Stockumites procedens ( Korn, 1984 ) 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 : 60-61

Stockumites procedens ( Korn, 1984) comb. nov.

Fig. 36 ; Tables 30–31

Acutimitoceras procedens Korn, 1984: 80 , pl. 4 figs 24–25, text-fig. 5b.

Acutimitoceras procedens – Korn 1994: 47, text-figs 36b–c, 71f.

Acutimitoceras prorsum prorsum – Korn 1981: 519, text-figs 3a, 4a–d.

Diagnosis

Species of Stockumites with a conch reaching 40 mm diameter. Conch at 5 mm dm thinly pachyconic, subevolute (ww/dm ~0.65; uw/dm ~0.35); at 15 mm dm thinly pachyconic, involute (ww/dm ~0.65; uw/dm ~0.12); at 30 mm dm thickly discoidal, involute (ww/dm ~0.55; uw/dm ~0.00). Whorl profile at 30 mm dm weakly depressed (ww/wh ~1.05); coiling rate very high (WER ~2.35). Venter very broadly rounded, umbilical margin broadly rounded. Growth lines fine, with convex course in the middle growth stage and slightly biconvex course in the adult stage. Ornament with shallow constrictions on the shell surface; with coarse internal shell thickenings.

Material examined

Holotype

GERMANY • Müssenberg , trench 1; Hangenberg Limestone , bed 3c; Korn 1980 Coll.; illustrated by Korn (1984: pl. 4 fig. 24) and Korn (1994: text-fig. 36b); GZG.INV.131.

Paratypes

GERMANY • 1 specimen; Müssenberg, trench 1; Hangenberg Limestone, bed 3c; Korn 1980 Coll.; GZG.INV.132 • 3 specimens; same collection data as for preceding; SMF 43141 – SMF 43143 .

Additional material

GERMANY • 1 specimen; Rhenish Mountains , Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 6b; Weyer 1993–1994 Coll.; MB.C.31088 .

Description

Holotype GZG.INV.131 is a very well preserved, complete specimen with 30 mm conch diameter. The conch is thickly discoidal (ww/dm = 0.58); it has a funnel-shaped umbilicus that is completely closed. The aperture is very high and leads to a coiling rate of WER = 2.34. The shell bears very fine, biconvex growth lines with only weakly developed lateral sinus. Four shell constrictions are developed at 90 degrees to each other, restricted to the venter and ventrolateral flank area.

The cross-section of the smaller paratype GZG.INV.132 has slightly wider whorls (ww/dm = 0.60) at 16 mm conch diameter. The umbilicus is still open at this stage (uw/dm = 0.12).

Specimen MB.C.31088 has a conch diameter of 17.5 mm ( Fig. 36A ) and shows the transition from the middle growth stage to the adult stage, which is characterised by a considerable increase in aperture height. The conch is thinly pachyconic with a slightly opened umbilicus (ww/dm = 0.66; uw/dm = 0.12) and a high coiling rate (WER = 2.02). The shell surface has initially rather coarse and lamellar but later fine growth lines, which extend with a convex arc across the flank and form a shallow subangular sinus on the venter. Weak shell constrictions extend parallel to the growth lines.

Remarks

Stockumites procedens can easily be distinguished from the other species of the genus by the very high coiling rate in the adult stage. With a value of about 2.35, it exceeds by far the rate (usually under 2.00) known from the other species. Another character to distinguish S. procedens from the other species of the genus is the development of shell constrictions, which extend across the venter with a subangular sinus.

Figures

Fig. 36. Stockumites procedens (Korn, 1984). A. Specimen MB.C.31088 (Weyer 1993–1994 Coll.) from the Oberrödinghausen railway cutting, bed 6b. B. Cross section of paratype GZG.INV.132 (Korn 1980 Coll.) from the Hangenberg Limestone at Müssenberg(from Korn 1984). C–E. 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 specimens from the type locality Stockum and from Oberrödinghausen. Scale bar units = 1 mm.