Mimimitoceras perditum, Korn & Weyer, 2023

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Oberrödinghausen (Early Carboniferous; Rhenish Mountains, Germany), European Journal of Taxonomy 882, pp. 1-230 : 20-25

Mimimitoceras perditum sp. nov.

urn:lsid:zoobank.org:act:FD477B07-4D4B-4C66-8704-F1EA9479CABB

Figs 12–15 ; Tables 2–3

Imitoceras varicosum – Vöhringer 1960: 122, pl. 2 fig. 1, text-fig. 4.

Mimimitoceras varicosum – Korn 1992a: 33; 1994: 22, text-figs 19a, c, 20c–d, 21f, 22c, 64e–f. — Becker 1996: 35, pl. 1 figs 4–5. — Sprey 2002: 52, text-fig. 17a. — Korn & Weyer 2003: text-fig. 14a.

Diagnosis

Species of Mimimitoceras with a conch reaching about 90 mm diameter. Conch thinly globular and subinvolute juvenile stage (ww/dm ~0.90; uw/dm ~0.20 at 2 mm dm), thinly pachyconic and involute in the adult stage (ww/dm ~0.60 at 45 mm dm); umbilicus closed at 8 mm dm. Whorl profile depressed in all stages up to 50 mm dm; coiling rate very low or low (WER ~1.50) up to 20 mm dm, thereafter slowly increasing to 1.75. Ornament with fine and sharp, narrow-standing growth lines with nearly linear course in the subadult stage but weakly convex course in the adult stage. Weak shell constrictions with nearly straight course up to 20 mm dm; adult stage only with internal shell thickenings. Suture line with very narrow, lanceolate external lobe and very narrow, symmetric, V-shaped adventive lobe.

Etymology

From the Latin ‘ perditum ’ = ‘lost, hopeless’; named after the position of the species near the extinction of the clade.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 2; Vöhringer Coll.; illustrated by Korn (1994: text-fig. 19a), re-illustrated here in Fig. 12A ; GPIT-PV- 63861 .

Paratypes

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting ; Hangenberg Limestone , bed 1 ; Vöhringer Coll.; GPIT-PV- 63863 , GPIT-PV- 64013 • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting ; Hangenberg Limestone , bed 2; Vöhringer Coll.; GPIT- PV- 63864 , GPIT-PV- 63873 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting ; Hangenberg Limestone , bed 5 ; Vöhringer Coll.; GPIT-PV- 63862 • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting ; Hangenberg Limestone , bed 1; Vöhringer Coll.; MB.C. 31051.1– 2 • 3 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 2; Vöhringer Coll.; MB.C. 31052.1–3 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5; Vöhringer Coll.; MB.C. 31053 • 3 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting ; Hangenberg Limestone , loose material; Vöhringer Coll.; MB.C.31054.1– 3 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting ; Hangenberg Limestone , loose material; Korn 1982 Coll.; MB.C. 31055 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone , upper part; Weyer 1993–1994 Coll.; MB.C. 31056 • 1 specimen; Rhenish Mountains , Oese, old quarry; Hangenberg Limestone , bed 11; Weyer & Korn 2000 Coll.; MB.C. 5263 .

Description

Holotype GPIT-PV-63861 is a fairly well-preserved internal mould with 62 mm diameter. It has a thickly discoidal conch morphology (ww/dm = 0.52) and a closed umbilicus ( Fig. 12A ). The conch is widest near the rounded umbilical margin; from there the flanks slowly converge towards the broadly rounded venter. The coiling rate is low (WER = 1.70). The internal mould shows two constrictions 90 degrees apart. Remains of the shell show delicate growth lines with weakly biconvex course and a shallow shell constriction without an apertural shell bulge.

Paratype GPIT-PV-63873 is, with 82 mm conch diameter, the largest of the present specimens ( Fig. 13 ). Corresponding to the general ontogenetic trend towards more slender conchs, it is thickly discoidal at this growth stage (ww/dm = 0.49); the whorl profile is weakly compressed in this growth stage (ww/wh = 0.87).

Paratype GPIT-PV-63864 shows the subadult morphology at 19 mm diameter ( Fig. 12B ). The conch is pachyconic (ww/dm = 0.72) with broadly rounded flanks and venter and has a low coiling rate (WER = 1.56). The shell bears constrictions at 90 degree intervals; they extend with an almost straight course across the flanks and the venter. These fine constrictions are accompanied on the apertural side by a low radial bulge.

The suture line of paratype GPIT-PV-64013 is characterised by very narrow lobes ( Fig. 14F ). Both the lanceolate external lobe and the almost symmetrical, V-shaped adventive lobe have only about one third of the width of the broadly rounded, somewhat asymmetrical ventrolateral saddle.

