Abstract
Abstract 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.
Highlights
Ammonites are iconic fossils fundamental to Mesozoic biostratigraphy, yet surprisingly little is known about their soft tissues and, their biology
Exceptional taphonomic pathways led to soft tissue preservation in some Jurassic coleoids through phosphatization or carbonization (Allison, 1988; Wilby et al, 2004; Klug et al, 2015)
In contemporaneous ammonoids soft part anatomy is primarily inferred from muscle attachment scars inside the shell (Mironenko, 2014) and hard parts housed in the buccal mass and associated with feeding (Keupp and Mitta, 2013; Klug and Lehmann, 2015; Smith et al 2021); indigestible remnants suggest a crop and stomach (Jäger and Fraaye, 1997)
Summary
Ammonites are iconic fossils fundamental to Mesozoic biostratigraphy, yet surprisingly little is known about their soft tissues and, their biology. Exceptional taphonomic pathways led to soft tissue preservation in some Jurassic coleoids through phosphatization or carbonization (Allison, 1988; Wilby et al, 2004; Klug et al, 2015). In contemporaneous ammonoids soft part anatomy is primarily inferred from muscle attachment scars inside the shell (Mironenko, 2014) and hard parts housed in the buccal mass and associated with feeding (Keupp and Mitta, 2013; Klug and Lehmann, 2015; Smith et al 2021); indigestible (skeletal) remnants suggest a crop and stomach (Jäger and Fraaye, 1997). The only known soft tissues from Jurassic ammonites are fragments of mantle and possibly gills in a cadocerid (Mironenko, 2015) and detached, flattened body masses of jaws and organs outside perisphinctid shells (Klug et al, 2021). Substantive three-dimensional preservation of ammonite soft parts is hitherto unreported
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