Epidomatoceras ebbighausenorum, Korn & Klug, 2023

Korn, Dieter & Klug, Christian, 2023, Early Carboniferous coiled nautiloids from the Anti-Atlas (Morocco), European Journal of Taxonomy 885, pp. 156-194 : 174-177

Epidomatoceras ebbighausenorum sp. nov.

urn:lsid:zoobank.org:act:E0A17693-5DCD-463E-B266-0D4AFE16AB29

Figs 13–14 ; Table 5

Diagnosis

Species of Epidomatoceras with thickly pachyconic, subevolute conch (ww/dm ~0.68; uw/dm ~0.32); whorl profile moderately depressed, rounded-rectangular (ww/wh ~ 1.65), venter flattened, ventrolateral shoulder subangular, in the middle growth stages marked by a spiral ridge. Whorls very weakly embracing. Ornament with very fine growth lines. Septa simply concave towards the venter, suture line with shallow lobes on venter and flanks.

Etymology

Named after Volker Ebbighausen (1941–2011) and Rodion Ebbighausen, who collected the majority of the type material.

Material examined

Holotype MOROCCO • Anti-Atlas, north of Gara el Itima , 35 km east-northeast of Taouz ; basal Hamou-Rhanem Formation; Ebbighausen, Ebbighausen & Korn 2004 Coll.; illustrated in Fig. 13A ; MB.C.31294.1 .

Paratypes MOROCCO • 18 specimens; same collection data as for holotype; Ebbighausen, Ebbighausen & Korn 2004 Coll.; MB.C.31294.2–31295.19 .

Description

Holotype MB.C.31294.1 is a nearly complete but somewhat corroded specimen with a conch diameter of 67 mm; it allows the study of two whorls ( Fig. 13A ). The conch is widely wheel-shaped with a moderately wide umbilicus (ww/dm=0.68; uw/dm=0.33). The profile of the last whorl is depressed (ww/wh =1.64) and subtrapezoidal with a nearly flat, steep umbilical wall, a subangular umbilical margin, slightly flattened, weakly converging flanks, a broadly rounded ventrolateral shoulder and a flattened venter with a shallow concave depression. Half a volution earlier, at 41 mm dm, the venter is weakly convex and the ventrolateral shoulders are marked by three longitudinal ridges, of which the inner one is the most prominent and separated from the middle one by a distinct groove. The outer of the three ridges is the weakest and located on the venter. The shell surface appears to be smooth. The internal mould of the inner whorls is visible in the umbilicus; they show a pronounced umbilical margin and a rather steep, flattened umbilical wall.

Paratypes MB.C.31294.2 ( Fig. 13B ) and MB.C.31294.3 ( Fig. 13C ) have a conch morphology that is very similar to that of the holotype. Specimen MB.C.31294.3 is less well preserved in the outer whorl but provides a good insight of the earlier growth stage. Already at 15 mm diameter, an angular and slightly raised umbilical margin is developed, on which the growth lines are strengthened and form delicate plications.

Paratype MB.C.31294.4 yielded a conch cross section that allows the study of two and a half volutions ranging from 9 mm to 46 mm conch diameter ( Fig. 14A ). It is not clear from the specimen whether some parts of the inner whorls are dissolved; the umbilical opening appears to have a width of 3.7 mm. The earliest preserved whorl profile (1.66 mm high, 2.53 mm wide) is rounded triangular with a flattened venter, but half a volution later (9 mm dm), a pronounced, subangular ventrolateral shoulder is developed and the nearly parallel, weakly converging flanks are bordered umbilically by a broadly rounded umbilical margin. From this growth stage, the whorl profile is rounded pentangular. Half a volution later, at 15 mm dm, this whorl profile is still present, but the umbilical margin is sharpened by a longitudinal crest formed by the shell. This whorl does not embrace the preceding one. Another half of a whorl later, at 26 mm dm, the umbilical wall is flattened and steep, while the umbilical margin is rounded. In this stage, the ventrolateral shoulder is characterised by a longitudinal groove accompanied by weak crests on both sides. Finally, at 46 mm dm, the ventrolateral and umbilical shoulders are narrowly rounded; venter, flanks and umbilical wall are flattened. The cross section demonstrates that the conch proportions do not change significantly during ontogeny. There is, between 9 and 46 mm conch diameter, a slight increase of the ww/dm ratio (from 0.58 to 0.66) paralleled by a slight decrease of the uw/dm ratio (from 0.43 to 0.29). The coiling rate increases from about 2.65 to 3.10 ( Fig. 14C–E ).

It appears that intraspecific variation ranges within rather narrow limits and ontogenetic changes are small. Specimens between 36 and 67 mm conch diameter, for instance, range in their ww/dm ratio between 0.60 and 0.70 with a weak tendency to become stouter with increasing diameter. In the same growth interval, the umbilical width ratio shows a very weak decrease. Only the coiling rate appears to decrease more significantly from a WER value of about 3.15 at 50 mm dm to 2.70 at 67 mm dm ( Fig. 14C–E ).

Paratype MB.C.31294.5 is a rather poorly preserved fragment of a large specimen (37 mm whorl width), but it shows the surface of a septum. This is continuously domed; the lobes in the suture line ( Fig. 14B ) are thus cut-out shapes of the septum and are not caused by septal inflexions. The siphuncle has a subcentral position and is slightly shifted from the centre towards the venter.

Remarks

Epidomatoceras ebbighausenorum sp. nov. differs from the other species of the genus in its stout conch and depressed whorl profile and the narrower umbilicus. The type species of the genus, Epidomatoceras planotergatum , for instance, has a nearly quadratic whorl profile (ww/wh ~ 1.05), while it is depressed in E. ebbighausenorum (ww/wh ~ 1.60). The umbilicus is rather wide in E. planotergatum (uw/dm ~ 0.37) but narrower in E. ebbighausenorum (uw/dm ~0.30). Similar differences occur when E. ebbighausenorum is compared with other species of Epidomatoceras , although E. doohylense possesses a slightly depressed whorl profile (ww/wh ~ 1.10).

Figures

Fig. 13. Epidomatoceras ebbighausenorum sp. nov. from N of Gara el Itima (all Ebbighausen, Ebbighausen & Korn 2004 Coll.). A. Holotype MB.C.31294.1; dorsal and lateral views. B. Paratype MB.C.31294.2; ventral and lateral views. C. Paratype MB.C.31294.3; lateral and apertural views. Scale bar units =1 mm.

Fig. 14. Epidomatoceras ebbighausenorum sp. nov. from N of Gara el Itima (all Ebbighausen, Ebbighausen & Korn 2004 Coll.). A. Cross section of paratype MB.C.31294.4.B. Suture line of paratype MB.C.31294.5, at ww =30.5 mm, wh= 22.5 mm. C–E. Ontogenetic trajectories of the cardinal conch parameters. Scale bar units=1 mm.