Maenioceras afroterebratum, Bockwinkel & Korn, 2024

Bockwinkel, Jürgen & Korn, Dieter, 2024, Ammonoids of the Middle Devonian family Maenioceratidae in the Anti-Atlas of Morocco, European Journal of Taxonomy 921 (1), pp. 1-35 : 7-8

Maenioceras afroterebratum sp. nov.

urn:lsid:zoobank.org:act:A50A8AFC-D773-4DF9-BAF2-3DA91E3E0250

Fig. 4 ; Table 1

Maenioceras terebratum – Termier & Termier 1950: pl. 143 fig. 28. — Petter 1959: 122, pl. 6 figs 18, 22, text-figs 16, 31a. — Aboussalam & Becker 2001: 89, pl. 1 figs 10–11. — Becker et al. 2004: pl. 1 figs 17–18.

Maenioceras terebratum terebratum – Bensaïd 1974: 103, pl. 1 figs 12, 12a (? fig. 11), pl. 2 figs 1, 1a, 4, pl. 3 fig. 5, text-fig. 12b 1, (non b 2).

Maenioceras (Maenioceras) terebratum – Göddertz 1987: 176, pl. 12 figs 3–9, text-figs 27a, 28a–e.

? Maenioceras terebratum – Termier & Termier 1950: pl. 143 figs 25–27. — Petter 1959: 122, pl. 6 fig. 10.

Diagnosis

Species of Maenioceras with thickly discoidal and subinvolute conch at 10 mm dm (ww/dm ~0.50; uw/dm ~ 0.25); conch thinly discoidal and involute at 17 mm dm (ww/dm ~ 0.40; uw/dm ~ 0.13) with weakly depressed whorl profile (ww/wh~ 0.90) and low coiling rate (WER~ 1.70). Whorl profile at 17 mm conch diameter trapezoidal with flattened, nearly parallel, weakly convergent flanks, narrowly rounded ventrolateral shoulder and broad, nearly flat venter. Ventrolateral shoulder with a faint spiral groove in juvenile and deeper groove in adult specimens. Growth lines coarse, lamellar. Suture line with very wide external lobe, shallow, asymmetrically rounded E 2 lobe, narrowly rounded ventrolateral saddle and blunt, V-shaped lateral lobe.

Etymology

Named because of the resemblance with M. terebratum and the origin from North Africa.

Type material

Holotype

MOROCCO • Anti-Atlas , Oued Mzerreb-W ; bed 6, middle Givetian ; Bockwinkel and Ebbighausen 2004 Coll.; MB.C.31947.1 (illustrated in Fig. 4A ).

Paratypes

MOROCCO • 22 specs; same collection data as for holotype; MB.C.31947.2 to MB.C.31947.23 • 25 specs; Anti-Atlas , Oufrane-W ; bed 3, middle Givetian ; Bockwinkel and Ebbighausen 2002–2004 Coll.; MB.C.31948.1 to MB.C.31948.25 • 5 specs; Anti-Atlas , Tiguisselt ; bed 3, middle Givetian ; Bockwinkel and Ebbighausen 2002–2004 Coll.; MB.C.31949.1 to MB.C.31949.5 .

Description

Holotype MB.C.31947.1 is a partly corroded limonitic internal mould with 17 mm conch diameter ( Fig. 4A ). The conch is thinly discoidal and involute (ww/dm = 0.40; uw/dm = 0.13) with a low coiling rate (WER ~1.70). The whorl profile is weakly compressed (ww/wh = 0.90); the flanks are slightly flattened and converge towards the pronounced ventrolateral shoulder, which is accompanied by a faint spiral groove, which begins at about 10 mm conch diameter and becomes distinct at 15 mm diameter. The venter is broadly convex and becomes flattened during the last preserved volution. Growth lines are only poorly preserved as imprints on the internal mould, which appears to be smooth. The phragmocone consists of about 18 chambers in the last preserved volution; there is a rather clear trend toward shorter chambers.

Paratype MB.C.31947.2 is a well-preserved specimen with 10.5 mm conch diameter ( Fig. 4C ). The conch is thickly discoidal and subinvolute (ww/dm = 0.50; uw/dm = 0.24) with moderately high coiling rate (WER ~1.82). The whorl profile is weakly depressed (ww/wh = 1.16); it shows very slightly convex flanks that converge towards the subangular ventrolateral shoulder. This is accompanied by a faint spiral groove. Traces of growth lines are visible on the internal mould; they are coarse, lamellar with strongly biconvex course. The suture line has, at approximately 10 mm conch diameter, an extremely wide external lobe with a very shallow, asymmetrically rounded E 2 lobe. On the flank follow a narrowly rounded ventrolateral saddle and a V-shaped, blunt lateral lobe ( Fig. 4E ). The phragmocone consists of about 14 chambers in the last preserved volution.

Paratypes MB.C.31948.1 (13.5 mm dm; Fig. 4B ) and MB.C.31948.2 (10 mm dm; Fig. 4D ) agree, in their conch morphology, with the holotype and the above described paratype. However, they possess only very weak longitudinal grooves.

Paratype MB.C.31947.3 was used for a cross section ( Fig. 4F ). It shows, at 12 mm conch diameter, a thickly discoidal and subinvolute conch (ww/dm = 0.51; uw/dm = 0.24) with low coiling rate (WER ~1.62). The whorl profile is weakly depressed (ww/dm = 1.11); it is remarkable for its flattened flanks, which are even weakly concave near the umbilicus. The flanks are weakly convergent and the venter is slightly flattened.

Remarks

Maenioceras afroterebratum sp. nov. is similar to M. terebratum and M. ornatum from the Rhenish Mountains. However, the lateral lobe is broadly rounded and the ventrolateral saddle is narrowly rounded in the new species, while both are subacute in M. terebratum ( Korn & Bockwinkel 2021: text-fig. 4b) and even acute in M. ornatum ( Holzapfel 1895: pl. 6 fig. 6c). Maenioceras afroterebratum differs by the much stouter conch and the more widely rounded venter from M. oufranense sp. nov. (ww/ dm ~ 0.35). Finally, M. afroterebratum is more slender and more involute (ww/dm~ 0. 50; uw/dm~0.25) than M. mzerrebense sp. nov. (ww/dm~ 0.65; uw/dm~0.30) at a comparable size.

Figures

Fig. 4. Maenioceras afroterebratum sp. nov. A. Holotype MB.C.31947.1 from Oued Mzerreb-W, bed 6. B. Paratype MB.C.31948.1 from Oufrane-W, bed 3. C. Paratype MB.C.31947.2 from Oued Mzerreb-W, bed 6. D. Paratype MB.C.31948.2 from Oufrane-W, bed 3. E. Paratype MB.C.31947.2 from Oued Mzerreb-W, bed 6; suture line at dm = 10.0 mm. F. Paratype MB.C.31947.3 from Oued Mzerreb-W, bed 6; cross section. Scale bar units = 1 mm.