Thrincoceras devolvere, Korn & Miao & Bockwinkel, 2022

Korn, Dieter, Miao, Luyi & Bockwinkel, Jürgen, 2022, The nautiloids from the Early Carboniferous Dalle à Merocanites of Timimoun, western Algeria, European Journal of Taxonomy 789, pp. 104-129 : 115-116

Thrincoceras devolvere sp. nov.

urn:lsid:zoobank.org:act:DFAFE2FC-2688-426E-B543-D0F61D31C354

Fig. 7 ; Table 3

Diagnosis

Species of Thrincoceras with weakly depressed, rounded-trapezoidal whorl profile (ww/wh ~ 1.40), venter slightly flattened, ventrolateral shoulder broadly rounded. Whorls slightly embracing, above 40 mm dm with the tendency for slow uncoiling. Ornament with about 20 spiral lines on the flank and few delicate spiral lines on the outer part of the venter, main area of the venter without spiral lines. Growth lines concavo-convex with shallow lateral sinus and deep, rounded V-shaped ventral sinus.

Etymology

After the Latin verb ‘ devolvere ’, meaning ‘devolve’, referring to the coiling.

Type material

Holotype ALGERIA • Gourara , Sebkha de Timimoun 14.5 km west-southwest of Timimoun; “ Dalle à Merocanites ” (Tournaisian-Viséan boundary interval); illustrated in Fig. 7 ; MB.C.30380 .

Description

Holotype MB.C.30380 is an incomplete specimen with 43 mm diameter of the conch ( Fig. 7 ). It is thinly discoidal and evolute (ww/dm = 0.44; uw/dm = 0.48), with a weakly depressed, rounded-trapezoidal whorl profile (ww/wh = 1.37) and a high coiling rate (WER = 2.14). It can be seen on the specimen that uncoiling starts at the last quarter of the last whorl, as the depth of the imprint zone decreases slightly.

The ornament consists of about 20 spiral lines on the flank, which are raised to faint ridges in the region of the middle flank and become weaker towards both the umbilicus and the venter. These spirals are crossed by finer growth lines and together form a conspicuous granulation on the penultimate volution. Most part of the venter does not have spiral lines; there are delicate growth lines, which form a deep and wide, rounded V-shaped ventral sinus. It appears that one volution earlier, spirals are also developed on the venter, but the specimen is poorly preserved in this area.

Remarks

Thrincoceras devolvere sp. nov. differs from the two North American species T. kentuckiense and T. depressum first in the size of the conch, which in the two latter species reaches well over 100 mm ( Hyatt 1893). T. kentuckiense , however, has a very similar whorl profile to T. devolvere sp. nov., but shows no detachment of the whorls from the preceding one.

The species known from Ireland, T. hyatti and T. hibernicum, have more subangular ventrolateral shoulders and flattened flanks ( Foord 1891, 1900) and therefore differ in whorl profile from T. devolvere sp. nov.

Figures

Fig. 7. Thrincoceras devolvere sp. nov., holotype MB.C.30380. Scale bar units = 1 mm.