Nicimitoceras acre ( Vöhringer, 1960 )

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Oberrödinghausen (Early Carboniferous; Rhenish Mountains, Germany), European Journal of Taxonomy 882, pp. 1-230 : 100-101

Nicimitoceras acre ( Vöhringer, 1960)

Fig. 61 ; Tables 57–58

Imitoceras acre Vöhringer, 1960: 121 , pl. 1 fig. 6, text-fig. 3.

Imitoceras acre – Bartzsch & Weyer 1988a: 134, text-fig. 2/1.

Nicimitoceras acre – Korn 1994: 56, text-figs 58e, 60d. — Sprey 2002, pl. 4 fig. 1.

? Aganides carinatus – Schmidt 1925: 535, pl. 19 fig. 3.

non 1981 Imitoceras (Imitoceras) acre – Ruan: 72, pl. 15 figs 3–5.

Diagnosis

Species of Nicimitoceras with a conch reaching 70 mm diameter. Conch thinly discoidal and involute at 30 mm dm (ww/dm ~0.35; uw/dm = 0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~0.60); whorl expansion in the adult stage rate very high (WER ~2.30). Venter narrowly rounded in the preadult stage, acute in the adult stage. Growth lines extremely fine, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with V-shaped external lobe and twice as deep, symmetrically V-shaped adventive lobe with angular base.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 2 ; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 1 fig. 6), Korn (1994: text-fig. 58e) and Sprey (2002: pl. 4 fig. 1); re-illustrated here in Fig. 61A ; GPIT-PV-63856.

Paratypes

GERMANY • 3 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3b; Vöhringer Coll.; GPIT-PV-63857–GPIT-PV-63859 .

Additional material

GERMANY • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 2 ; Vöhringer Coll.; MB.C.31131 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3a ; Korn 1991 Coll.; MB.C.31132 • 1 specimen; Rhenish Mountains, Oberrödinghausen , railway cutting ; Hangenberg Limestone , bed 3d1b; Weyer 1993–1994 Coll.; MB.C.31133.

Description

Holotype GPIT-PV-63856, 49 mm in diameter ( Fig. 61A ), is a lenticular conch (ww/dm = 0.29) with a closed umbilicus and a very high coiling rate (WER = 2.31). On the last whorl, the transition from a narrowly rounded venter to an oxyconic venter takes place. Only small portions of the shell are preserved, they show extremely fine growth lines that extend with a low ventrolateral projection across the outer flank. There are neither shell constrictions nor internal shell thickenings. The suture line has a narrowly V-shaped external lobe with somewhat convex flanks. The lobe is little more than half as deep as the adventive lobe, which is V-shaped with convex flanks ( Fig. 61C ).

Paratype GPIT-PV-63859 is a medium-sized specimen (30 mm dm) that still shows the non-acute venter of the preadult stage. It is extremely discoidal (ww/dm = 0.35) with the umbilicus completely closed. The venter is narrowly rounded ( Fig. 61B ). No shell remains are preserved in this specimen.

Remarks

Nicimitoceras acre is easily distinguished from the other species of the genus by its oxyconic conch form. Species of the genus Acutimitoceras have a superficially similar conch, but unlike N. acre , they are characterised by an attached keel. The suture lines of Acutimitoceras species have a deep external lobe, in contrast to the short external lobe in Nicimitoceras .

Figures

Fig. 61. Nicimitoceras acre (Vöhringer, 1960), from the Oberrödinghausen railway cutting, bed 2. A. Holotype GPIT-PV-63856 (Vöhringer Coll.). B. Paratype GPIT-PV-63859 (Vöhringer Coll.). C. Suture line of holotype GPIT-PV-63856, at ww = 11.0 mm, wh=15.5 mm. Scale bar units =1 mm.