Stockumites nonaginta Korn & Weyer 2023, sp. nov.

Stockumites nonaginta sp. nov.

urn:lsid:zoobank.org:act: 4325F28E-A831-4D8E-8248-AFCFE79608CE

Figs 19–20; Tables 12–13

Species of Stockumites with a conch reaching 70 mm diameter. Conch at 5 mm dm thinly pachyconic, subinvolute (ww/dm ~ 0.70, uw/dm ~ 0.16); at 15 mm dm thinly pachyconic, involute (ww/dm ~ 0.65, uw/dm ~ 0.00); at 30 mm dm thinly discoidal, involute (ww/dm ~ 0.50, uw/dm ~ 0.00). Whorl profile at 30 mm dm weakly compressed (ww/wh ~ 0.85); coiling rate high (WER ~ 2.05). Venter rounded, umbilical margin rounded. Growth lines very fine, narrow-standing, with convex course. Without constrictions on the shell surface; without internal shell thickenings. Suture line with lanceolate external lobe and widely V-shaped adventive lobe.

From the Latin ‘ nonaginta ’, meaning ‘90’ and referring to the shape of the adventive lobe.

Holotype GERMANY • Upper Franconia, 400 m north-west of Kirchgattendorf; bed 21 (“ Gattendorfia Limestone ”); Schindewolf 1934 Coll.; illustrated in Fig. 19; BGRB X13403.

Paratypes GERMANY • 2 specimens; Upper Franconia, 400 m north-west of Kirchgattendorf; bed 21c–d (“ Gattendorfia Limestone ”); Korn 1989 Coll.; MB.C.31270.1–2.

Holotype BGRB X13403 is 51 mm in diameter (Fig. 19), which allows for the study of the conch morphology as well as areas of the shell surface and the suture line. The conch is thinly discoidal with an almost completely closed umbilicus (ww/dm= 0.37; uw/dm =0.02). The whorl profile is compressed (ww/dm= 0.67); it is widest on the inner flank at a short distance from the rounded umbilical margin. The flanks slowly converge towards the continuously rounded venter. It appears that the shell is completely smooth; small remnants of shell show only very faint growth lines. The suture line has a very narrow and deep, parallel-sided external lobe, which continues into a broadly rounded ventrolateral saddle. The adventive lobe is almost right-angled V-shaped (Fig. 20C).

The two sectioned paratypes MB.C.31270.1 and MB.C.31270.2 show very similar conch proportions and virtually the same ontogenetic trajectories (Fig. 20A–B). The inner whorls are subevolute up to 2 mm conch diameter (the uw/dm reaches 0.36); then the umbilicus is slowly closed. The whorl profile is kidney-shaped to a conch diameter of 2 mm; thereafter it is C-shaped up to a conch diameter of 10 mm. With a diameter of 10 mm the conch flattens out and with a diameter of 30 mm the conch is thickly discoidal (ww/dm =0.50).

The growth trajectories of the cardinal conch parameters show different courses (Fig. 20D–F). A biphasic course can be seen in the ww/dm ratio. Between 1 and 10 mm conch diameter the ww/dm value increases slowly from 0.60 to 0.70; thereafter there is a continuous decrease to 0.50 at 30 mm dm. The course of the ww/wh trajectory is monophasic with a smooth decline from 2.00 at 1 mm dm to 0.85 at 30 mm.

Stockumites nonaginta sp. nov. and S. hofensis sp. nov. are very similar species and can only be reliably distinguished by the suture line. While the adventive lobe of S. nonaginta is broadly V-shaped and almost rectangular, it is narrowly V-shaped in S. hofensis.

To distinguish the two species within the assemblage, the suture lines of several specimens were exposed. Unfortunately, this was not successful for all sectioned specimens, so that it is not entirely certain whether the suture lines of the two sectioned specimens actually correspond to the holotype.