Shell wall structure and sharp-edged apertural shell margin in the Callovian Quenstedtoceras (Cephalopoda, Ammonoidea)

Abstract

The structure of the shell wall, particularly at the apertural margin, is described in the Callovian ammonoid Quenstedtoceras from Lukow, Poland. Ventral and lateral sides of the shell wall show periodic interruptions in shell secretion. As shown in the present study, these interruptions divide the shell wall into calcareous sublayers that are separated by thin intermediate layers of organic substance. In places, the organic layers increase considerably in thickness. The presence of organic layers between the calcareous sublayers increases the amount of organic substance in the shell wall. The shell aperture has a ventral rostrum. At the shell aperture, the shell wall decreases in thickness and forms a sharp-edged, thin, apertural margin that probably retains its shape in the adult stage. In comparison with Nautilus, the shell wall of Quenstedtoceras is much thinner at the aperture, has a longer growth zone and probably does not form a gerontic thickening. The shape of the apertural margin of Quenstedtoceras is also compared with that of the Callovian ammonoid Kosmoceras and of scaphitid ammonoids. Its significance in ammonoid anatomical design and function is discussed.

Keywords::

• apertural margin
• shell wall structure
• ventral rostrum
• ammonoids
• Quenstedtoceras

Acknowledgements

The author thanks Dr P. Weaver, North Carolina Museum of Natural Sciences, USA, for linguistic and stylistic corrections of the manuscript, Dr C. Klug, Paläontologisches Institut und Museum, Zürich, Switzerland, and Dr N. H. Landman, American Museum of Natural History, New York, USA, for valuable comments and improvement of the manuscript.