Acutimitoceras paracutum, 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 : 88-90

Acutimitoceras paracutum sp. nov.

urn:lsid:zoobank.org:act:CBBCC2D2-8B60-491F-9F4F-875625DFFFC7

Fig. 54 ; Table 50

Diagnosis

Species of Acutimitoceras with a thinly discoidal and involute adult stage at 20 mm dm (ww/dm ~0.40; uw/dm ~0.12). Whorl cross section at 20 mm dm compressed (ww/wh ~0.75); coiling rate low (WER ~1.70). Venter narrowly rounded at 10 mm dm and subacute at 20 mm dm. Lamellar growth lines with slightly biconvex course. Weak constrictions on the shell surface; they stand in distances of about 75 degrees and largely follow the course of the growth-lines.

Etymology

Named because of the morphological similarity to Acutimitoceras acutum .

Material examined

Holotype

GERMANY • Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6a; Weyer 1993–1994 Coll.; illustrated in Fig. 54B ; MB.C.31127.

Paratype

GERMANY • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6a; Korn 1989 Coll.; MB.C.31126 .

Description

Holotype MB.C.31127 has a conch diameter of 20 mm and is a well-preserved specimen ( Fig. 54B ). It is discus-shaped and involute (ww/dm = 0.40; uw/dm = 0.12) and possesses a whorl profile with a rounded umbilical margin, from where the convex flanks converge towards the subacute venter. The ww/wh ratio is 0.77 and the coiling rate is low (WER = 1.72). The last whorl of the specimen is increasingly overlapping upon the umbilicus; therefore, the uw/dm ratio has a value of 0.17 at about 15.5 mm conch diameter.

The entire specimen is covered with shell, which possesses nearly equidistant lamellar growth lines, five of which are counted on one millimetre on the outer flank. They have a weakly biconvex course with a low dorsolateral projection in some distance from the umbilicus on the inner flank, a very shallow lateral sinus, a low ventrolateral projection and a shallow, broadly V-shaped but closely rounded ventral sinus. The specimen has five shell constrictions, which are almost regularly spaced. They are slightly oblique in their course so that they cross the growth lines at a very acute angle.

Paratype MB.C.31126 has 24 mm conch diameter and corresponds to the holotype with regard to conch geometry ( Fig. 54A ). However, the umbilicus is almost closed in this specimen. The shape of the venter is subacute with an attached keel. The shell bears widespread, lamellar growth lines running across the keel and four constrictions at 90-degree intervals.

Remarks

Acutimitoceras paracutum sp. nov. differs from the other species of the genus by the low coiling rate and the rapid closure of the umbilicus at a large conch diameter. While the holotype of A. paracutum has a whorl expansion rate of only ~1.72 at 20 mm conch diameter, the other species of Acutimitoceras show a much higher value (normally above 2.00). The similar A. acutum , for instance, shows a much more rapid coiling (WER ~2.05) at the same growth stage. The second difference to A. acutum is the wider umbilicus in A. paracutum (uw/dm ~0.12 at 20 mm dm in A. paracutum but only about ~ 0.05 in A. acutum ).

Figures

Fig. 54. Acutimitoceras paracutum sp. nov. from the Oberrödinghausen railway cutting. A. Paratype MB.C.31126 (Korn 1989 Coll.) from bed 6a. B. Holotype MB.C.31127 (Weyer 1993–1994 Coll.) from bed 6a. Scale bar units= 1 mm.