Mimimitoceras varicosum

Mimimitoceras varicosum (Schindewolf, 1923)

Fig. 4; Table 1

Postprolobites varicosus Schindewolf, 1923: 405, text-fig. 13b.

Imitoceras varicosum – Vöhringer 1960: 122, pl. 2 fig. 1c. — Weyer 1977: 170, pl. 2 fig. 2. — Bartzsch & Weyer 1982: 19.

Mimimitoceras varicosum – Korn 1994: 22, text-fig. 64e–f.

non Aganides varicosus – Schmidt 1925: 533, pl. 23 fig. 1.

non Prionoceras varicosum – Lange 1929: 60, pl. 1 figs 13, 13a. — Schindewolf 1952: 294, pl. 2 figs 3–4. — Petter 1959: 250, pl. 18 fig. 12, text-fig. 56k.

non Imitoceras varicosum – Vöhringer 1960: 122, pl. 2 fig. 1a–b, text-fig. 4.

non Mimimitoceras varicosum – Korn 1992a: 33; 1994: 22, text-figs 19a, c, 20c–d, 21f, 22c. — Becker 1996: 35. — Korn & Weyer 2003: text-fig. 14a. — Bockwinkel & Ebbighausen 2006: 94, text-figs 7a–b, 8.

Species of Mimimitoceras with thickly pachyconic and subinvolute conch at 20–30 mm dm (ww/ dm =0.80–0.85). Whorl cross section depressed; coiling rate low (WER ~ 1.70). Ornament with rather coarse and sharp, narrow-standing growth lines with weakly biconvex course. Prominent shell constrictions with biconvex course.

Lectotype GERMANY • Upper Franconia, old quarry 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone”); Schindewolf 1916 Coll.; illustrated by Schindewolf (1923: text-fig. 13b), Vöhringer (1960: pl. 2 fig. 1c) and Korn (1994: text-fig. 64E), re-illustrated here in Fig. 4B; SMF Mbg.4706.

Paralectotype GERMANY • 1 specimen; Upper Franconia, old quarry 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1916 Coll.; illustrated in Fig. 4A; SMF Mbg.7560.

Lectotype SMF.Mbg.4706 is a moderately well-preserved, weakly deformed specimen with 19 mm conch diameter, but offering a fairly good impression of the conch geometry and ornament (Fig. 4B). The conch is thickly pachyconic (ww/dm=0.83) with an obviously closed umbilicus. Flanks and venter form an almost semi-circular arch; the coiling rate is low (WER =1.68). The shell bears constrictions which, like the coarse growth lines, are weakly biconvex in their course; the lateral sinus is very shallow and the slightly deeper ventral sinus is rather narrow. On the flanks, they are accompanied by a barely visible shell bulge on the apertural side.

Paralectotype SMF.Mbg.7560 with 28 mm conch diameter is very similar to the holotype in conch shape and ornamentation (Fig. 4A). The course of the constrictions, which are less pronounced in this specimen, is almost linear.

The species name “ Imitoceras varicosum ” was used by Vöhringer (1960) for globular specimens from the Hangenberg Limestone of Oberrödinghausen; this view was also followed by subsequent researchers of assemblages of the same age (Korn 1994; Becker 1996; Korn & Weyer 2003). However, when reexamining the material from Gattendorf, it must be noted that material from the two localities differ; therefore, Korn & Weyer (2023) introduced the new species Mimimitoceras perditum for the material from the Rhenish Mountains. Mimimitoceras varicosum differs in the stouter conch from M. perditum (ww/dm ~ 0.80 in M. varicosum but ~ 0.70 in M. perditum). Mimimitoceras varicosum has biconvex growth lines with rather distinct lateral sinus, while M. perditum has finer growth lines with nearly straight course. Most probably, the type material of Mimimitoceras varicosum originates from the Acutimitoceras ucatum Zone (Korn & Weyer 2023), as younger strata of the Gattendorfia Stufe are not known from the Gattendorf locality. In contrast, M. perditum occurs not frequently in the upper part of the Hangenberg Limestone (Eocanites delicatus Zone).

Mimimitoceras varicosum is superficially similar to the co-occurring Stockumites kleinerae (Korn, 1984) in the stout conch with a ww/dm ratio of 0.80 at 20 mm diameter. However, M. varicosum has distinct shell constrictions at this stage, which are not present in S. kleinerae. In addition, the growth lines of M. varicosum are clearly biconvex but convex in S. kleinerae.