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 unrivalled information on soft body organization interpreted through correlative neutron and X-ray tomography. 3D-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 defence strategy and necessitates a distinct swimming mechanism of hyponome propulsion, a trait that we infer evolved early in the ammonoid–coleoid lineage.
Kwa sababu ya nadra sana ya kuhifadhi tishu laini katika ammonoidi, kuna maswali mengi kuhusu mambo ya msingi ya biolojia yao. Ammoniiti ya Jurassic ya Kati iliyohifadhiwa kwa njia ya kipekee na habari isiyo na kifani juu ya shirika la mwili laini, iliyofafanuliwa kupitia neutron na tomografia ya X-ray. 3D-imaging ya misuli na viungo vya mwili kwa mara ya kwanza katika kundi hili la ikoni ya visukuku hutoa ufahamu muhimu katika morphology ya kazi. Kwa mfano, kuna misuli ya mgongo ambayo huunganisha mwili na kifuniko, badala ya kutumia funnel kama ilivyo katika Nautilus. Hii inaonyesha mwili wa kuhamisha, retractable kama mkakati wa ulinzi na inahitaji tofauti kuogelea utaratibu wa hyponome propulsion, tabia ambayo sisi infer ilibadilika mapema katika ammonite-coleoid ukoo.