Stockumites hofensis, Korn & Weyer, 2023

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 : 32-35

Stockumites hofensis sp. nov.

urn:lsid:zoobank.org:act:0776DC38-0887-4CEB-98A7-99A3408F2F14

Figs 16–18 ; Tables 10–11

Imitoceras Gürichi – Schindewolf 1923: 331, pl. 15 fig. 1, pl. 16 fig. 1, text-fig. 4d.

Diagnosis

Species of Stockumites with a conch reaching 90 mm diameter. Conch at 5 mm dm thickly discoidal, subinvolute (ww/dm ~ 0.58, uw/dm ~ 0.18); at 15 mm dm thickly discoidal, involute (ww/dm ~ 0.55, uw/dm ~ 0.02); at 30 mm dm thinly discoidal, involute (ww/dm ~ 0.40, uw/dm ~ 0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~ 0.65); coiling rate high (WER ~ 2.05). Venter narrowly rounded, umbilical margin rounded. Growth lines fine, narrow-standing, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with lanceolate external lobe and narrowly V-shaped adventive lobe.

Etymology

After the city of Hof, seven kilometres west of the Gattendorf outcrop.

Material examined

Holotype GERMANY • Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21c–d (“ Gattendorfia Limestone ”); Korn 1989 Coll.; illustrated in Fig. 16C ; MB.C.31269.1.

Paratypes GERMANY • 6 specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; BGRB X13397 – BGRB X13402 • 15 specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21c–d (“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31269.2–16.

Additional material examined GERMANY • 55 specimens; Upper Franconia , 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; BGRB unnumbered .

Description

Specimen MB.C.31269.1 is selected as the holotype because it shows the suture line and the shell ornamentation in addition to the only slightly distorted conch shape ( Fig. 16C ); it was also taken at a well-documented stratigraphic position in the section. Therefore, it can best be taken as representative of the species. It has a diameter of 30 mm and is thinly discoidal (ww/dm =0.43) with a depressed whorl profile (ww/wh = 0.70). The conch is widest at the rounded umbilical margin, from where the somewhat flattened flanks converge to the narrowly rounded venter. The coiling rate is moderately high (WER =1.96). Some remains of the shell show very fine, lamellar-like, quite widely spaced growth lines with a convex course. The suture line has a narrow, lanceolate external lobe, a broadly rounded, nearly symmetric ventrolateral saddle and a V-shaped, symmetric adventive lobe ( Fig. 17C ).

The paratypes BGRB X13397, BGRB X13398, BGRB X13399 and BGRB X13400 ( Fig. 16A–B, D–E ) agree, in their conch proportions, broadly with the holotype; the intraspecific variation is quite small. However, it must be taken into account that the conch undergoes an ontogenetic change towards a more slender adult shape.

The cross sections of the paratypes MB.C.31269.2, MB.C.31269.3 and MB.C.31269.4 show rather similar ontogenetic developments of the conch geometry ( Fig. 18A–C ). The best preserved of these is specimen is MB.C.31269.3; it allows the study from the initial stage up to a diameter of 31 mm ( Fig. 18B ). The innermost whorls up to 2.5 mm conch diameter have a kidney-shaped profile; they embrace each other very little. After that, the whorls begin to increase in height and at a conch diameter of about 10 mm, they have a horseshoe-shaped profile. After that, more intensive growth in height leads to the formation of the characteristic adult stage with a compressed whorl profile; this is characterised by strongly convergent flanks and a slightly rounded venter.

The ontogenetic trajectories show a very simple biphasic or monophasic course ( Fig. 18D–F ). The ww/ dm ratio remains constant between 1 mm and 10 mm conch diameter at a value of 0.60; in the later stage it decreases continuously down to a value of 0.35 at 70 mm diameter. The uw/dm ratio is high at 1.5 mm conch diameter (uw/dm =0.45) with continuous reduction thereafter. The umbilicus is almost closed already at a conch diameter of around 10 mm. The ww/wh trajectory is monophasic with a continuous decline from about 2.00 at 1 mm diameter to 0.60 at 70 mm diameter. In contrast, the evolution of the coiling rate is biphasic; a rapid ontogenetic increase means that the value of WER=2.00 is already reached at a conch diameter of 7 mm. After that, the value remains stable in the following ontogeny.

The suture lines show a slight variation in their course. While the external lobe is always deep and narrowly lanceolate, the V-shaped adventive lobe shows differently shaped flanks ranging from almost straight to sigmoidal ( Fig. 17C–E ).

Remarks

Stockumites hofensis sp. nov. resembles S. nonaginta sp. nov.; both species are difficult to distinguish without knowledge of the suture line. This differs with regard to the adventive lobe, which is narrowly V-shaped in S. hofensis , but wide V-shaped and almost rectangular in S. nonaginta . The whorl profile also shows some differences; in S. hofensis the conch is widest in close proximity to the umbilicus, while in S. nonaginta the widest point is some distance from the umbilicus on the inner flank.

Stockumites hofensis sp. nov. differs from S. subbilobatus by the more slender conch; at 30 mm dm the ww/dm ratio is about 0.42 in S. hofensis but more than 0.50 in S. subbilobatus . In addition, S. subbilobatus has lamellar growth lines in contrast to S. hofensis with very fine growth lines.

Figures

Fig. 16. Stockumites hofensis sp. nov. from Gattendorf. A. Paratype BGRB X13397 (Schindewolf 1934 Coll.) from bed 21. B. Paratype BGRB X13398 (Schindewolf 1934 Coll.) from bed 21. C. Holotype MB.C.31269.1 (Korn 1989 Coll.) from bed 21c–d.D. Paratype BGRB X13399 (Schindewolf 1934 Coll.) from bed 21. E. Paratype BGRB X13400 (Schindewolf 1934 Coll.) from bed 21. Scale bar units=1 mm.

Fig. 17. Stockumites hofensis sp. nov. from Gattendorf. A. Cross section of paratype MB.C.31269.5 (Korn 1989 Coll.) from bed 21c–d. B. Cross section of paratype MB.C.31269.6 (Korn 1989 Coll.) from bed 21c–d. C. Suture line of holotype MB.C.31269.1 (Korn 1989 Coll.) from bed 21c–d, at ww =9.6 mm, wh= 10.8 mm. D. Suture line of paratype BGRB X13398 (Schindewolf 1934 Coll.) from bed 21, at dm =28.8 mm, ww =11.4 mm, wh =15.1 mm. E. Suture line of paratype BGRB X13397 (Schindewolf 1934 Coll.) from bed 21, at ww =12.6 mm, wh=15.7 mm. Scale bar units =1 mm.

Fig. 18. Stockumites hofensis sp. nov. from Gattendorf, bed 21c–d. A. Cross section of paratype MB.C.31269.2 (Korn 1989 Coll.). B. Cross section of paratype MB.C.31269.3 (Korn 1989 Coll.). C. Cross section of paratype MB.C.31269.4 (Korn 1989 Coll.). 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.