Nicimitoceras Korn, 1993

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 : 93-94

Genus Nicimitoceras Korn, 1993

Type species

Imitoceras subacre Vöhringer, 1960: 120 ; original designation.

Genus diagnosis

Genus of the Acutimitoceratinae with a discoidal conch with moderately high to high coiling rate (WER = 1.90–2.20); inner whorls subinvolute to evolute to variable degree. Ornament with convex or slightly biconvex growth lines. Suture line with short, V-shaped or lanceolate external lobe (0.50 to 0.75 depth of the adventive lobe).

Genus composition

Central Europe ( Schmidt 1924; Vöhringer 1960; Korn 1984): Aganides carinatus Schmidt, 1924 ; Imitoceras trochiforme Vöhringer, 1960 ; Imitoceras subacre Vöhringer, 1960 ; Imitoceras acre Vöhringer, 1960 ; Imitoceras heterolobatum Vöhringer, 1960 ; Acutimitoceras caesari Korn, 1984 .

North Africa ( Bockwinkel & Ebbighausen 2006): Acutimitoceras mfisense Bockwinkel & Ebbighausen, 2006 .

Remarks

Vöhringer (1960) placed his three new species “ Imitoceras . trochiforme ”, “ I. subacre ” and “ I. acre ” in the species group of “ Imitoceras lineare ”, which was putatively characterised by a conch that is involute in all growth stages and by the absence of shell constrictions. This concept can no longer be followed for several reasons:

(1) “ Imitoceras lineare ” from the Late Devonian is a species of the genus Mimimitoceras and possesses shell constrictions in the juvenile and middle growth stage (Korn 1994). The species has hardly any features in common with the three Early Carboniferous species and therefore cannot be considered closely related. Mimimitoceras lineare differs markedly in the low coiling rate (WER = 1.50) and the deep external lobe, which has the same depth when compared with the adventive lobe. Species of Nicimitoceras have a very characteristic, short external lobe.

(2) Vöhriger’s assumption that the three species have a conch that is involute at all growth stages is apparently based on the cross section of specimen GPIT-PV-638645 ( N. trochiforme ). However, this cross section is off-centre and does not allow the study of the innermost whorls; there is no evidence for the assumption of involute whorls. In fact, other preparations show that the inner whorls are subinvolute or subevolute.

The following genera must be discussed in the context of Nicimitoceras :

Follimitoceras , introduced by Ruan (1995) as a subgenus of Acutimitoceras with the type species Imitoceras (Imitoceras) folliforme Ruan, 1981 . – The subgenus was established for discoidal prionoceratid ammonoids with a deep, slightly pouched external lobe. The conch characters are very similar to Nicimitoceras , with which it shares the lenticular conch shape and the high coiling rate. However, Follimitoceras differs in the deeper, pouched external lobe. Follimitoceras should therefore be regarded as a valid, independent genus. Kullmann (2009) regarded Follimitoceras , with a question mark, as a synonym of Zadelsdorfia . Such an attribution as proposed by Kullmann is very unlikely because of the conch shape and the open umbilicus in the adult stage in Zadelsdorfia .

Acrimitoceras , also introduced by Ruan (1995) as a subgenus of Acutimitoceras with the type species Imitoceras acre Vöhringer, 1960 . – The subgenus separates the acute end-member of an evolutionary lineage, not paying attention to the evolutionary lineage leading to it, i.e., possibly through Nicimitoceras subacre , the type species of Nicimitoceras . Acrimitoceras should therefore be regarded as synonym of Nicimitoceras .

Streeliceras, introduced by Becker (1996) as a subgenus of Acutimitoceras with the type species Imitoceras heterolobatum Vöhringer, 1960 . – The subgenus was introduced for species that possess evolute inner whorls in combination with a short external lobe. Separation from Nicimitoceras was only justified by the higher number of evolute juvenile whorls. However, numerous studies of cross-sections of acutimitoceratin ammonoids have shown that the juvenile umbilical width changes gradually between species of the different groups. In contrast, the short external lobe is a very constant character of the acutimitoceratin ammonoids of the “ Gattendorfia Stufe ” and is always accompanied by lenticular conchs and high coiling rates. Therefore, “ Imitoceras heterolobatum ” does hardly fulfil the criteria for an own subgenus and is thus kept in Nicimitoceras here.

Nicimitoceras differs from Stockumites by the short external lobe and, in most cases, by the higher coiling rate (WER usually above 2.00, but usually below this value in Stockumites ).

A genus with similar species is Imitoceras , but in the species of this genus the external lobe is pouched in contrast to Nicimitoceras with a lanceolate or V-shaped external lobe. In the stratigraphically earliest members of the genus this feature may still be weakly developed ( Bockwinkel & Ebbighausen 2006), but the shape of the adventive lobe, the dorsal of which is curved inwards in Imitoceras , is another distinguishing criterion.