Acanthoclymenia planorbis (Sandberger & Sandberger, 1851)

Korn, Dieter & Bockwinkel, Jürgen, 2022, The early gephuroceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea), European Journal of Taxonomy 823, pp. 10-47 : 42-44

Acanthoclymenia planorbis (Sandberger & Sandberger, 1851)

Figs 21–22

Goniatites planorbis Sandberger & Sandberger, 1851: 96 , pl. 9 fig. 3.

Gephyroceras planorbis – Wedekind 1918: 122.

Ponticeras planorbis – Matern 1931: 83.

Acanthoclymenia planorbis – Korn & Klug 2002: 119, text-fig. 123h.

Diagnosis

Species of Acanthoclymenia reaching about 30 mm conch diameter with small, extremely discoidal, evolute conch between12 and 32 dm (uw/dm decreasing from ~ 0.50 to ~ 0.45). Whorl profile compressed; umbilical wall shallow and rounded, whorl profile pear-shaped, ventrolateral margin angular, venter flat. Growth lines very fine, strongly biconvex.

Material examined

Lectotype GERMANY • Rhenish Mountains, Oberscheld ; middle Frasnian (Red Ironstone); Wiesb. 46a. Illustrated by Sandberger & Sandberger (1850 –1856: pl. 9 fig. 3a); re-illustrated here ( Fig. 21A ).

Paralectotype GERMANY • 1 specimen; Rhenish Mountains , Oberscheld ; middle Frasnian (Red Ironstone); Wiesb. 46b. Illustrated by Sandberger & Sandberger (1850 –1856: pl. 9 fig. 3) re-illustrated here ( Fig. 21B ) .

Additional material

GERMANY • 1 specimen; Rhenish Mountains , Oberscheld (Grube Königszug); middle Frasnian (Red Ironstone); Kauth Coll.; MB.C.30438 • 1 specimen; Rhenish Mountains , Oberscheld; middle Frasnian (Red Ironstone); Koch Coll.; MB.C.30439 .

Description

Four specimens are selected for description and illustration:

Lectotype Wiesb. 46a: incomplete specimen with 32 mm conch diameter in haematitic ironstone ( Fig. 21A ).

Paralectotype Wiesb. 46b: incomplete specimen with 22 mm conch diameter in haematitic ironstone ( Fig. 21B ).

MB.C.30438: slab and counterslab with two specimens (13 and 7 mm conch diameter) co-occuring with Mesobeloceras kayseri in haematite ore ( Fig. 22A ).

MB.C.30439: moderately well-preserved specimen with 12 mm conch diameter in haematitic iron ore ( Fig. 22B ).

The specimens contribute only little to our knowledge of the species. All are embedded on rock slabs ( Fig. 22 ), which prevents precise measurement of the whorl width, but it is clear that the conch is extremely discoidal. It is also clear that the whorl profile is pear-shaped with a rounded umbilical wall, convex flanks, a subangular ventrolateral shoulder and a flattened venter. The umbilicus has nearly exactly half the value of the conch diameter.

Remarks

Acanthoclymenia planorbis differs from the other species of the genus from Oberscheld, A. forcipifera , in the much wider umbilicus at 12 mm conch diameter (uw/dm ~ 0.50 in A. planorbis but ~ 0.30 in A. forcipifera ).

Figures

Fig. 21. Acanthoclymeniaplanorbis (Sandberger & Sandberger, 1851).A. Lectotype 46a in theWiesbaden collection from Oberscheld; photograph and reproduction of the figure of Sandberger & Sandberger (1850–1856: pl. 9 fig. 3a). B. Paratype 46b in the Wiesbaden collection from Oberscheld; photograph and reproduction of the figure of Sandberger & Sandberger (1850–1856: pl. 9 fig. 3). C. Probably lost specimen; photograph and reproduction of the figure of Sandberger & Sandberger (1850–1856: pl. 9 fig. 3d, e). Scale bar units = 1 mm.

Fig. 22. Acanthoclymenia planorbis (Sandberger & Sandberger, 1851). A. Specimen MB.C.30438 (Koch Coll.) from Oberscheld. B. Specimen MB.C.30439 (Kauth Coll.) from Oberscheld (Königszug Mine). Scale bar units = 1 mm.