Pseudarietites subtilis Vöhringer, 1960

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 : 183-186

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 ).

Figures

Fig. 113. Pseudarietites subtilis Vöhringer, 1960 from the Oberrödinghausen railway cutting, both Vöhringer Coll. A. Holotype GPIT-PV-63984 from bed 3c. B. Paratype GPIT-PV-63986 from bed 3c. Scale bar units= 1 mm.

Fig. 114. Pseudarietites subtilisVöhringer, 1960from theOberrödinghausen railwaycutting.A. Specimen MB.C.31221 (Korn 1977 Coll.), bed unknown. B. Cross section of paratype GPIT-PV-63959 (Vöhringer Coll.) from bed 3c. C. Growth line course of holotype GPIT-PV-63984 (Vöhringer Coll.) from bed 3c, at dm =26.5 mm, ww =8.1 mm, wh =8.6 mm.D –F. Ontogenetic development of the conch width index (ww/dm), umbilical width index (uw/dm), whorl width index (ww/wh) and whorl expansion rate (WER) of selected specimens. Scale bar units =1 mm.