New Kimmeridgian ammonite fauna from east Spitsbergen and its phyletic significance

Abstract

A rich Kimmeridgian ammonite fauna is described from the Janusfjellet Subgroup at Holmgardfjellet, Agardhbukta, east Spitsbergen. The fauna collected from a narrow stratigraphic interval represents a single ammonite assemblage showing a wide but continuous range of variability, with end-forms strongly resembling Amoeboceras (Hoplocardioceras) decipiens Spath and A. (Amoebites) elegans Spath, respectively. Accepting a horizontal classification in ammonite taxonomy, the assemblage may easily be placed into a single species for which the name Amoeboceras (Amoebites) uralense Mesezhnikov, 1973, is available, as this form corresponds to the intermediate and the most common morphotype. The fauna with A. uralense is possibly directly below that with A. elegans and A. decipiens, but above that with A. kochi. Close phyletic relations are suggested between the A. elegans - A. uralense group (traditionally placed in the subgenus Amoebites) and the subgenera Hoplocardioceras (type species A. decipiens), and Euprionoceras (type species A. kochi).