Stockumites Becker, 1996

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 : 46-47

Genus Stockumites Becker, 1996

Type species

Imitoceras intermedium Schindewolf, 1923: 333 ; original designation.

Genus diagnosis

Genus of the subfamily Acutimitoceratinae with a discoidal to globular conch with low to high coiling rate (WER = 1.70–2.10 and rarely up to 2.35); inner whorls subinvolute to very evolute to variable degree. Venter broadly or narrowly rounded. Ornament usually with convex or rarely with biconvex growth lines, shell surface with or without constrictions. Suture line with deep, lanceolate external lobe (as deep as the adventive lobe).

Genus composition

Central Europe ( Münster 1839; Schindewolf 1923; Schmidt 1925; Vöhringer 1960; Korn 1984): Goniatites subbilobatus Münster, 1839 ; Imitoceras intermedium Schindewolf, 1923 ;? Gattendorfia involuta Schindewolf, 1924 ; Aganides prorsus Schmidt, 1925 ; Imitoceras prorsum antecedens Vöhringer, 1960 ; Imitoceras prorsum convexum Vöhringer, 1960 ; Imitoceras depressum Vöhringer, 1960 ; Imitoceras liratum exile Vöhringer, 1960 ; Imitoceras liratum simile Vöhringer, 1960 ; Imitoceras undulatum Vöhringer, 1960 ; Acutimitoceras kleinerae Korn, 1984 ; Acutimitoceras procedens Korn, 1984 ; Acutimitoceras stockumense Korn, 1984 ; Stockumites parallelus sp. nov.; Stockumites voehringeri sp. nov.; Stockumites hofensis Korn & Weyer, 2023 ; Stockumites nonaginta Korn & Weyer, 2023 .

North Africa (Korn & Klug 2002; Ebbighausen et al. 2004; Bockwinkel & Ebbighausen 2006; Ebbighausen & Bockwinkel 2007): Acutimitoceras hilarum Korn in Korn & Klug, 2002; Acutimitoceras algeriense Ebbighausen, Bockwinkel, Korn & Weyer, 2004 ; Acutimitoceras sinulobatum Ebbighausen, Bockwinkel, Korn & Weyer, 2004 ; Acutimitoceras hollardi Bockwinkel & Ebbighausen, 2006 ; Acutimitoceras occidentale Bockwinkel & Ebbighausen, 2006 ; Acutimitoceras posterum Bockwinkel & Ebbighausen, 2006 ; Acutimitoceras endoserpens Ebbighausen & Bockwinkel, 2007 ; Acutimitoceras pentaconstrictum Ebbighausen & Bockwinkel, 2007 ; Acutimitoceras sarahae Ebbighausen & Bockwinkel, 2007 ; Stockumites marocensis sp. nov.

South Urals ( Barskov et al. 1984; Kusina 1985; Nikolaeva 2020): Acutimitoceras mugodzharense Kusina in Barskov et al., 1984; Acutimitoceras pulchrum Kusina, 1985 ; Acutimitoceras alabasense Nikolaeva, 2020 ; Acutimitoceras dzhanganense Nikolaeva, 2020 .

Central Asia ( Librovitch 1940): Imitoceras rotiforme Librovitch, 1940 .

South China ( Sun & Shen 1965; Ruan 1981): Imitoceras inequalis Sun & Shen, 1965 ; Imitoceras sinense Sun & Shen, 1965 ; Imitoceras (Imitoceras) crassum Ruan, 1981 .

North America (questionable species) ( Rowley 1895; Moore 1928): Goniatites louisianensis Rowley, 1895 ; Aganides compressus Moore, 1928 .

Remarks

Stockumites was introduced by Becker (1996) as a subgenus of Acutimitoceras to separate the species with rounded venter from the acute species (such as A. acutum and A. wangyuense ). This difference alone would probably not justify two genera. However, a closer examination of the material from various regions (Rhenish Mountains, Upper Franconia, Thuringia, Guizhou) shows that the acute venter is not the only character that distinguishes Acutimitoceras from Stockumites . A good additional distinguishing character is the attached keel, which gives the venter a galeate profile in cross-section in Acutimitoceras . Therefore, Acutimitoceras is defined here to accommodate the forms that possess these two characters, and the genus Stockumites is accepted for the forms with rounded venter and without an attached keel.

The Central European species of Stockumites can be classified into different categories based on their morphology:

(1) Conch size: some of the species ( S. kleinerae , S. intermedius , S. voehringeri sp. nov., S. subbilobatus ) attain a diameter of 70 mm; most of the others remain smaller (up to about 40–50 mm).

(2) Adult conch shape: within the genus Stockumites , the general conch shape varies from thickly discoidal (most of the species) to thickly pachyconic ( S. kleinerae , S. depressus ).

(3) Juvenile conch shape: in the juvenile stage, the conch shape varies between subinvolute ( S. depressus ) and evolute ( S. convexus , S. antecedens ). Within the genus, a temporal morphological trend from evolute to subinvolute can be observed; the umbilicus is particularly wide in the inner whorls of the stratigraphically older species.

(4) Growth line strength: some species have lamellar growth lines ( S. intermedius , S. undulatus ) and others very fine or barely visible growth lines.

(5) Growth line course: most species have convex growth lines, but these are weakly biconvex in S. parallelus sp. nov. and distinctly biconvex in S. undulatus .

(6) Constrictions: some species ( S. similis , S. exilis , S. parallelus sp. nov.) have shell constrictions, others ( S. kleinerae , S. voehringeri sp. nov., S. subbilobatus , S. convexus ) have inner shell thickenings and still others ( S. intermedius , S. depressus , S. undulatus , S. antecedens ) have neither.