Maenioceras ornatum, Korn & Bockwinkel, 2021

Korn, Dieter & Bockwinkel, Jürgen, 2021, The pharciceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea), European Journal of Taxonomy 771, pp. 1-79 : 9-11

Maenioceras ornatum sp. nov.

urn:lsid:zoobank.org:act:D40EFEA4-C79A-4A01-A3AD-4DDB2AF923C5

Figs 6–7 ; Table 2

Maeneceras terebratum – Holzapfel 1895: 107, pl. 4 figs 14, 18, pl. 6 figs 6–7, 9 (only). — Foord & Crick 1897: 123, text-fig. 57. — Frech 1897: pl. 32a fig. 17a–b; 1902: 54, text-fig. 15. — Wedekind 1918: 114, pl. 16 fig. 7, text-fig. 23a.

Maenioceras terebratum – Walliser 1970: 121, pl. 2 figs 1–2, text-fig. 5d. — House 1971: 28, pl. 2 fig. 5. — Walliser et al. 1995: 111, text-fig. 6d. — Korn & Klug 2002: text-figs 132, 133a.

Diagnosis

Maenioceras with thinly discoidal, involute conch at 25 mm dm (ww / dm ~0.45; uw/ dm ~0.05). Whorl profile slightly compressed (ww / wh ~ 0.90); flanks flattened and nearly parallel, venter broadly rounded. Growth lines coarse, lamellar, strongly biconvex; ventrolateral shoulder with a distinct spiral groove.

Etymology

Named after the Latin ‘ ornatum ’, meaning ‘decorated’, because of the shell ornament.

Material examined

Holotype GERMANY • Rhenish Mountains , Oberscheld (possibly Caroline Mine ); middle Givetian (Red Ironstone); MB.C.30229.1. ( Fig. 6C )

Paratypes GERMANY • 13 specimens; Rhenish Mountains , Oberscheld (possibly Caroline Mine ); middle Givetian (Red Ironstone); MB.C.30229.2 to MB.C.30229.14 • 1 specimen; Rhenish Mountains , Oberscheld ( Caroline Mine , 128 m level); middle Givetian (Red Ironstone); Möbus Coll.; MB.C.30230 .

Description

The description is based on three specimens:

Holotype MB.C.30229.1: weakly deformed, comparatively well-preserved specimen with 23 mm conch diameter in red ironstone ( Fig. 6C ).

Paratype MB.C.30229.2: laterally deformed, incomplete, rather poorly preserved specimen with about 35 mm conch diameter in red ironstone ( Fig. 6A ).

Paratype MB.C.30229.3: laterally deformed, rather poorly preserved specimen with about 25 mm conch diameter in red ironstone ( Fig. 6B ).

The three type specimens listed above allow the study of conch geometry and ornament between 20 and 35 mm conch diameter. They show thinly discoidal conchs with parallel flanks; the whorl profile is widest near the midflank, from where the flanks slowly converge towards the umbilicus and a little faster towards the venter. Flanks and venter are separated by a narrowly rounded shoulder that is accompanied, on the flank side, by a shallow longitudinal groove.

Particularly in the holotype MB.C.30229.1, the ornament is well-preserved and shows lamellar growth lines with a broadly rounded dorsolateral projection, a slightly narrower lateral sinus, a very high and narrow ventrolateral projection and a very deep, narrow ventral sinus. Remains of the wrinkle-layer are preserved in the dorsal whorl area; they show very delicate lines.

Remarks

The new species was previously described by Holzapfel (1895) from well-preserved material from the Roteisenstein from Adorf ( Fig. 7A ). The large specimen illustrated by Holzapfel agrees very well with the specimens from Oberscheld in terms of conch form and ornament. Holzapfel already recognized the differences between the specimens from the Stringocephalus Limestone of Villmar ( M. terebratum ) and from the Roteisenstein of Oberscheld ( M. ornatum sp. nov.) in the strength of the growth lines. However, he considered both forms to belong to M. terebratum .

The new species differs from Maenioceras terebratum in the stouter conch (ww /dm = 0.40–0.45 in M. ornatum sp. nov. at 30 mm conch diameter but 0.25–0.30 in M. terebratum ), the wider and slightly flattened venter and the much coarser ornament with lamellar growth lines.

Figures

Fig. 6. Maenioceras ornatum sp. nov., all probably from the Caroline Mine near Oberscheld.A. Paratype MB.C.30229.2. B. Paratype MB.C.30229.3. C. Holotype MB.C.30229.1. Scale bar units = 1 mm.

Fig. 7. Maenioceras ornatum sp. nov. A. Reproduction of the figure in Holzapfel (1895: pl. 6 fig. 9) of a specimen from Martenberg near Adorf. B. Reproduction of the figure in Holzapfel (1895: pl. 4 fig. 14) of a specimen from Ense near Bad Wildungen. Scale bar units = 1 mm.

Tables

Table 2. Conch dimensions and ratios of selected specimens of Maenioceras ornatum sp. nov.

Specimen	dm	ww	wh	uw	ah	ww / dm	ww / wh	uw / dm	WER	IZR
MB.C.30229.2	32.9	13.0	16.4	1.8	–	0.40	0.79	0.05	–	–
MB.C.30229.3	24.9	11.0	12.5	1.3	–	0.44	0.88	0.05	–	–
MB.C.30229.1	23.1	10.2	11.4	1.6	5.4	0.44	0.89	0.07	1.70	0.53