3 D - Analysis of a non - planispiral ammonoid from the Hunsrück - Slate : natural or 1 pathological variation ? 2

9 We herein examine the only known non-planispirally coiled early Devonian ammonoid, the 10 holotype specimen of Ivoites opitzi, to investigate if the host was encrusted in vivo and if 11 these sclerobionts were responsible for the trochospiral coiling observed in this unique 12 specimen. To test if the presence of runner-like sclerobionts infested the historically 13 collected specimen of Ivoites opitzi during its life, we used microCT to produce a three14 dimensional model of the surface of the specimen. Our results indicate that sclerobionts 15 grew across the outer rim (venter) on both sides of the ammonoid conch at exactly the 16 location where the deviation from the planispiral was recognized, and where subsequent 17 ammonoid growth would likely preclude encrustation. This indicates in vivo encrustation of 18 the I. opitzi specimen, and represents the earliest documentation of the phenomenon. 19 Further, this suggests that non-planispiral coiling in I. opitzi was pathologically induced and 20 does not represent natural morphological variation in the species. Despite the observed 21 anomalies in coiling, the specimen reached adulthood and retains important identifying 22 morphological features, suggesting the ammonoid was minimally impacted by encrustation 23 in life. As such, appointing a new type specimen – as suggested by some authors – for the 24 species is not necessary. In addition, we identify the sclerobionts responsible for modifying 25 the coiling of this specimen as hederelloids, a peculiar group of sclerobionts likely related to 26 phoronids. Hederelloids in the Devonian are commonly found encrusting on fossils collected 27 in moderately deep environments within the photic zone and are rarely documented in 28 dysphotic and aphotic samples. This indicates that when the ammonoid was encrusted it 29 lived within the euphotic zone and supports the latest interpretations of the Hunsrück Slate 30 depositional environment. 31