Ammonoid Buccal Mass and Jaw Apparatus

Abstract

Current knowledge on the ammonoid buccal mass and jaw apparatus is synthesized based on in situ fossil records from 109 genera that are distributed in 30 superfamilies of 8 suborders of Devonian to Cretaceous age. As in those of modern cephalopods, the jaw apparatus of ammonoids consists of upper and lower elements. Comparative anatomical examination of the attachment scars of chitin-secreting cells (beccublasts) on the jaw lamellae allow us to reconstruct the buccal mass structure of ammonoids. The jaw apparatuses of ammonoids can be classified into the normal, anaptychus, aptychus, intermediate, and rhychaptychus types based mainly on the differences in overall morphology and lamellar composition in the lower jaws. The fairly large variation of the jaw morphology and the variety of food remains preserved in the digestive tract and buccal cavities in Mesozoic ammonoids may reflect diversity of feeding and diets ranging from predatory via scavenging to microphagous habits.