Emsian Ammonoidea and the age of the Hunsrück Slate (Rhenish Mountains, Western Germany)

The ammonoids from the well-studied German Fossillagerstatte of the Hunsruck Slate have the reputation of being the oldest known representatives of this cephalopod group. This material is of great interest not only because of the global scarcity of the earliest ammonoids, but also because it includes the first record of stratigraphically-controlled specimens, which could be assigned to the middle Kaub Formation in the Bundenbach/Gemunden area. Accordingly, some of the Hunsruck Slate ammonoids are indeed the stratigraphically oldest ammonoids because they are associated with the index dacryoconarid Nowakia praecursor and thus derive from the Nowakia zlichovensis Biozone of the early Emsian (Zlichovian), while younger dacryoconarids and other ammonoid samples (Mimagoniatites fecundus, Mimosphinctes tripartitus) indicate the Nowakia barrandei to N elegans biozones. In spite of this special importance, these Early Devonian cephalopods have never been revised comprehensively. Our study includes more than 300 Hunsruck Slate specimens from both public and private collections. For the first time, ammonoids from the Altlay Hunsriick Slate in the Northern Hunsruck/Mosel region are reported, while all the materials from older collections derive from the middle Kaub Formation of the Central Hunsriick Basin (central Hunsriick, Taunus). These early ammonoids thus prove to be a valuable source of information for biostratigraphic correlation within the Hunsriick Slate and with early Emsian occurrences in other regions. Based on conch characters (geometry, ornament, suture lines) and ontogenetic traits, we describe the species Metabactrites fuchsi n. sp., Ivoites hunsrueckianus (ERBEN 1960), Ivoites schindewolfi n. sp., ?Ivoites sp., Ivoites opitzi n. sp., Anetoceras mittmeyeri n. sp., Erbenoceras advolvens (ERBEN 1960), Erbenoceras solitarium (BARRANDE 1865), Chebbites sp., Mimosphinctes primigenitus (ERBEN 1965), Mimosphinctes tripartitus EICHENBERG, 1931, Gyroceratites heinricherbeni n. sp., ?Teicherticeras sp., and Mimagoniatites fecundus (BARRANDE 1865). Supposedly endemic species of the Hunsriick Slate such as "Anetoceras recticostatum ERBEN, 1962" and "Mimagoniatites falcistria (FucHs 1915)" are here synonymized with the widely distributed species Erbenoceras solitarium and Mimagoniatites fecundus, both known to occur also outside Europe. Furthermore, we studied their taphonomy and assigned them to seven groups of preservation. The results of this taphonomic study corroborate previous interpretations of the depositional environment and diagenesis. We also discuss the evolution of the shell in the earliest ammonoids and their closest relatives as well as structures ("Opitzian pits") possibly caused by parasitic infestations of these early Emsian ammonoids.