Acutimitoceras Librovitch, 1957

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 : 41-42

Genus Acutimitoceras Librovitch, 1957

Type species

Imitoceras acutum Schindewolf, 1923, p. 333 ; original designation.

Genus diagnosis

Genus of the Acutimitoceratinae with a discoidal, lenticular conch with low to high coiling rate (WER =1.70–2.20); inner whorls subevolute or evolute. Venter narrowly rounded or oxyconic with an attached ventral keel. Ornament with biconvex growth lines, shell with constrictions. Suture line with deep and broad, V-shaped external lobe (as deep as the adventive lobe).

Genus composition

Central Europe ( Schindewolf 1923; Korn & Weyer 2023): Imitoceras acutum ( Schindewolf, 1923) ; Acutimitoceras ucatum Korn & Weyer, 2023 ; Acutimitoceras paracutum Korn & Weyer, 2023 .

South China ( Sun & Shen 1965): Imitoceras wangyuense Sun & Shen, 1965 .

Remarks

Acutimitoceras was established by Librovitch (1957) for prionoceratids with a sharpened venter. The genus was subsequently not considered further but was revived and broadened by Korn (1981, 1984) to include those early Tournaisian prionoceratid species that differ from the genus Mimimitoceras by more widely umbilicate inner whorls. However, this definition also includes species that are now placed in other genera (e.g., Stockumites , Nicimitoceras , Hasselbachia ).

Acutimitoceras was then restricted to the oxyconic forms by Becker (1996), who established the subgenus Stockumites for those forms with a rounded venter throughout ontogeny. This concept was either not supported (e.g., Korn & Klug 2002; Korn & Weyer 2003; Korn 2006; Korn & Feist 2007) or accepted on the condition that Stockumites remains only a subgenus of Acutimitoceras ( Kullmann 2009) . However, a closer examination of the material from the various regions (Rhenish Mountains, Upper Franconia, Thuringia, Guizhou) shows that the acute venter is not the only character to distinguish Acutimitoceras from Stockumites ( Korn & Weyer 2023) . An additional good distinguishing character is the attached keel, which gives the external side a galeate profile in cross-section even when the venter is not acute. Therefore, Acutimitoceras is reduced here to the forms with these two characters and the genus Stockumites is accepted for the forms without an attached keel.