Vestinautilus kesslerae sp. nov.

urn:lsid:zoobank.org:act: 73A42507-23FD-4390-9DEE-8FC07C72A7BC

Fig. 9; Table 3

Diagnosis

Species of Vestinautilus with thickly pachyconic, subevolute conch (ww/dm ~ 0.68; uw/dm ~0.38); whorl profile moderately depressed, oval (ww/wh ~1.75), venter broadly arched, ventrolateral shoulder subangular with three weak spiral ridges. Whorls not embracing, just touching the preceding one. Ornament with coarse growth-lines forming a moderately deep, broadly arched ventral sinus.

Etymology

Named after Birgit Kessler (Berlin), who found the holotype.

Material examined

Holotype MOROCCO • Anti-Atlas, South-east of Gara el Itima, 31.5 km east-northeast of Taouz; Zrigat Formation; Kessler 2007 Coll.; illustrated in Fig. 9; MB.C.31287.

Description

Holotype MB.C.31287 is an entirely chambered steinkern specimen with 41 mm phragmocone diameter; it comprises one and a quarter whorl (Fig. 9). At the largest diameter of the specimen, the conch shape is thinly pachyconic and subevolute (ww/dm ~ 0.68; uw/dm ~0.38). The whorl profile is depressed (ww/ wh ~1.74) and has the shape of a symmetric oval. The coiling rate is very high (WER ~2.72) and the last whorl just touches the preceding one. The umbilical window has a diameter of 11.5 mm.

The initial chamber is bluntly cone-shaped with a length of 0.6 mm, a height of 2.0 mm and a width of about 4 mm. Very early in ontogeny, the whorl profile becomes depressed and already immediately after the protoconch, an angular ventrolateral shoulder is formed. This angular shoulder becomes more pronounced to form a spiral ridge, which is, at a conch diameter of 25 mm, accompanied by one less well-developed ridge on each side. The outer of these ridges fades out and is only barely visible at a conch diameter of 42 mm. Only some traces of the shell ornament are visible in the dorsal portion. They show coarse growth lines and three of them are arranged at a millimetre distance. They form a broadly arched ventral sinus.

The siphuncle has a subcentral position with a slight shift towards the venter. The suture line forms a wide and shallow external lobe, a very shallow and wide lateral lobe and a small, rounded internal lobe.

Remarks

Vestinautilus kesslerae sp. nov. has a somewhat marginal position in the morphological spectrum of the genus. It differs from the other species because of the combination of a number of conch and sculpture characters: the advolute conch shape even at a conch diameter of 40 mm, the comparatively weak development of only a few longitudinal ridges and the convex venter.