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

Abstract

We herein examine the only known non-planispirally coiled early Devonian ammonoid,the holotype specimen of Ivoites opitzi, to investigate if the host wasencrusted in vivo and if these sclerobionts were responsible for the trochospiralcoiling observed in this unique specimen. To test if the presence of runner-likesclerobionts infested the historically collected specimen of Ivoites opitzi during itslife, we used microCT to produce a three-dimensional model of the surface ofthe specimen. Our results indicate that sclerobionts grew across the outer rim(venter) on both sides of the ammonoid conch at exactly the location where thedeviation from the planispiral was recognized, and where subsequent ammonoidgrowth would likely preclude encrustation. This indicates in vivo encrustation of theI. opitzi specimen, and represents the earliest documentation of the phenomenon.Further, this suggests that non-planispiral coiling in I. opitzi was likely pathologicallyinduced and does not represent natural morphological variation in the species.Despite the observed anomalies in coiling, the specimen reached adulthood andretains important identifying morphological features, suggesting the ammonoid wasminimally impacted by encrustation in life. As such, appointing a new type specimen-as suggested by some authors-for the species is not necessary. In addition, we identifythe sclerobionts responsible for modifying the coiling of this specimen as hederelloids, apeculiar group of sclerobionts likely related to phoronids. Hederelloids in the Devonianare commonly found encrusting on fossils collected in moderately deep environmentswithin the photic zone and are rarely documented in dysphotic and aphotic samples.This indicates that when the ammonoid was encrusted it lived within the euphotic zoneand supports the latest interpretations of the Hunsrück Slate depositional environmentin the Bundenbach-Gemünden area.