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 was encrusted in vivo and if these sclerobionts were responsible for the trochospiral coiling observed in this unique specimen. To test if the presence of runner-like sclerobionts infested the historically collected specimen of Ivoites opitzi during its life, we used microCT to produce a three-dimensional model of the surface of the specimen. Our results indicate that sclerobionts grew across the outer rim (venter) on both sides of the ammonoid conch at exactly the location where the deviation from the planispiral was recognized, and where subsequent ammonoid growth would likely preclude encrustation. This indicates in vivo encrustation of the I. opitzi specimen, and represents the earliest documentation of the phenomenon. Further, this suggests that non-planispiral coiling in I. opitzi was likely pathologically induced and does not represent natural morphological variation in the species. Despite the observed anomalies in coiling, the specimen reached adulthood and retains important identifying morphological features, suggesting the ammonoid was minimally 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 identify the sclerobionts responsible for modifying the coiling of this specimen as hederelloids, a peculiar group of sclerobionts likely related to phoronids. Hederelloids in the Devonian are commonly found encrusting on fossils collected in moderately deep environments within 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 zone and supports the latest interpretations of the Hunsrück Slate depositional environment in the Bundenbach-Gemünden area.

Morphology of the Early Jurassic Arietitidae and the effects of syn vivo serpulid infestations

“…The ammonite specimen was facing three major problems when infested by the serpulid: The extra weight possibly caused a more downward orientation of the aperture because of a relative shift in the centres of mass and buoyancy (e.g. Saunders and Shapiro, 1986;Naglik et al, 2015;Tajika et al, 2015;Naglik et al, 2016). This probably occurred because the worm tube aperture stayed close to the aperture of the ammonite conch, thereby accumulating additional mass at the greatest possible distance from the centre of mass.…”

“…According to Checa et al (2002), the anomaly compensating growth of the specimen serves not only to maintain the equilibrium position in the water column, but also to ensure the correct aperture orientation, which is a prerequisite for the ammonite's buoyancy pursuing a nektonic lifestyle (Klug et al, 2004(Klug et al, , 2010Stilkerich et al, 2017).…”

“…Similarly, remains of other organisms settling on ammonoid conchs are quite common (e.g. Seilacher, 1960Seilacher, , 1982Stilkerich et al, 2017). Evidence for an organism settling on an ammonoid conch syn vivo can mainly be found in the reaction to this epizoan by the ammonoid animal and the specific orientation of the epizoan (Seilacher, 1960;Meischner, 1968;Keupp et al, 2011).…”

“…Unsurprisingly, it was the same people who also discovered remains of other organisms attached to the ammonite conchs, either syn vivo or post mortem. Among Jurassic ammonites, representatives of the Arietitidae are remarkable because of the combination of their often large size (up to almost one metre ;Stevens, 1988), great abundance (and correspondingly their ecological importance), and also their role in the Early Jurassic rediversification (e.g. Donovan et al, 1981;Neige et al, 2013).…”

“…1). Worms identified as epizoans (Keupp et al, 2011) have been widely described in the literature (Ehrenberg, 1932;Lange, 1932;Schindewolf, 1934;Merkt, 1966;Holland, 1971;Buys, 1973;Landman and Waage, 1986;Nicosia, 1986;Turek, 1987;Jäger, 1991;Rakús and Zítt, 1993;Fraaye and Jäger, 1995;Checa et al, 2002;Hoffmann and Keupp, 2015;Tajika et al, 2015) as well as post mortem epicoles (e.g Seilacher, 1982;Schmid-Röhl and Röhl, 2003).…”