Occluded Umbilicus In The Pinacitinae (Devonian) And Its Palaeoecological Implications

Eifelian (Middle Devonian) ammonoids of the Pinacitinae Hyatt, 1900 ( Exopinacites , Pinacites )with preserved shell structures from the eastern Anti–Atlas (Morocco) have revealed unusual morphological features. The Pinacitinae belong to the earliest ammonoids which closed their umbilici. As an approach to an interpretation of these structures, the representatives of the subfamily Pinacitinae ( Exopinacites singularis , Pinacites jugleri , P. eminens ) are compared with other ammonoids, e.g. Acrimeroceras , Araucanites , Clistoceras , Gaudryceras , Nathorstites , Prolobites , and Synpharciceras , which produced umbilical plugs and covers. Some of these are comparable in structure to Nautilus pompilius and N. belauensis . In contrast to all of these taxa, the lateral shell wall of the Pinacitinae reached the centre of the umbilicus and formed an umbilical lid. The umbilical shell wall rests on the umbilical lid of the previous whorl. This construction probably had the advantage that it improved the hydrodynamic properties of the conch, along with the oxyconic conch shape and the approximately horizontal orientation of the aperture.