Maenioceras oufranense, 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 : 11-13

Maenioceras oufranense sp. nov.

urn:lsid:zoobank.org:act:C6A0EE25-33C2-48E1-97BD-3C45967DC002

Fig. 6 ; Table 3

Maenioceras terebratum var. tenue – Petter 1959: 124, pl. 6 fig. 15, text-fig. 31b.

Maenioceras terebratum tenue – Bensaïd 1974: 104, text-fig. 12a.

? Maenioceras terebratum – Belka et al. 1999: pl. 4 figs 7–8. — Aboussalam & Becker 2011: text-fig. 8.1–2.

? Bensaidites n. sp. Becker et al. 2004: pl. 1 fig. 22.

Diagnosis

Species of Maenioceras with thickly discoidal and subinvolute conch at 8 mm dm (ww/dm ~0.55; uw/dm ~ 0.25); conch thinly discoidal and involute at 15 mm dm (ww/dm ~ 0.40; uw/dm ~ 0.10) with weakly compressed whorl profile (ww/wh~ 0.75) and low coiling rate (WER~ 1.60). Whorl profile at 15 mm conch diameter narrow horseshoe-shaped with convex, weakly convergent flanks, rounded ventrolateral shoulder and narrowly rounded venter. Ventrolateral shoulder with a faint spiral groove in juvenile and deeper groove in adult specimens. Growth lines coarse, strongly biconvex. Suture line with very wide external lobe, shallow, asymmetrically rounded E 2 lobe, narrowly rounded ventrolateral saddle and blunt, tongue-shaped lateral lobe.

Etymology

Named after the type locality Oufrane.

Type material

Holotype

MOROCCO • Anti-Atlas , Oufrane-W ; bed 3, middle Givetian ; Bockwinkel and Ebbighausen 2002– 2004 Coll.; MB.C.31951.1 (illustrated in Fig. 6A ).

Paratypes

MOROCCO • 47 specs; same collection data as for holotype; MB.C.31951.2 to MB.C.31951.48 .

Description

Holotype MB.C.31951.1 is a limonitic, dessert-polished internal mould; it is fully septate and, with 20 mm conch diameter, the largest available specimen ( Fig. 6A ). It is an extremely discoidal and involute conch (ww/dm = 0.33; uw/dm = 0.04) with an apparently low coiling rate. The whorl profile is compressed (ww/wh = 0.58); the flanks stand almost parallel in the umbilical area, but becoming convergent towards the rather narrowly rounded venter. The ventrolateral shoulder shows a pronounced spiral groove. The growth lines, which are visible as impressions, were probably coarse and clustered. They extend with strongly biconvex course. The suture lines are difficult to trace, but it is clear that the last chambers of the phragmocone are very closely spaced.

Paratype MB.C.31951.2 is a well-preserved limonitic internal mould of a phragmocone ( Fig. 6B ) with 12.5 mm diameter. It is a thinly discoidal and involute conch (ww/dm = 0.41; uw/dm = 0.13) with low coiling rate (WER = 1.61). The whorl profile is weakly compressed (ww/wh = 0.80); the flanks are flatly convex and converge towards the narrowly rounded venter. The ventrolateral shoulder possesses a faint spiral groove. The growth lines are apparently coarse and strongly biconvex.

Paratypes MB.C.31951.4 (7.5 mm conch diameter; Fig. 6D ) and MB.C.31951.3 (13 mm conch diameter; Fig. 6C ) demonstrate the transition from the juvenile stage with slow but constant opening of the umbilicus to the intermediate growth stage, in which the umbilicus becomes narrower by increased overlap of the inner flank area upon the umbilicus.

The cross section of paratype MB.C.31951.6 demonstrates the transformation from the juvenile to the preadult conch morphology ( Fig. 6F ). This ontogenetic transformation shows that the C-shaped whorl profile of the juvenile stage with a ww/wh ratio of about 2.00 is, beginning at a conch diameter of 5 mm, rapidly reduced to a value of 0.77 at 14 mm conch diameter. Parallel to this development, the uw/dm ratio is reduced markedly caused by overlap upon the umbilicus. The conch is thinly discoidal and involute at 14.2 mm dm (ww/dm = 0.40; uw/dm = 0.11); the flanks are almost parallel near the umbilicus and converge towards the narrowly rounded venter.

The suture line of paratype MB.C.31951.5 shows a very wide external lobe, a very shallow, asymmetrically rounded E 2 lobe and a narrowly rounded, small ventrolateral saddle; the asymmetric lateral lobe has a convex ventral flank and a steeper dorsal flank ( Fig. 6E ).

Remarks

Maenioceras oufranense sp. nov. is characterised by the extremely discoidal and involute conch in specimens with about 20 mm conch diameter (ww/dm~ 0.35; uw/dm ~0.05) and the narrowly rounded venter, which is broadly rounded in M. afroterebratum sp. nov. and nearly flat in M. mzerrebense sp. nov.

Maenioceras oufranense sp. nov. shows close similarity to M. tenue in the conch proportions, but differs in the narrowly rounded venter from Maenioceras tenue , which has an almost completely flat venter.

Figures

Fig. 6. Maenioceras oufranense sp. nov. from Oufrane-W, bed 3.A. Holotype MB.C.31951.1. B. Paratype MB.C.31951.2. C. Paratype MB.C.31951.3. D. Paratype MB.C.31951.4. E. Paratype MB.C.31951.5; suture line at dm = 12.0 mm. F. Paratype MB.C.31951.6; cross section. G–I. Ontogenetic trajectories of the cardinal conch parameters. Scale bar units = 1 mm.