Triimitoceras, Korn & Bockwinkel & Ebbighausen & Klug, 2003

Korn, Dieter, Bockwinkel, Jürgen, Ebbighausen, Volker & Klug, Christian, 2003, Palaeobiogeographic and evolutionary meaning of an early Late Tournaisian ammonoid fauna from the Tafilalt of Morocco, Acta Palaeontologica Polonica 48 (1), pp. 71-92 : 78-79

publication ID	https://doi.org/ 10.5281/zenodo.13272527
persistent identifier	https://treatment.plazi.org/id/03BE87DE-5A77-FFCF-FCF8-FE9D753BFB30
treatment provided by	Felipe
scientific name	Triimitoceras
status	gen. nov.

Treatment

Subfamily Imitoceratinae Ruzhencev, 1950 Triimitoceras gen. nov.

Derivation of name: After the triangularly coiled inner whorls, and the similarity with Imitoceras .

Type species: Triimitoceras epiwocklumeriforme sp. nov.

Diagnosis.— Imitoceratidae with discoidal conch that is triangularly coiled in juveniles. Aperture and whorl expansion rate low ( WER 1.50 to 1.70). Suture line with slightly pouched external lobe and large, V−shaped adventive lobe.

Comparisons.— Triimitoceras has a suture line typical for the family Imitoceratinae . As in Imitoceras Schindewolf, 1923 , theexternallobeisslightlypouched,theventrolateralsaddleis strikingly asymmetrical, and the adventive lobe is much deeper than the external lobe. However, the conch of Triimitoceras is thinner with lower aperture, and the conspicuous steinkern constrictions of the type species (see below) are not known from Imitoceras . The cardinal difference, of course, is the triangular coiling of the inner whorls of Triimitoceras , not known from any other Carboniferous prionoceratid.

Occurrence.—Only known so far from the vicinity of Jebel Ouaoufilal;OuedZnaïguiFormation,earlyLateTournaisian. Triimitoceras epiwocklumeriforme sp. nov.

Figs. 11D–F View Fig , 12 View Fig , 13 View Fig .

Derivation of name: After the similarity in the conch to the Devonian Epiwocklumeria .

Holotype: Specimen MB.C.3910 (coll. Bockwinkel), Fig. 11E View Fig .

Type locality and horizon: Taouz, Jebel Ouaoufilal, locality A east of Ksar Bouhamed (Tafilalt, Morocco); Oued Znaïgui Formation, early Late Tournaisian.

Material.— 59 specimens between 3 and 14 mm in diameter. Most of them are smaller than 10 mm.

Diagnosis.— Triimitoceras with pachyconic conch at 6 mm diameter (ww/dm 0.60) and thickly discoidal at 12 mm diameter (ww/dm 0.50). Steinkern with three strongly sinuous constrictions per volution. Suture line with subparallel−sided, slightly pouched external lobe, narrow ventrolateral saddle, and large V−shaped adventive lobe.

Description.—Juveniles are relatively more globose than later stages ( Fig. 12 View Fig ). Specimens up to 6 mm conch diameter are, with three prominent constrictions, triangularly coiled. In this respect, the conch resembles that of some Late Devonian wocklumeriid clymeniids.

Three strong constrictions per volution are visible in all specimens (steinkerns). In small individuals (dm 6 mm), such as the paratype MB.C.3915, they form a weak ventrolateral projection, but larger specimens (dm 13 mm), such as the holotype MB.C.3910 display a prominent tongue−shaped extension of the ventrolateral salient and a deep ventral sinus. The conch interior between the constrictions is smooth .

The suture of the holotype MB.C.3910 resembles, in the general outline, that of Imitoceras : the external lobe is slightly pouched, it is not as deep as the large adventive lobe, and the ventrolateral saddle is strongly asymmetric ( Fig. 13 View Fig ). The lateral saddle is very wide and overspans almost two thirds of the flank .

Dimensions.—(in mm). Comparisons.—There is no similar Tournaisian ammonoid known. Such combination of features is only known among the Late Devonian prionoceratids. A triangular juvenile conch occurs in some species of Mimimitoceras Korn, 1988 , but these are always globose. The shape of the steinkern constrictions is similar to that in some species of the genus Balvia Lange, 1929 , but these show sutures with rounded lobes.