Ponticeras materni, Korn & Bockwinkel, 2022

Korn, Dieter & Bockwinkel, Jürgen, 2022, The early gephuroceratid ammonoids from the Roteisenstein Formation of Dillenburg (Cephalopoda, Ammonoidea), European Journal of Taxonomy 823, pp. 10-47 : 25-28

Ponticeras materni sp. nov.

urn:lsid:zoobank.org:act:498BFB7B-FB0D-46D1-8BE5-89323D1C8628

Figs 11–12

Gephyroceras aequabile var. nov. – Wedekind 1913: 70, pl. 6 fig. 14, text-fig. 14b.

Diagnosis

Species of Ponticeras reaching about 80 mm conch diameter with thinly discoidal, subevolute conch at 5 mm dm (ww/dm ~ 0.45; uw/dm ~ 0.42), with extremely discoidal, subevolute conch at 15 mm dm

(ww/dm ~ 0.32; uw/dm ~ 0.33) and with extremely discoidal, subevolute conch at 50 mm dm (ww/dm ~ 0.25; uw/dm ~ 0.40); coiling rate usually low to moderately high (WER = 1.70–1.85). Whorl profile depressed in early ontogeny (ww/wh ~ 1.40 at 5 mm dm) and compressed in late ontogeny (ww/wh ~ 0.70 at 50 mm dm); umbilical wall oblique and broadly rounded, venter weakly flattened. Growth lines fine, rhythmically strengthened to weak plications, strongly biconvex. Sometimes with a spiral ridge on the outer side of the umbilicus parallel to the umbilical seam. Suture line with V-shaped external lobe with strongly diverging flanks; median saddle comparatively high, E 2 lobe deep, pointed, ventrolateral saddle and lateral lobe rounded.

Etymology

Named after Hans Matern to honour his work on Devonian ammonoids.

Material examined

Holotype GERMANY • Rhenish Mountains , Oberscheld (Grube Volpertseiche); late Givetian (Red Ironstone); Koch Coll.; MB.C.22159 . Illustrated in Fig. 11A .

Paratypes GERMANY • 1 specimen; Rhenish Mountains , Dillenburg; late Givetian (Red Ironstone); Kauth Coll.; MB.C.22178 • 1 specimen; Rhenish Mountains , Oberscheld (?); late Givetian (Red Ironstone); Dannenberg Coll.; MB.C.4297 • 1 specimen; Rhenish Mountains , Oberscheld (?); late Givetian (Red Ironstone); MB.C.22158 • 1 specimen; Rhenish Mountains , Oberscheld (Grube Prinzkessel ; late Givetian (Red Ironstone); Zimmermann 1936 Coll.; MB.C.22175 • 1 specimen; Rhenish Mountains, Oberscheld (Sessacker) ; late Givetian (Red Ironstone); Dannenberg Coll.; MB.C.30427 .

Description

Five specimens are selected for description and illustration:

MB.C.22159: incomplete but rather well-preserved specimen with 49 mm conch diameter in haematitic ironstone ( Fig. 11A ).

MB.C.22175: moderately preserved specimen with 38 mm conch diameter in haematitic ironstone; the last quarter of the last volution belongs to the body chamber ( Fig. 11B ).

MB.C.22178: well-preserved specimen with 25 mm conch diameter in haematitic limestone ( Fig. 11C ).

MB.C.22158: specimen with 68 mm conch diameter in micritic haematite-rich limestone; cut for a cross section ( Fig. 12A ).

MB.C.4297: incomplete specimen with approximately 80 mm conch diameter in micritic haematite-rich limestone.

Holotype MB.C.22159 is, at 49 mm conch diameter, extremely discoidal and subevolute ( Fig. 11A ); the whorl profile shows a flattened, oblique umbilical wall, a rounded umbilical margin, weakly convergent flanks with a faint longitudinal ventrolateral depression and a rounded venter that is separated from the flanks by a weakly angular shoulder. The shell ornament is not well-preserved, but it appears that growth lines are fine and only strengthened rhythmically on the umbilical wall. The penultimate whorl shows very weak, short plications on the umbilical margin. Only a small portion of the ventral suture line is visible in the specimen; it shows the shape of the external lobe typical for Ponticeras .

