Stockumites kleinerae ( Korn, 1984 )

Korn, Dieter & Weyer, Dieter, 2023, The ammonoids from the Gattendorfia Limestone of Gattendorf (Devonian-Carboniferous boundary; Upper Franconia, Germany), European Journal of Taxonomy 883, pp. 1-61 : 16-19

Stockumites kleinerae ( Korn, 1984)

Figs 6–7 ; Table 3

Acutimitoceras kleinerae Korn, 1984: 74 , pl. 1 figs 1–5, pl. 3 fig. 23, text-figs 4c–d, 5a.

Acutimitoceras kleinerae – Korn 1992b: 16, pl. 2 figs 4–5; 1994: 47, text-figs 36e, 38a–e, 40b, 44g –h, 45e, 48c–d, 56a–b, 57d–e. — Schoenlaub et al. 1992: pl. 5 figs 4–5. — Kullmann 2000: text-fig. 4h. — Korn & Klug 2002: 197, text-fig. 173a.

Stockumites kleinerae – Korn & Weyer 2023: 47, figs 29–30.

Imitoceras Denckmanni – Schindewolf 1923: 336, pl. 15 figs 5–6, text-fig. 4h.

Imitoceras substriatum – Vöhringer 1960: 128, pl. 3 fig. 1, text-fig. 9.

non Acutimitoceras kleinerae – Sheng 1989: 110, pl. 33 figs 3–5.

Diagnosis

Species of Stockumites with a conch reaching 70 mm diameter. Conch at 5 mm dm pachyconic to globular, subinvolute to subevolute (ww/dm =0.60–0.90; uw/dm=0.20–0.40); at 15 mm dm pachyconic to globular, involute (ww/dm =0.80–0.90; uw/dm=0.05–0.15); at 30 mm dm thickly pachyconic, involute (ww/dm ~ 0.75; uw/dm=0.00–0.05). Whorl profile at 30 mm dm weakly depressed (ww/wh ~ 1.35); coiling rate moderately high (WER ~ 1.80). Venter very broadly rounded, umbilical margin broadly rounded. Growth lines fine to coarse, wide-standing, with convex course. Without constrictions on the shell surface; with weak internal shell thickenings. Suture line with narrow external lobe and narrowly V-shaped adventive lobe (from Korn & Weyer 2023).

Material examined

Holotype GERMANY • Rhenish Mountains , forestry road cutting 900 m east of Stockum ; Stockum Limestone ( Stockumites prorsus Zone ); Korn 1982 Coll.; illustrated by Korn (1984: pl. 1 fig. 3) and Korn (1994: text-fig. 38b); SMF 43001 .

Paratypes GERMANY • 15 specimens; Rhenish Mountains , forestry road cutting, 900 m east of Stockum ; Stockum Limestone; Korn 1982 Coll.; SMF 43002 – SMF 43016 • 13 specimens; Rhenish Mountains , trench II , 950 m east of Stockum ; Stockum Limestone; Korn 1982 Coll.; SMF 43017 – SMF 43029 • 10 specimens; Rhenish Mountains , Müssenberg ; Hangenberg Limestone, bed 3c; Korn 1980 Coll.; SMF 43030 – SMF 43039.

Additional material GERMANY • 3specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1916 Coll.; SMF Mbg.3108, SMF Mbg.7561– SMF Mbg.7562 • 1 specimen; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); GZG. INV.141 • 4 specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; BGRB X13381 – BGRB X13383 , BGRB unnumbered specimen • 1 specimen; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21a(“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31261 • 2 specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21c–d (“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31262.1– MB.C.31262.2 .

Description

Specimen MBG3108 is a comparatively well-preserved, only slightly deformed conch with a diameter of 67 mm ( Fig. 6A ). It is thickly discoidal (ww/wh nearly 0.60) with an almost completely closed umbilicus.The whorl width slightly exceeds the whorl height (ww/wh = 1.04); the coiling rate is moderate (WER =1.87). The whorl profile shows a very broadly rounded venter. It seems to be somewhat narrowed at the aperture, but this is apparently caused by deformation of the body chamber. The ornament consists of coarse growth lines with a convex course.

Specimens BGRB X13381 ( Fig. 6C ), BGRB X13382 ( Fig. 6D ) and BGRB X13383 ( Fig. 6E ) show similar conch morphologies between 18 and 32 mm conch diameter; only the ww/dm ratio decreases from 0.84 to 0.78.

The small specimen GZG.INV.141 has a globular shape (ww/dm=0.86) with a slightly opened umbilicus (uw/dm =0.14) at 10 mm conch diameter ( Fig. 6B ). It shows coarse growth lines that curve forward on the flank and form a broad, flat ventral projection. A shallow shell constriction is present at a conch diameter of about 7 mm.

The cross sections of specimens MB.C.31261, SMF Mbg.7561, SMF Mbg.7562 and MB.C.31262.1 show the ontogenetic changes of the conch geometry from the innermost whorls up to a conch diameter of 34 mm ( Fig. 7A–D ). They show that the early juvenile evolute stage with a crescent-shaped whorl profile is present up to a conch diameter of about 5 mm. After this, rapid increase of the whorl width and stagnation of opening of the umbilicus leads to a pachyconic, subinvolute or involute conch with a C-shaped whorl profile already at 8 mm diameter.

The growth trajectory of the ww/dm ratio shows a distinct triphasic ontogeny ( Fig. 7E ), while the ww/ wh trajectory is weakly biphasic with a nearly continuous decrease from a value of 2.00 at 1 mm conch diameter to 1.00 at 70 mm diameter ( Fig. 7F ). The coiling rate is rather low in all growth stages and exceeds a WER value of 1.80 only in the adult stage at about 60 mm conch diameter ( Fig. 7G ).

Remarks

Stockumites kleinerae is, next to Mimimitoceras varicosum , the stoutest ammonoid species in the ammonoid assemblage from bed 21 of Gattendorf. However, both species can easily be distinguished by the presence ( M. varicosum ) or absence ( S. kleinerae ) of shell constrictions. S. kleinerae differs from S. intermedius by the coarser growth lines, which are dense and sharply formed in the adult stage of S. kleinerae , whereas in S. intermedius they are widely spaced and lamellar.

Figures

Fig. 6. Stockumites kleinerae (Korn, 1984) from Gattendorf, bed 21. A. Specimen SMF Mbg.3108 (Schindewolf 1916 Coll.). B. Specimen GZG.INV.141. C. Specimen BGRB X13381 (Schindewolf 1934 Coll.). D. Specimen BGRB X13382 (Schindewolf 1934 Coll.). E. Specimen BGRB X13383 (Schindewolf 1934 Coll.). Scale bar units =1 mm.

Fig. 7. Stockumites kleinerae (Korn, 1984) from Gattendorf. A. Cross section of specimen MB.C.31261 (Korn 1989 Coll.) from bed 21a. B. Cross section of specimen SMF Mbg.7561 (Schindewolf 1916 Coll.) from bed 21. C. Cross section of specimen SMF Mbg.7562 (Schindewolf 1916 Coll.) from bed 21. D. Cross section of specimen MB.C.31262.1 (Korn 1989 Coll.) from bed 21c–d. E–G. 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.