Shell ultrastructure, muscle-scars, and buccal apparatus in ammonoids

Abstract

(1) The Cretaceous genus Aconeceras has large ventro-lateral muscle-scars in addition to smaller dorsal and ventral scars. This indicates that some ammonoid groups at least probably had strong retractor muscles to the head and funnel, and that their muscle system was more complex than that in the Recent Nautilus. (2) Radula of Aconeceras has multicuspid teeth, known among recent cephalopods only in bathypelagic octopods. (3) In the Cretaceous heteromorphic genus Ptychoceras, repetition of shell layers and mode of truncation of initial portion of shell indicate that the shell surface was covered by soft tissue, at least in apertural region. (4) Shape of shell aperture in many ammonoids differs considerably from that in Nautilus by lack of hyponomic and ocular sinuses. This may indicate principal morphological differences in the head-funnel region of the body between ammonoids and Nautilus. (5) Transition from retro- to prochoanitic septal necks, which took place with different evolutionary rates in different ammonoid orders, improved the osmotic pumping system, and made it possible to form a new portion of siphuncle prior to secretion of next septum.