Acutimitoceras ucatum, 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 : 86-88

Acutimitoceras ucatum sp. nov.

urn:lsid:zoobank.org:act:93EC6766-E4CB-4811-A919-79F87CF41BAE

Figs 52–53 ; Tables 48–49

Aganides acutus – Schmidt 1925: 534, pl. 19 fig. 5.

Imitoceras acutum – Vöhringer 1960: 137, pl. 1 fig. 7, text-fig. 16.

Acutimitoceras acutum – Korn 1994: 42, text-figs 49j–k, 50b, 51a, 53c–d, 56h, 57a.

Acutimitoceras acutum acutum – Bartzsch & Weyer 1996: 95, text-fig. 1.

Acutimitoceras (Acutimitoceras) acutum – Becker 1996: 36.

Diagnosis

Species of Acutimitoceras with thickly discoidal and subinvolute conch at 6 mm dm (ww/dm ~0.50; uw/ dm ~0.25), thinly discoidal and involute conch at 12 mm dm (ww/dm ~0.40; uw/dm ~0.12) and thinly discoidal and involute conch at 24 mm dm (ww/dm ~0.35; umbilicus closed). Whorl cross section at 24 mm dm compressed (ww/wh ~0.60); coiling rate high (WER ~2.10). Venter subacute at 8 mm dm and acute at 15 mm dm. Lamellar growth lines with slightly biconvex course. Weak shell constrictions; they largely follow the course of the growth lines.

Etymology

An anagram of acutum, referring to the close resemblance of the two species.

Material examined

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen , railway cutting; Hangenberg Limestone , bed 6; Vöhringer Coll.; illustrated by Korn (1994: text-fig. 49k); re-illustrated here in Fig. 52B ; GPIT- PV-63900.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6; Vöhringer Coll.; GPIT-PV-63901, GPIT-PV-63902 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6; Vöhringer Coll.; GPIT-PV-63899 • 2 specimens; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 4; Vöhringer Coll.; MB.C.31121.1–2 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5; Vöhringer Coll.; MB.C.31122 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 5a2; Weyer 1993–1994 Coll.; MB.C.31123 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6a; Weyer 1993–1994 Coll.; MB.C.31124 • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6b2; Weyer 1993–1994 Coll.; MB.C.31125 • 1 specimen; Rhenish Mountains , Oese, old quarry; Hangenberg Limestone, bed 10; Becker 1988 Coll.; MB.C.3374 .

Description

Holotype GPIT-PV-63900 is the better preserved of the two illustrated specimens collected by Vöhringer (1960). It has a conch diameter of 22 mm and is widely covered with shell remains ( Fig. 52B ). The conch has the shape of a discus (ww/dm = 0.35) and the umbilicus is almost completely closed. The whorl profile shows convex flanks, which are almost parallel in the dorsolateral region and, in the ventrolateral region, converge rapidly towards the acute venter. An attached keel supports the sharpening of the venter.

The shell surface of the holotype shows lamellar, regularly spaced growth lines with a weakly biconvex course ( Fig. 53C ). Both lateral projections are low and the ventral sinus is very shallow. Very weak shell constrictions extend parallel to the growth lines.

Paratype GPIT-PV-63899 has a 25 mm conch diameter and is very similar to the holotype in its conch proportions and also in its shell ornament ( Fig. 52A ). In this specimen, too, the umbilicus is not yet completely closed. The suture line has a wide, V-shaped external lobe with nearly straight flanks, a rounded ventrolateral saddle and an asymmetric adventive lobe. This is pointed at its base and possesses a strongly convex ventral flank and a weakly convex dorsal flank ( Fig. 53B ).

The sectioned paratype GPIT-PV-63902 allows the study of its conch ontogeny up to a diameter of 24 mm ( Fig. 53A ). There are very noticeable changes in conch geometry; the whorl profile begins kidney-shaped in the early juvenile stage, followed by an almost circular profile, which changes to a galeate and finally oxyconic shape with increasing whorl height. The coiling rate shows an almost continuous increase during ontogeny. Between 2 and 3 mm conch diameter it is only about 1.70 and gradually increases to a maximum value of WER = 2.11 at 25 mm diameter.

Remarks

For the separation of Acutimitoceras ucatum sp. nov. and A. acutum , cross-sections of the conchs are best suited. Paratype GPIT-PV-63902 (with the penultimate half of the whorl reconstructed by interpolation) and a cross-section of a paratype of “ A. acutum oxynotum ” (= A. acutum ) from Saalfeld show especially the differences in the shape of the flanks. While the flanks are convex in A. ucatum , they are flattened and more strongly convergent in A. acutum . The growth diagrams show that A. acutum and A. ucatum hardly differ in the ontogenetic courses. The two species can also be separated by the strength of the growth lines, which are rather coarse in A. ucatum but very delicate in A. acutum .

Figures

Fig. 52. Acutimitoceras ucatum sp. nov. from the Oberrödinghausen railway cutting, both Vöhringer Coll. A. Paratype GPIT-PV-63899 from bed 5. B. Holotype GPIT-PV-63900 from bed 6. Scale bar units =1 mm.

Fig. 53. Acutimitoceras ucatum sp. nov. from the Oberrödinghausen railway cutting. A. Cross section of paratype GPIT-PV-63902 from bed 6. B. Suture line of paratype GPIT-PV-63899 from bed 5, at ww =7.2 mm, wh=8.5 mm. C. Growth line course of holotype GPIT-PV-63900 from bed 6, at ww =7.3 mm, wh=8.8 mm. D–F. 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.