Girtyoceras ibergense Korn, 1992

Girtyoceras ibergense Korn, 1992

Fig. 20 View Fig ; Table 12 View Table 12

Goniatites discus Roemer, 1852: 95 , pl. 13 fig. 35.

Girtyoceras ibergense Korn in Gischler & Korn, 1992: 278, text-figs 4m –n, 5b, 6g –h.

Homoceras discus – Schmidt 1925: 577, pl. 21 fig. 16, pl. 24 figs 10–11.

non Goniatites discus Roemer, 1850: 39 , pl. 6 fig. 7.

Diagnosis

Species of Girtyoceras with a discoidal, subinvolute conch at 15 mm diameter (ww/dm ~ 0.45; uw/dm ~ 0.15) and a thinly, involute conch at 35 mm diameter (ww/dm ~ 0.35; uw/dm ~ 0.10). Umbilical margin rounded. Venter rounded up to 12 mm conch diameter; sharpening of the venter begins at 30 mm dm. Internal mould in the preadult stage with very weak, slightly biconvex and rectiradiate constrictions. Without riblets.

Type material

Holotype

GERMANY – Harz Mountains • ‘ Grund’ (most probably the Iberg near Bad Grund ); “ Posidonienkalk ” ( Late Viséan ); Roemer Coll.; GPIC Nr. 420 ; illustrated by Roemer (1852: pl. 13 fig. 35) and Gischler & Korn (1992: text-fig. 7a).

Material examined

MOROCCO – Central Meseta • 2 specs; Sidi Lamine , sample 3; 2010; Korn and Ebbighausen leg.; MB.C. 32210.1 , MB.C. 32210.2 .

Description

Holotype GPIC420 ( Fig. 20A View Fig ) has a conch diameter of 35 mm. It is thinly discoidal and involute (ww/ dm = 0.34; uw/dm = 0.11) with an extremely high coiling rate (WER = 2.51). At this stage, the venter is not yet acute, and the rounded umbilical margin shows no indication of a ridge. In some areas, remains of the shell are preserved; the fine growth lines follow a biconvex course with a pronounced ventrolateral projection. Constrictions appear as shallow notches on the flank. The suture line shows the typical course for Girtyoceras , with a V-shaped external lobe, a median saddle reaching half the height of the ventrolateral saddle and a V-shaped adventive lobe.

Specimen MB.C.32210.1 ( Fig. 20C View Fig ) is an incomplete conch with a diameter of 16 mm. It is thickly discoidal and subinvolute (ww/dm = 0.47; uw/dm = 0.15) with a moderate coiling rate (WER = 1.98). The internal mould is nearly smooth except for one rather weak constriction with a biconvex course.

Remarks

Roemer described species named Goniatites discus two times ( Roemer 1850, 1852). These are demonstrably different species: the species established in 1850 comes from the Iberg Limestone of the Frasnian stage and is a gephuroceratid ammonoid; the form described in 1852 is a Girtyoceras . Thus, Goniatites discus Roemer, 1852 represents a homonym of Goniatites discus Roemer, 1850 and was revised under the new name Girtyoceras ibergense by Korn in Gischler & Korn (1992).

Although only a few specimens are available, it is possible to distinguish G. ibergense from other species of Girtyoceras . Stratigraphically younger species, such as G. brueningianum ( Schmidt, 1925) and G. goii Korn, 1988 , always display a distinct umbilical ridge, weak ribbing and a considerably earlier sharpening of the venter.

It is more challenging to differentiate it from species known from the Bowland Shales of England. Despite similarly weak ornamentation, G. premeslerianum Moore, 1946 and G. platyforme Moore, 1946 exhibit a 1.5 times wider umbilicus. Girtyoceras simplex Moore 1946 is distinguishable from G. ibergense by the tapering of the external side starting at a conch diameter of 23 mm.