Arnsbergites undetermined

Arnsbergites sp. Fig. 14; Tables 8, A 4 Material examined MOROCCO – Central Meseta • 1 spec.; Sidi Amar, southern side of the Tabaïnout mountain ridge; Late Viséan; 2011; Korn and Ebbighausen leg.; MB.C. 32216. Description The sectioned specimen MB.C.32216 is rather poorly preserved but shows delicate spiral lines across the flanks and venter. It provides insight into conch ontogeny from the initial stage up to a conch diameter of 17.6 mm (Fig. 14A). The specimen passes through two stages of conch ontogeny, with the transition occurring at approximately 2–2.5 mm in diameter. During the first stage, the whorl profile is kidney-shaped and strongly depressed, while in the second stage, it becomes C-shaped with a very high overlap rate. Notably, the umbilicus gradually closes from a conch diameter of around 5 mm, with the uw/dm ratio reaching just 0.08 at a diameter of 17.6 mm (Fig. 14B). Remarks The cross section of specimen MB.C.32216 cannot be attributed to any of the species of Arnsbergites from the Chebket el Hamra site (Jerada region, north-eastern Morocco). In contrast to MB.C.32216, all specimens from that locality display an umbilicus that continues to open, albeit slowly, during later stages of ontogeny (Korn & Ebbighausen 2008). This characteristic is also seen in material from other regions, such as southern Portugal (Korn 1997), Ireland (Moore & Hodson 1958) and the Rhenish Mountains (Korn 1990).