Oxyconic members of the Dimeroceratidae (Goniatitida, Tornoceratina) are included in a new subfamily, the
Paratornoceratinae n. subfam. The group occurred globally in almost all rich ammonoid faunas of the higher
part of Upper Devonian II (UD II-F to II-I, Nehdenian).  Twosystematically excavated populations from
Dar Kaoua in the central part of the Tafilalt (eastern Anti-Atlas, Morocco) are morphometrically
analyzed as an example to clarify the significance of intraspecific variability, potential chronomorphoclines
and of distribution patterns in a supposedly species-poor but variable oxyconic ammonoid group.  Results
showed that there are two clearly distinct species, Acrimeroceras falcisulcatum BECKER and Acri. stella
n. sp. which immigrated the region episodically within one zone.  Taxonomic complexity and diversity is
much higher in the group than previously assumed.  Shell features and restricted variability support an
autecological interpretation as fully pelagic active swimmers which could evolve gradually from suprabenthic
Dimeroceratinae after the extinction of oxyconic Tornoceratidae. Size distributions suggest that shell growth
continued throughout life without reaching a final maximum diameter unlike in other ammonoids.