Acutimitoceras acutum ( Schindewolf, 1923 )

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Gattendorf (Devonian-Carboniferous boundary; Upper Franconia, Germany), European Journal of Taxonomy 883, pp. 1-61 : 42-45

Acutimitoceras acutum ( Schindewolf, 1923)

Figs 23–24 ; Tables 14–15

Imitoceras acutum Schindewolf, 1923: 338 , pl. 15 figs 3–4.

Acutimitoceras acutum oxynotum Bartzsch & Weyer, 1996: 96 , text-figs 2–4.

Imitoceras acutum – Weyer 1976: 841, text-figs 1–3; 1977: 172, pl. 1 figs 5–6; 1979: pl. 1 fig. 3, pl. 3 fig. 7, pl. 5 fig. 13.

Acutimitoceras acutum – Bartzsch & Weyer 1986: pl. 2 fig. 4. — Korn 1994: 42, text-figs 56h, 57a; 2006: text-fig. 3a.

non Aganides acutus – Schmidt 1925: 534, pl. 19 fig. 5.

non Imitoceras acutum – Vöhringer 1960: 137, pl. 1 fig. 7, text-fig. 16.

non Acutimitoceras acutum – Korn 1992b: 15, pl. 1 figs 19–20; 1994: 42, text-figs 49j–k, 50b, 51a, 53c–d. — Schönlaub et al. 1992: pl. 4 figs 19–20.

non Acutimitoceras acutum acutum – Bartzsch & Weyer 1996: 95, text-fig. 1.

non Acutimitoceras (Acutimitoceras) acutum – Becker 1996: 36.

Diagnosis

Species of Acutimitoceras with a thickly discoidal and subinvolute conch at 6 mm dm (ww/dm ~ 0.52; uw/dm ~ 0.25), thinly discoidal and involute conch at 12 mm dm (ww/dm ~ 0.40; uw/dm ~ 0.10) and thinly discoidal and involute conch at 24 mm dm (ww/dm ~ 0.35; umbilicus closed). Whorl profile at 24 mm dm compressed (ww/wh ~ 0.60); coiling rate high (WER ~ 2.20). Venter subacute at 12 mm dm and acute at 20 mm dm. Fine growth lines with weakly biconvex course. Weak constrictions on the shell surface; they largely follow the course of the growth-lines.

Material examined

Holotype GERMANY • Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1916 Coll.; illustrated by Schindewolf (1923: pl. 15 fig. 3), Korn (1994: text-fig. 56h) and Korn (2006: text-fig. 3a); re-illustrated here in Fig. 23A ; SMF Mbg.3105 .

Paralectotype GERMANY • Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1916 Coll.; SMF Mbg.3106 .

Additional material

GERMANY • 1 specimen; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21f; Korn 1989 Coll.; MB.C.31283 • 1 specimen; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21d; Weyer 1995 Coll.; MB.C.31284 .

Description

Lectotype SMF Mbg.3105 is a rather well-preserved specimen 16.6 mm in diameter; it is fully covered with shell ( Fig. 23A ). Its conch shape is lenticular with an almost closed umbilicus (ww/dm =0.35; uw/ dm =0.04) and a high coiling rate (WER= 2.10). The whorl profile forms a narrow triangle; the flanks converge, beginning at the umbilical margin, slowly towards the venter but in the outer flank area, they converge more rapidly towards the raised ventral keel. The shell surface shows fine, convex growth lines which form a low ventrolateral projection and a deep V-shaped ventral sinus ( Fig. 24C ). Shell constrictions follow the course of the growth lines and are spaced at 90 degrees.

The smaller specimen MB.C.31283 shows the morphology of an intermediate growth stage with 11 mm conch diameter ( Fig. 23B ). The conch is thinly discoidal and subinvolute (ww/dm =0.42; uw/dm=0.16). It illustrates the transition from the early stage with a rounded venter to the older stage with an acute venter, showing a clear narrowing of the venter at a conch diameter of around 10 mm. The suture line has a V-shaped external lobe, a narrow, narrowly rounded ventrolateral saddle, and a large, asymmetrical adventive lobe with a convexly curved ventral flank and almost straight dorsal flank ( Fig. 24B ).

Paralectotype SMF.Mbg.3106 was sectioned already by Schindewolf; it shows a partially distorted cross-sectional image that did not hit the specimen perfectly in the centre ( Fig. 24A ). Nevertheless, the specimen gives a good overview of the ontogenetic changes from the evolving juvenile stage to the involute, disk-shaped adult stage with a sharpened venter.

Remarks

Bartzsch & Weyer (1996) introduced the subspecies Acutimitoceras acutum oxynotum to separate forms that later in ontogeny gain the character of an acute venter than in the nominate species. However, this comparison was done with the material from the Rhenish Mountains. The lectotype from Upper Franconia is obviously identical in shell geometry with the specimens from Thuringia. They also share the character of convergent flanks. For these reasons, Acutimitoceras acutum oxynotum must be regarded as a junior synonym of A. acutum .

Acutimitoceras ucatum from the Rhenish Mountains has convex flanks; the whorls are widest in some distance from the umbilicus ( Korn & Weyer 2023). The growth lines have a weakly biconvex course in A. acutum , but show stronger undulation in A. ucatum . Acutimitoceras paracutum from the Rhenish Mountains has a similar conch morphology and ornament, but differs in having a much lower coiling rate (WER ~ 1.70) in contrast to ~ 2.10 in A. acutum ).

Figures

Fig. 23. Acutimitoceras acutum (Schindewolf, 1923) from Gattendorf. A. Lectotype SMF Mbg.3105 (Schindewolf 1916 Coll.) from bed 21. B. Specimen MB.C.31283 (Korn 1989 Coll.) from bed 21f. Scale bar units= 1 mm.

Fig. 24. Acutimitoceras acutum (Schindewolf, 1923) from Gattendorf. A. Cross section of paralectotype SMF Mbg.3106 (Schindewolf 1916 Coll.) from bed 21. B. Suture line of specimen MB.C.31283 (Korn 1989 Coll.) from bed 21f, at dm =11.5 mm, ww =4.8 mm, wh =5.5 mm. C. Growth line and constriction course inlectotype SMF.Mbg.3105 (Schindewolf 1916Coll.) frombed 21, at dm = 16.4 mm, ww =5.0mm, wh=9.0 mm.D–F. Ontogenetic development of the conch width index (ww/dm), umbilical width index (uw/dm), whorl width index (ww/wh) and whorl expansion rate (WER) of selected specimens. Grey dots =specimen MB.C.1986 from the Pfaffenberg near Saalfeld, from Bartzsch & Weyer (1996). Scale bar units =1 mm.