Goniatites amarensis, Korn & Ebbighausen, 2025

Goniatites amarensis sp. nov.

urn:lsid:zoobank.org:act:D1A90F56-8651-4F54-9D82-C59E55D4F0C3

Figs 5–6 View Fig View Fig ; Tables 1–2, A1 View Table 1 View Table 2

Goniatites crenistria View in CoL – Korn & Ebbighausen 2008: 97, text-figs 16–21.

Diagnosis

Species of Goniatites reaching a conch diameter of 120 mm. Conch globular at 10 mm diameter (ww/ dm ~ 1.00), thickly pachyconic at 30 mm diameter (ww/dm ~ 0.80) and thinly pachyconic at 50 mm diameter (ww/dm ~ 0.65). Umbilicus slightly opened in all stages (uw/dm ~ 0.20 between 5 and 20 mm dm), in the adult stage slightly narrower; umbilical wall rounded. Coiling rate low, increasing in the adult stage (WER = 1.75–1.90 between 50 to 100 mm dm). Ornamentation with crenulated, weakly biconvex and rectiradiate growth lines with low dorsolateral and ventrolateral projections; external sinus deep. Without spiral lines. Suture line with a V-shaped, moderately narrow external lobe (~ 0.60 of the external lobe depth, 1.40 of the adventive lobe width), and a moderate median saddle (~ 0.40 of the external lobe depth). Flanks of the external lobe almost straight, ventrolateral saddle subacute, adventive lobe V-shaped with weakly sinuous flanks.

Etymology

Named after the type locality Sidi Amar.

Type material

Holotype

MOROCCO – Central Meseta • Sidi Amar, southern side of the Tabaïnout mountain ridge; coquina at the top of the Tabaïnout limestone (Late Viséan); 2011; Korn and Ebbighausen leg.; MB.C. 32215.1 ; illustrated in Fig. 5A View Fig .

Paratypes

MOROCCO – Central Meseta • 3 specs; same data as for the holotype; MB.C. 32215.2 to MB.C. 32215.4 .

Description

Holotype MB.C.32215.1 is a fully chambered conch with a diameter of 54 mm ( Fig. 5A View Fig ), giving a total size, including the body chamber, of approximately 95 mm. The conch is thinly pachyconic and involute (ww/dm = 0.65; uw/dm = 0.14); the flanks and venter form a broad convex arch and the umbilical margin is uniformly rounded. The coiling rate is moderately high (WER = 1.86). Shell remains are not preserved.

The suture line shows a V-shaped external lobe with nearly straight flanks; the median saddle has a height of 0.40 of the external lobe depth. The external lobe is notably wide, with an external lobe/ adventive lobe ratio of approximately 1.40. The subacute ventrolateral saddle and the adventive lobe have the same width. The adventive lobe is nearly symmetrical and has weakly sinuous flanks ( Fig. 6B View Fig ).

Paratype MB.C.32215.3 has a conch diameter of 22.5 mm and is preserved without shell remains ( Fig. 5C View Fig ). The conch is thickly pachyconic and involute (ww/dm = 0.79; uw/dm = 0.13); the flanks and the venter form a very broad convex arch and the umbilical margin is uniformly rounded. The phragmocone has about 20 chambers in the last preserved volution.

Paratype MB.C.32215.4 was sectioned, allowing the study of conch ontogeny from the initial stage up to a diameter of 29 mm ( Figs 5D View Fig , 6A View Fig , Table A1 View Table 1 ). At this diameter, nine and a half volutions are already present. The ontogenetic changes in conch geometry are relatively simple, consisting mainly of the reduction in whorl width relative to both the conch diameter and the whorl height. As a result, the two conch parameters ww/dm and ww/wh appear as nearly monophasic trajectories ( Fig. 6C–D View Fig ). The ratio of umbilical width to conch diameter remains stable throughout ontogeny ( Fig. 6C View Fig ), which is also evident in the cross-sectional illustration of the continuous opening of the umbilicus. The coiling rate changes only slightly, maintaining a value of about 1.50 between 2 and 23 mm conch diameter ( Fig. 6E View Fig ). It appears to increase slightly in the last half evolution, reaching a value of 1.59.

Remarks

The specimens from Sidi Amar closely resemble those from the Jerada Basin in eastern Morocco in terms of conch morphology, and are therefore considered conspecific. The Jerada specimens were previously attributed to G. crenistria by Korn & Ebbighausen (2008). However, they reach a much larger conch size of up to 120 mm in diameter compared to G. crenistria from the Rhenish Mountains, which only attain about 70 mm in diameter.Another difference between G. crenistria and G. amarensis is observed in their conch ontogeny, now better understood from the material from Sidi Amar. Goniatites amarensis shows a stairway-like opening of the umbilicus, with the uw/dm ratio remaining nearly constant throughout ontogeny. In contrast, G. crenistria performs a near-closure of the umbilicus during the growth interval between 5 and 15 mm in conch diameter. Additionally, the rapid increase in the coiling rate in late ontogeny distinguishes G. amarensis from G. crenistria . In G. amarensis , the coiling rate rises from 1.50 to 1.85 between 20 and 55 mm in conch diameter, while in G. crenistria , it increases from around 1.60 to 1.70 over the same ontogenetic interval.