Volker Dietze

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

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.

The hansrieberi biohorizon (Aalenian; Opalinum Zone) in the Opalinuston Formation of Donzdorf-Grünbach (Eastern Swabian Alb, Germany)

Abstract The lithostratigraphy and ammonite fauna of the Early Aalenian Opalinuston Formation (Zillhausen Member) at Donzdorf-Grünbach is described. A new chronospecies of Leioceras, L. hansrieberi, is erected. It characterizes the hansrieberi biohorizon of the Opalinum Subzone (Opalinum Zone). The macroconchs of L. hansrieberi represent the largest specimens hitherto described within the genus Leioceras. In addition, the associated macrofauna is described which allows to draw some conclusions on the palaeoecology of this locality.

First find of aptychi of Leioceras and Bredyia (Ammonoidea, Hildoceratoidea) in the Aalenian of Northern Caucasus, Russia

Aptychi are reported from the Lower Aalenian Leioceras opalinum Zone and Subzone of the Khussa-Kardonik section in Karachay-Cherkessia (Kuban River Basin). They are associated with conchs of the ammonites genera Leioceras and Bredyia (superfamily Hildoceratoidea) and interpreted as their lower jaws. The Hildoceratoidea aptychi are referred to a form group defined by Trauth as ‘cornaptychi’. Wide valves with a straight apical angle (morph I) probably belong to Bredyia. Aptychi with an acute apical angle (wide in the basal part; morph II) and relatively narrow ones (morph III) belong to macroconchs and microconchs of Leioceras, respectively. Aptychi assignable to Leioceras opalinum are herein also reported from Southern Germany.