites Zone of Dorset. Similar but distinct forms of Mollistephanus in the succeeding Ovale Zone are named M. cockroadensis sp. nov. [M], The acme of the genus lies in the Trigonalis Subzone of the Laeviuscula Zone, the type horizon of its type species M. mollis in ammonite faunal horizon Bj-8b of the current biozonal classification. It is accompanied there by rare Albarracinites albarraciniensis Fernandez-Lopez [m]. Younger transients of Mollistephanus range upwards in the Laeviuscula Subzone, Bj-10, and into the lower Sauzei Zone, Bj-1 la. The assemblage from each horizon is monobiospecific. The origins of the genus and its relations to the parallel phyletic mainstem of the Stephanoceatidae, Stephanoceras s. l. , including another dwarf genus Phaulostephanus in the overlying Humphriesianum Zone, are discussed but remain unresolved.