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

The extreme rarity of soft-tissue preservation in ammonoids has meant there are open questions regarding fundamental aspects of their biology. We report an exceptionally preserved Middle Jurassic ammonite with unrivaled information on soft-body organization interpreted through correlative neutron and X-ray tomography. Three-dimensional imaging of muscles and organs of the body mass for the first time in this iconic fossil group provides key insights into functional morphology. We show that paired dorsal muscles withdrew the body into the shell, rather than acting with the funnel controlling propulsion as in Nautilus. This suggests a mobile, retractable body as a defense strategy and necessitates a distinct swimming mechanism of hyponome propulsion, a trait that we infer evolved early in the ammonoid-coleoid lineage.

Automated summary

The study of a Middle Jurassic ammonite with exceptionally preserved soft-body structure provides valuable insights into the functional morphology of these iconic fossil group. The findings suggest a defense strategy of retracting the body into the shell with paired dorsal muscles, rather than propulsion through the funnel as seen in Nautilus. This indicates a distinct swimming mechanism of hyponome propulsion early in the evolution of ammonoid-coleoid lineage.