Correlative tomography of an exceptionally preserved Jurassic ammonite implies hyponome-propelled swimming

Abstract

The discovery of exceptional soft-tissue preservation in the rear mantle of a Jurassic Sigaloceras ammonite (Cherns et al., 2021) represents an extremely significant window into ammonoid anatomy. However, it has implications beyond those explored in Cherns et al.’s paper, which relate largely to mobility, that should also be examined. This exceptional preservation has implications for the origin of the complex ammonite suture. The formation of ammonite sutures has been the subject of speculation for decades. This debate reached an impasse after two prevailing hypotheses—tie point and viscous fingering—became viewed as equally plausible (García-Ruiz et al., 1990). A third hypothesis was proposed by Inoue and Kondo (2016) in which the frilled pattern is generated by an uneven rear-mantle structure at the back of the soft tissue of the ammonite. Inoue and Kondo (2016) based this hypothesis on the growth patterns of sea slugs. Cherns et al.’s discovery sheds light on which of these theories can still be considered plausible and which should be discarded.