Maenioceras terebratum

Maenioceras terebratum (Sandberger & Sandberger, 1851) Figs 4–5; Table 1 Goniatites terebratus Sandberger & Sandberger, 1851: 99, pl. 5 fig. 3a, c. Maenioceras terebratum – Walliser 1970: 121, pl. 2 figs 1–2, text-fig. 5d. — House 1971: 28, pl. 2 fig. 5. — Walliser et al. 1995: 111, text-fig. 6d. — Korn & Klug 2002: text-figs 132, 133a, e–g. ? Maenioceras terebratum – Termier & Termier 1950: pl. 143 fig. 28 (non figs 25–27= Afromaenioceras sulcatostriatum (Bensaïd, 1974)). — Petter 1959: 122, pl. 6 figs 10, 18, 22?, text-figs 16, 31a (non fig. 17 = Afromaenioceras sulcatostriatum). — Belka et al. 1999: pl. 4 figs 7–8. — Aboussalam & Becker 2001: 89, pl. 1 figs 10–11. — Becker et al. 2004: 42, pl. 1 figs 17–18. ? Maenioceras terebratum terebratum – Bensaïd 1974: 103, pl. 1 figs 11–12, pl. 2 fig. 1, pl. 3 fig. 5, pl. 6 figs 2–3, text-fig. 12b. — Wissner & Norris 1991: 66, pl. 3.1 figs 6–8, pl. 3.2 figs 1–5, 13, 14, text-fig. 3.16. ? Maenioceras terebratum tenue – Bensaïd 1974: 104, text-fig. 10a. — Wissner & Norris 1991: 67, pl. 3.1 figs 9–11, text-fig. 3.14–3.17. non Maeneceras terebratum – Holzapfel 1895: 107, pl. 4 figs 14, 18, pl. 6 figs 6–7, 9 (only). — Foord & Crick 1897: 123, text-fig. 57. — Frech 1897: pl. 32a fig. 17a–b; 1902: 54, text-fig. 15. — Wedekind 1918: 114, pl. 16 fig. 7, text-fig. 23a. Diagnosis Maenioceras with extremely discoidal, involute conch at 40 mm dm (ww /dm ~0.25; uw / dm ~0.05); whorl profile strongly compressed (ww/ wh=0.50). Flanks flattened and nearly parallel with subumbilical depression, venter narrowly rounded. Growth lines fine, strongly biconvex; ventrolateral shoulder with a distinct spiral groove. Suture line with very wide external lobe with acute ventrolateral saddle and a low median saddle, and a V-shaped lateral lobe and two U-lobes. Material examined Lectotype GERMANY • Rhenish Mountains, Villmar; middle Givetian (“ Stringocephalenkalk ”); 33c (Wiesbaden Museum); figured by Sandberger & Sandberger (1850 –1856: pl. 5 fig. 3a), re-illustrated here in Fig. 4A; Wiesbaden Museum nr. 33c. Paratypes GERMANY • 2 specimens; Rhenish Mountains, Villmar; middle Givetian (“ Stringocephalenkalk ”); 33a–b (Wiesbaden Museum), illustrated here in Fig. 4C–D; Wiesbaden Museum nr. 33a–b. Additional material GERMANY • 4 specimens; Rhenish Mountains, Wetzlar (Philippswonne Mine); middle Givetian (Red Ironstone); von Buch Coll.; MB.C.4976, MB.C.4977.1 to MB.C.4977.3 • 2 specimens; Rhenish Mountains, Wetzlar (Philippswonne Mine); middle Givetian (Red Ironstone); MB.C.30228.1, MB.C.30228.2. Description The material from Wetzlar contributes little to the knowledge of the species. Specimen MB.C.4977.1 is a weakly deformed, comparatively well-preserved specimen with 45 mm conch diameter in haematitic ironstone and the best available specimen that allows the study of the adult conch (Fig. 5A). It is, at nearly 45 mm diameter, extremely discoidal with a very narrow umbilicus (ww /dm =0.24; uw / dm =0.06). The whorl profile shows that the conch is widest in the midflank area; the flanks stand parallel and are weakly convex. They converge towards the umbilicus to form a shallow groove, from which the umbilical margin is elevated like a low rim. The flanks also converge towards the narrowly rounded venter, which is separated from the flanks by a rather deep ventrolateral groove. The shell ornament is visible in a small area near the aperture and shows very weak growth lines. MB.C.4976 is a rather poorly preserved specimen of 42 mm diameter, embedded in an ironstone slab (Fig. 5B). It shows the same conch proportions as specimen MB.C.4977.1. Remarks The type material from the Stringocephalus Limestone of Villmar is poorly preserved. It hardly allows an accurate description of the conch shape and the ornament. In addition, there is the rather small size of the lectotype which is 25 mm in diameter. The identification of the specimens from the Roteisenstein is therefore mainly based on the compressed conch shape of the type material. It is not certain whether the findings reported from North Africa and Canada actually belong to M. terebratum. Göddertz (1987) described sickle-shaped furrows on the inner half of the flank of larger specimens from Algeria; apparently this feature does not appear in the specimens from the Rhenish Mountains. However, it is not clear whether the furrows are actually shell constrictions or internal shell thickenings. Maenioceras terebratum differs from M. ornatum sp. nov. in the slenderer conch, the much narrower and rounded venter and the much weaker ornament with finer growth lines. Already Holzapfel (1895) discussed the morphological differences of specimens attributed by him to “ Maeneceras terebratum ” between the various occurences. However, he considered the difference in the weaker ornament of specimens from the Stringocephalus Limestone compared to the occurrence in the cephalopod limestone as caused by preservation. This is obviously not the case, as the specimens from the Red Ironstone of Wetzlar share the weak ornament with the specimens from the Stringocephalus Limestone of Villmar, but differ from the Red Ironstone specimens from Oberscheld.

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