Trimaenioceras eculeus, 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 : 27-29

Trimaenioceras eculeus gen. et sp. nov.

urn:lsid:zoobank.org:act:9EBD0D9B-BFCE-4218-B3DA-A78113DF0399

Fig. 14 ; Table 11

Diagnosis

Species of Trimaenioceras gen. nov. with thickly discoidal and subinvolute conch at 15 mm dm (ww/ dm ~ 0.60; uw/dm ~ 0.25); conch thinly discoidal and subinvolute at 28 mm dm (ww/dm ~ 0.40; uw/ dm ~ 0.10) with weakly compressed whorl profile (ww/wh ~ 0.85) and low coiling rate (WER ~ 1.65). Whorl profile at 28 mm conch diameter horseshoe-shaped with convex, weakly convergent flanks, rounded ventrolateral shoulder and rounded venter. Ventrolateral shoulder without spiral groove.

Growth lines very fine. Suture line with wide external lobe, moderately deep, cuneiform, strongly asymmetric E 2 lobe, rounded ventrolateral saddle and V-shaped, symmetric, acute lateral lobe.

Etymology

From the Latin nomen ‘ eculeus ’ = ‘a little horse’, because of the horseshoe-shaped whorl profile.

Type material

Holotype

MOROCCO • Anti-Atlas , south-eastern Tafilalt , Hassi Nebech , section 2; middle Givetian ; Bockwinkel and Ebbighausen 2002–2004 Coll.; MB.C.31969.1 (illustrated in Fig. 14A ).

Paratypes

MOROCCO • 27 specs; same collection data as for holotype; MB.C.31969.2 to MB.C.31969.28 • 5 specs; same collection data as for holotype except collected by locals; MB.C.31970.1 to MB.C.31970.5 .

Description

Holotype MB.C.31969.1 is a well-preserved limonitic, fully chambered specimen with 28 mm conch diameter ( Fig. 14A ). It is thinly discoidal and involute (ww/dm = 0.41; uw/dm = 0.11) and shows that the umbilicus starts closing at about 25 mm conch diameter by overlap of the flanks. The whorl profile is weakly compressed (ww/wh = 0.84) and the coiling rate is low (WER = 1.64). The suture line shows a very wide external lobe that is subdivided into three parts, of which the E 1 lobe is wider and deeper than the asymmetric, cuneiform E 2 lobes. The median saddle reaches about two thirds of the depth of the external lobe. The ventrolateral saddle is nearly symmetric and rounded; the lateral lobe is V-shaped with gently convex flanks ( Fig. 14D ). The last phragmocone volution has 16 chambers.

Paratypes MB.C.31969.2 (20.5 mm conch diameter; Fig. 14B ) and MB.C.31969.3 (15 mm conch diameter; Fig. 14C ) demonstrate the ontogenetic transformation of the conch shape. Together with the holotype, they show that the conch width decreases, between 15 and 28 mm conch diameter, from a ww/ dm ratio of about 0.60 to about 0.40; parallel to this, the uw/dm ratio decreases from 0.25 to about 0.10. The last phragmocone volution of paratype MB.C.31969.2 has 13 chambers.

The suture line of paratype MB.C.31969.4 shows, at a whorl height of 9.5 mm, a very wide external lobe; the E 2 lobe is nearly V-shaped and blunt. The ventrolateral saddle is inverted U-shaped and the lateral lobe is V-shaped with a convex ventral flank ( Fig. 14E ).

Remarks

Trimaenioceras eculeus gen. et sp. nov. is similar to T. klugi gen. et sp. nov., but differs in the stouter and more widely umbilicate conch in comparative growth stages. At around 15 mm conch diameter, ww/ dm ~ 0.60 and uw/dm~ 0.25 in T. eculeus , but ~ 0. 50 and ~ 0.05 in T. klugi .

Figures

Fig. 14. Trimaenioceras eculeus gen. et sp. nov., from Hassi Nebech. A. Holotype MB.C.31969.1. B. Paratype MB.C.31969.2. C. Paratype MB.C.31969.3. D. Holotype MB.C.31969.1; suture line at wh = 13.5 mm. E. Paratype MB.C.31969.4; suture line at wh = 9.5 mm. F. Paratype JB HN 1; cross section. Scale bar units = 1 mm.