Paratype MB.C.22175 has an extremely discoidal and subevolute conch with moderate coiling rate at 38 mm diameter; the whorl profile is compressed with oblique, broadly rounded umbilical wall that continues into the broadly rounded umbilical margin ( Fig. 11B ). The flanks are subparallel and separated from the narrowly rounded venter by a weakly angular ventrolateral shoulder. The venter becomes subacute during the last volution. Few shell remains show biconvex growth lines that form a very high, narrow ventrolateral projection and a deep ventral sinus. The suture line has a V-shaped external lobe with a median saddle reaching almost half of the height of the lobe depth. The ventrolateral saddle is rounded; the lateral lobe reaches only 60% of the E lobe depth and is broadly rounded ( Fig. 12B ).

The smaller paratype MB.C.22178 with 25 mm conch diameter does not show the suture line, species assignment is thus not completely clear. However, it shows the shell ornament well-preserved with strongly concavo-convex growth lines, which form a high linguiform ventrolateral salient. As in the holotype, the growth lines are strengthened rhythmically on the umbilical wall and the umbilical margin has weak plications ( Fig. 11C ).

The specimens allow the study of a growth interval between 25 and nearly 60 mm diameter. The material is somewhat variable with respect to the shape of the whorl profile, which in specimens MB.C.22159 and MB.C.22158 (the sectioned one) has a slightly flattened, oblique umbilical wall and a more pronounced umbilical margin when compared with specimen MB.C.4297. The shape of the venter also shows some variation; it is more strongly flattened in specimen MB.C.22158 ( Fig. 12A ) than in specimen MB.C.22159 ( Fig. 11A ).

All of the specimens show the shell ornament, which consists of very fine but rhythmically strengthened growth lines. These have a biconvex course with a low dorsolateral projection, a wide and shallow lateral sinus, a high and narrow ventrolateral projection and a deep ventral sinus. In the immature stage between about 15 and 30 mm diameter, there are weak and short radial plications visible on the umbilical margin. In specimen MB.C.22158, a faint spiral ridge accompanies the umbilical seam.

Specimen MB.C.22158 was sectioned and allows the study of conch geometry from the initial stage up to 68 mm conch diameter ( Fig. 12A ). The ontogenetic trajectories of the cardinal conch parameters are strikingly different: (1) The ww/dm and the ww/wh ratios are nearly monophasic with nearly continuously decreasing values; between 1 and 68 mm conch diameter, the ww/dm ratio decreases from 0.80 to 0.24 and the ww/wh ratio from nearly 2.00 to 0.75. (2) The uw/dm trajectory is triphasic with early ontogenetic increase from 0.32 to 0.43 between 1 and 4.5 mm dm, followed by a decrease to 0.33 at 16 mm dm and an adult re-increase to 0.40 at 40 mm. The umbilical ratio then remains at this value. (3) The WER trajectory is also triphasic with an early juvenile decrease from 2.00 at 1 mm dm to 1.73 at 2 mm dm, followed by an accelerated increase to a maximum value of 2.06 at 16 mm dm and a terminal decrease to around 1.70 in the largest stage. The whorl profile is crescent-shaped in the early juvenile stage up to 2 mm diameter and then becomes increasingly compressed. At about 10 mm conch diameter, the ww/wh ratio becomes lower than 1.00. Already at 8 mm diameter, the umbilical wall becomes very flat and connects with the preceding whorl in a very small angle. At 22 mm diameter, the umbilical wall is weakly incurved, a character that is maintained until the terminal stage. The venter is narrowly rounded at 30 mm diameter, but in the last one and a half whorls, it becomes flattened and gets bordered from the flanks by a pronounced, subangular ventrolateral shoulder.

Remarks

Ponticeras materni sp. nov. is similar to P. aequabile but differs in the flattened venter, which is narrowly rounded in the latter species. Ponticeras materni sp. nov. differs from the other species with a flattened venter ( P. auritum ) in the much wider umbilicus (uw/dm = 0.35–0.40 in P. materni sp. nov. but only ~ 0.20 in P. auritum ).

Figures

Fig. 11. Ponticeras materni sp. nov. A. Holotype MB.C.22159 (Koch Coll.) from Oberscheld (Volpertseiche Mine).B. Paratype MB.C.22175 (Zimmermann 1936 Coll.) from Oberscheld (Prinzkessel Mine). C. Paratype MB.C.22178 (Kauth Coll.) from Dillenburg. Scale bar units = 1 mm.

Fig. 12. Ponticeras maternisp. nov. A. Cross section of paratype MB.C.22158, probably from Oberscheld. B. Suture line of paratype MB.C.22175 (Zimmermann 1936 Coll.) from Oberscheld (Prinzkessel Mine), at dm = 29.0 mm, ww = 7.4 mm, wh = 11.1 mm. C–F. Ontogenetic trajectories of the cardinal conch parameters. Scale bar units = 1 mm.