Pseudarietites subtilis Vöhringer, 1960

Figs 113–114; Tables 108–109

Pseudarietites westfalicus subtilis Vöhringer, 1960: 164, pl. 6 fig. 12.

Pseudarietites westfalicus subtilis – Bartzsch & Weyer 1982: 20, text-fig. 5; 1986: pl. 2 fig. 1. Pseudarietites subtilis – Korn 1994: 78, text-figs 70g –h, 71e, 72c.

Diagnosis

Species of Pseudarietites with a conch reaching 40 mm diameter. Conch at 15 mm dm thinly discoidal, subevolute (ww/dm ~0.40; uw/dm ~0.33). Whorl profile at 15 mm dm nearly circular (ww/wh ~1.00); coiling rate moderate (WER ~1.90). Venter broadly rounded with distinct keel between two distinct grooves, umbilical margin rounded. On the flank 30 sharp ribs with nearly linear course.

Holotype

GERMANY • Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3c; Vöhringer Coll.; illustrated by Vöhringer (1960: pl. 6 fig. 12) and Korn (1994: text-fig. 70g); re-illustrated here in Fig. 113A; GPIT-PV-63984.

Paratypes

GERMANY • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3c; Vöhringer Coll.; GPIT-PV-63986, GPIT-PV-63959 .

Additional material

GERMANY • 1 specimen; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3a; Weyer 1993–1994 Coll.; MB.C.31219 • 2 specimens; Rhenish Mountains, Oberrödinghausen, railway cutting; Hangenberg Limestone, bed 3c 1; Weyer 1993–1994 Coll.; MB.C.31220.1–2 • 1 specimen; Rhenish Mountains, Oberrödinghausen, west of railway cutting; Hangenberg Limestone, loose material; Korn 1977 Coll.; MB.C.31221.

Description

Holotype GPIT-PV-63984 is a fairly well-preserved but somewhat damaged specimen with 26 mm conch diameter (Fig. 113A). It is extremely discoidal and subevolute (ww/dm = 0.33; uw/dm = 0.39) and it seems that the umbilicus shows a little bit of opening during the last volution. The ww/wh ratio is nearly equal and the coiling rate is low (WER = 1.69). The sculpture consists of about 18 almost straight ribs on the flank of the penultimate last whorl. The ribs weaken out towards the aperture; the last half volution bear about 30 ribs, which are slightly forward directed but almost straight. Fine growth lines follow the course of the ribs on the flanks and form a rather narrow, shallow sinus on the venter (Fig. 114C). The venter has barely visible longitudinal grooves and a rounded keel between them.

Paratype GPIT-PV-63986 has 18 mm conch diameter (Fig. 113B) and corresponds, in conch shape and sculpture, to the penultimate whorl of the holotype. It shows ribs that are very sharp on the inner flank.

Specimen MB.C.31221 (Fig. 114A) is incomplete, but displays the characteristics of the species. It has a conch diameter of 17 mm and is thinly discoidal with a moderately wide umbilicus (ww/dm = 0.37; uw/dm = 0.36). It possesses more than 30 sharp ribs on the last volution; these ribs extend forward over the midflank area and diminish without an extra projection on the outer flank. The venter possesses two longitudinal grooves and a raised keel.

Paratype GPIT-PV-63959 was sectioned by Vöhringer; it allows the description of the ontogeny of the conch up to a diameter of 22 mm (Fig. 114B). The shape of the whorl profile goes through three ontogenetic stages. In the first stage, up to 3 mm dm, it is broadly crescent-shaped and depressed, followed by the second stage, up to 6 mm dm, in which the profile is rather circular. In the third stage, the ventral keel develops and the profile becomes compressed (ww/wh = 0.92 at 22 mm dm).

The sectioned paratype GPIT-PV-63959 shows that the ww/dm ratio has a monophasic ontogenetic trajectory with a continuous linear decrease from 1.00 at 0.8 mm dm to 0.35 at 22 mm. The ontogeny of the uw/dm ratio is triphasic; it increases in early ontogeny to 0.40 at 2 mm dm, then decreases to 0.30 at 8 mm dm and finally increases slightly to 0.34 at 22 mm dm. The ontogeny of the coiling rate is also triphasic; after an initial decrease of the WER to about 1.60 at 2.5 mm dm, it increases to a maximum value of 2.05 at 12 mm dm, followed by a decline in adulthood to 1.90 (Fig. 114D–F).

Remarks

Pseudarietites subtilis differs from P. westfalicus in the much narrower umbilicus (uw/dm = 0.35 in P. subtilis at 20 mm dm but about 0.45 in P. westfalicus).