Darkaoceras galeatum ( Matern, 1931 )

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 : 29-32

Darkaoceras galeatum ( Matern, 1931)

Figs 13–14

Koenenites galeatus Matern, 1931: 75 , pl. 2 fig. 11.

Hoeninghausia galeatus – Korn & Klug 2002: 118.

Darkaoceras galeatum – Bockwinkel et al. 2013: 261, text-figs 3–4.

Diagnosis

Species of Darkaoceras reaching about 60 mm conch diameter with extremely discoidal, subinvolute conch between 30 and 50 mm dm (ww/dm = 0.25–0.30; uw/dm = 0.15–0.25). Whorl cross section strongly compressed (ww/wh = 0.45–0.60); venter acute with pronounced keel. Growth lines fine, bundled around the umbilicus, strongly biconvex with moderately high dorsolateral projection and very high and narrow ventrolateral projection; ventrolateral shoulder with weak spiral groove. Suture line with high median saddle, V-shaped prongs of the E lobe, V-shaped L lobe, and shallow and rounded U 2 lobe.

Material examined

Holotype GERMANY • Rhenish Mountains , Oberscheld (Westfeld der Grube Königszug, Firste der 120–150 m-Sohle); late Givetian (Red Ironstone); Ahlburg Coll.; MB.C.3652 . Illustrated by Matern (1931: pl. 2 fig. 10a, d), re-illustrated here in Fig. 13C .

Paratypes GERMANY • 10 specimens; Rhenish Mountains , Oberscheld (Westfeld der Grube Königszug, Firste der 120–150 m-Sohle); late Givetian (Red Ironstone); Ahlburg Coll.; MB.C.3629 , MB.C.3633 , MB.C.3635 , MB.C.3645 , MB.C.3649 , MB.C.3650 , MB.C.3651 , MB.C.3657 , MB.C.3658 , MB.C.3665 .

Additional material

GERMANY • 1 specimen; Rhenish Mountains , Nanzenbach (Grube Königszug); late Givetian (Red Ironstone); Dannenberg Coll.; MB.C.30428 • 1 specimen; Rhenish Mountains , Oberscheld; late Givetian (Red Ironstone); Hubach Coll.; MB.C.3616 • 1 specimen; Rhenish Mountains , Oberscheld (Grube Gründchesseite); late Givetian (Red Ironstone); Lotz 1901–1902 Coll.; MB.C.22153 • 1 specimen; Rhenish Mountains , Oberscheld (Grube Prinzkessel); late Givetian (Red Ironstone); Fremdling Coll.; MB.C.30429 .

Description

Five specimens are selected for description and illustration:

MB.C.3658: well-preserved, slightly deformed specimen with 48 mm conch diameter in iron-rich micritic limestone; largely covered with shell ( Fig. 13A ).

MB.C.3635: laterally deformed specimen with 48 mm conch diameter in iron-rich micritic limestone; largely covered with shell ( Fig. 13B ).

MB.C.3633: deformed specimen with about 50 mm conch diameter in iron-rich micritic limestone; cut for a cross section but inner whorls not preserved. Outer suture line visible ( Fig. 14A ).

Holotype MB.C.3652: rather poorly preserved, slightly deformed specimen with about 42 mm conch diameter in iron-rich micritic limestone; specimen suffered from rough preparation ( Fig. 13C ).

MB.C.30429: incomplete, slightly deformed specimen with 24 mm conch diameter in iron-rich sparitic limestone; partly covered with shell ( Fig. 13D ).

The two specimens MB.C.3658 and MB.C.3635, both with about 48 mm conch diameter, have an extremely discoidal and subinvolute conch with very high coiling rate and strongly compressed whorl profile ( Fig. 13A–B ). The umbilical wall is very shallow and the flanks converge towards the subacute venter. The ornament consists of fine growth lines with biconvex course; the ventrolateral projection is high and narrow. Faint short riblets are present around the umbilicus.

The suture line of specimen MB.C.3633 has a wide external lobe; this is subdivided by a median saddle that has a height of two thirds of the external lobe depth ( Fig. 14A ). The prongs of the external lobe are slightly asymmetric and V-shaped. On the flank follows a V-shaped lateral lobe and, on the umbilial margin, a broadly rounded umbilical lobe.

Remarks

The specimens from Dillenburg are quite large; smaller specimens from 3 up to 38 mm conch diameter were described by Bockwinkel et al. (2013b) from Hagen-Herbeck, northern margin of the Rhenish Mountains.

A similar species to D. galeatum is D. meridionale , but this species differs in having slightly thicker inner whorls. D. velox Bockwinkel, Becker & Ebbighausen, 2013 also differs by the broader inner whorls.

Figures

Fig. 13. Darkaoceras galeatum (Matern, 1931). A. Specimen MB.C.3658 (Ahlburg Coll.) from Oberscheld (Königszug Mine). B. Specimen MB.C.3635 (Ahlburg Coll.) from Oberscheld (Königszug Mine). C. Holotype MB.C.3652 (Ahlburg Coll.) from Oberscheld (Königszug Mine). D. Specimen MB.C.30429 (Fremdling Coll.) from Oberscheld (Prinzkessel Mine). Scale bar units = 1 mm.

Fig. 14. Darkaoceras galeatum (Matern, 1931). A. Suture line of specimen MB.C.3633 (Ahlburg Coll.) from Oberscheld (Königszug Mine), at ww = 10.7 mm, wh = 16.5 mm. B–E. Ontogenetic trajectories of the cardinal conch parameters. Scale bar units = 1 mm.