Epitornoceras mithracoides ( Frech, 1888 )

Korn, Dieter & Bockwinkel, Jürgen, 2022, The tornoceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea), European Journal of Taxonomy 806, pp. 32-51 : 40-43

Epitornoceras mithracoides ( Frech, 1888)

Figs 6–7 ; Table 3

Goniatites (Tornoceras) mithracoides Frech, 1888: 30 , pl. 2 fig. 1.

Tornoceras (Epitornoceras) mithracoides – Frech 1902a: 174, text-fig. 2a; 1902b: 52, text-fig. 14b.

Goniatites (Tornoceras) mithracoides – Miller 1938: 141, text-fig. 30d.

Epitornoceras mithracoides – House 1978: 60, pl. 10 figs 2–3. — Korn & Klug 2002: 158. — Bockwinkel et al. 2013: 268, text-figs 11–12. — Bockwinkel & Korn 2015: 358, text-figs 9–10.

non Epitornoceras mithracoides – House et al. 1985: 5, text-figs 5a–b. — Dzik 2002: text-fig. 50n–o. — Korn & Klug 2002: text-fig. 144t. — Bockwinkel et al. 2009: 65, text-figs 1–2; 2013b: 57, textfigs 4d, 58–59; 2015: 148, text-figs 31–32; 2017: 346, text-figs 34–35.

Diagnosis

Epitornoceras reaching about 200 mm conch diameter. Thinly discoidal and involute adult stage (ww/dm ~ 0.35; umbilicus closed at 50 mm dm); whorl profile compressed (ww/wh ~ 0.65 at 50 mm dm); whorl expansion rate high (WER ~ 2.20). Flanks weakly convex, convergent, umbilical margin pronounced in the juvenile stage but continuously rounded in the adult stage. Without any ventrolateral grooves. Suture line with broadly rounded, asymmetric adventive lobe with convergent flanks; width of the adventive lobe equals the lobe depth.

Type material

Lectotype GERMANY • Rhenish Mountains, Oberscheld (Volpertseiche Mine); late Givetian (Red Ironstone); designated by Bockwinkel et al. (2009); figured by Frech (1888: text-fig. 1b, 1β) and House (1978: textfigs 2–3), re-illustrated here in Fig. 7A ; Koch Coll.; MB.C.469 .

Paralectotype GERMANY • Rhenish Mountains , Oberscheld (Volpertseiche Mine); late Givetian (Red Ironstone); figured by Frech (1888: text-fig. 1a, 1α), re-illustrated here in Fig. 7C ; Koch Coll.; MB.C.470 .

Description

Two specimens are selected for description and illustration.

Lectotype (MB.C.469) Completely chambered specimen with 53 mm conch diameter in haematitic limestone; it represents the inner portion of a phragmocone of a much larger specimen ( Figs 6 , 7A ). It is a disc-shaped involute conch about 53 mm in diameter (ww/dm = 0.35). It is widest at the closed umbilicus, from where the

flanks converge to the narrowly rounded venter. No shell remains are preserved; the specimen is covered by remains of the septa of the following volution. The suture line was already correctly illustrated by Frech (1888); it shows ( Fig. 6 ) a V-shaped external lobe and a very narrow, subacute ventrolateral saddle. The adventive lobe is weakly asymmetric and broadly rounded with distinctly divergent flanks ( Fig. 7B ).

Paralectotype (MB.C.470)

Fragment of a completely chambered specimen with an estimated phragmocone diameter of 100 mm haematitic limestone ( Figs 6 , 7C ). It is part of a phragmocone at least 100 mm in diameter; the total diameter together with the body chamber may therefore have been about 200 mm. The shape of the body corresponds to that of the lectotype. The paralectotype does not bear any shell remains either.

Remarks

Both specimens from Dillenburg are quite large; smaller specimens up to 15 mm in diameter were described by Bockwinkel et al. (2013b) and Bockwinkel & Korn (2015).

Epitornoceras mithracoides cannot really be confused with any other ammonoid species from the Rhenish Mountains because of the subacute ventrolateral saddle. Very similar forms have been reported from the Anti-Atlas of Morocco and identified as this species ( Bockwinkel et al. 2009, 2013a, 2015, 2017). These specimens have a very similar conch form to the red ironstone specimens, but show a suture line with a considerably deeper adventive lobe characterised by nearly parallel flanks. The differences become clear when the suture lines are morphometrically recorded. Measured at half the depth of the adventive lobe, the width of this lobe is 100% of the lobe depth in E. mithracoides , but only about 75% in the North African specimens ( Bockwinkel et al. 2009), which are described below as E. transmediterraneum . The shape of the umbilicus can be mentioned as a further criterion; the umbilical wall is evenly rounded in the adult stage in E. mithracoides , but it is obliquely flattened caused by an umbilical shell thickening in E. transmediterraneum , leading to a funnel-shaped umbilicus with a distinct umbilical edge of the internal mould.

Figures

Fig. 6. Epitornoceras mithracoides (Frech, 1888), reproduction in original size of the illustration of the type material; paralectotype (MB.C.470) and lectotype (MB.C.469) by Frech (1888).

Fig. 7. Epitornoceras mithracoides (Frech, 1888), specimens from the Volpertseiche Mine near Oberscheld (both Koch Coll.). A. Lectotype (MB.C.469). B. Suture line of lectotype (MB.C.469); at dm = 43.5 mm; ww = 15.0 mm; wh = 25.5 mm. C. Paralectotype (MB.C.470). Scale bar units = 1 mm.

Tables

Table 3. Conch dimensions and ratios of Epitornoceras mithracoides (Frech, 1888), lectotype (MB.C.469).

Specimen	dm	ww	wh	uw	ah	ww/dm	ww/wh	uw/dm	WER	IZR
MB.C.469	52.7	18.4	28.0	0	–	0.35	0.66	0.00	2.20	0.40