Vöhringer had already produced eight cross sections, but he only used one of them for his publication. All of them are now presented here ( Figs 14A–E , 15 ). These cross sections show very similar conch geometries and ontogenetic pathways; intraspecific variation is apparently rather low. It should be noted that the variation decreases during ontogeny for all four cardinal conch parameters.

All cross sections show very similar whorl profiles, which are already horseshoe-shaped in the juvenile stage at about 5 mm conch diameter. During ontogeny, however, the ww/wh ratio decreases continuously from an average value of 2.00 at 1 mm dm to a value of 1.35 at 20 mm dm ( Fig. 14I ). In the adult stage of 80 mm conch diameter, the value is only 0.90. The umbilicus is slightly open in the juvenile stage at 2 mm conch diameter with a uw/dm ratio reaching a value of 0.20, but the umbilicus is completely closed already at about 8 mm conch diameter. The coiling rate remains low during ontogeny; it increases only in the adult stage, but remains below a value of 1.75 ( Fig. 14J ).

Remarks

The material described here as the new species Mimimitoceras perditum sp. nov. was previously ( Vöhringer 1960; Korn 1994) attributed to the species originally described by Schindewolf (1923) as “ Postprolobites varicosus ”. However, Mimimitoceras varicosum has a stouter conch than M. perditum and biconvex growth lines and constrictions. M. varicosum is most probably from the latest Famennian or earliest Tournaisian part of the Gattendorfia Limestone , because at Gattendorf only assemblages corresponding to either the “Stockum level” or the basal part of the Gattendorfia Limestone are known. M. perditum , on the other hand, occurs at Oberrödinghausen predominantly in the upper portion of the Gattendorfia Limestone.

Mimimitoceras perditum sp. nov. differs from M. hoennense in the stouter conch (ww/dm ~0.65 at 30 mm dm in M. perditum but only ~ 0.50 in M. hoennense ). The new species differs from the Late Devonian species M. trizonatum Korn, 1988 , M. liratum ( Schmidt, 1924) and M. fuerstenbergi Korn, 1992 , which possess rather stout conchs, in the nearly straight course of the growth lines and the weaker shell constrictions.

Vöhringer (1960) already reported“ Imitoceras varicosum ” from near the base of the Hangenberg Limestone (bed 5), and, separated by a gap in the occurrence, from beds 2 and 1 at the top of the unit. Indeed, a few specimens from bed 5 are available in the Vöhringer collection. However, new collections did not produce any identifiable specimens of Mimimitoceras from the basal part of the Gattendorfia Limestone.

Figures

Fig. 12. Mimimitoceras perditum sp. nov., from the Oberrödinghausen railway cutting, both Vöhringer Coll. A. Holotype GPIT-PV-63861 from bed 2. B. Paratype GPIT-PV-63864 from bed 2. Scale bar units = 1 mm.

Fig. 13. Mimimitoceras perditum sp. nov., paratype GPIT-PV-63873 (Vöhringer Coll.) from the Oberrödinghausen railway cutting, bed 1. Photograph of the unwhitened specimen showing exemplary the often poorly preserved shell surface of the material from Oberrödinghausen. Scale bar units =1 mm.

Fig. 14. Mimimitoceras perditum sp. nov. from the Oberrödinghausen railway cutting, all Vöhringer Coll. A. Cross section of paratype MB.C.31054.1 from an unknown bed. B. Cross section of paratype MB.C.31052.1 from bed 2. C. Cross section of paratype MB.C.31052.2 from bed 2. D. Cross section of paratype MB.C.31054.2 from an unknown bed. E. Cross section of paratype MB.C.31052.3 from bed 2. F. Suture line of paratype GPIT-PV-64013 from bed 1, at dm =25.0 mm, ww =16.5 mm, wh =13.0 mm. G. Growth line course of paratype GPIT-PV-63862 from bed 5, at ww =26.0 mm, wh= 25.5 mm. H–J. Ontogenetic development of the conch width index (ww/dm), umbilical width index (uw/dm), whorl width index (ww/wh) and whorl expansion rate (WER) of selected specimens. Scale bar units =1 mm.

Fig. 15. Mimimitoceras perditum sp. nov. from the Oberrödinghausen railway cutting, all Vöhringer Coll. A. Cross section of specimen MB.C.31051.1 from bed 2. B. Cross section of specimen GPIT- PV-63862 from bed 5. C. Cross section of specimen MB.C.31053 from bed 2. Scale bar units =1 mm.