Pleydellia (Pleydellia) aalensis ( Zieten, 1830 )

Seyed-Emami, Kazem, Fürsich, Franz T., Wilmsen, Markus, Majidifard, Mahmoud R. & Shekarifard, Ali, 2008, Lower and Middle Jurassic ammonoids of the Shemshak Group in Alborz, Iran and their palaeobiogeographical and biostratigraphical importance, Acta Palaeontologica Polonica 53 (2), pp. 237-260 : 251-252

Pleydellia (Pleydellia) aalensis ( Zieten, 1830)

Fig. 5E.

1830 Ammonites aalensis ; Zieten 1830: pl. 28: 3.

2004 Pleydellia aalensis ( Zieten, 1830) ; Myczyński 2004: 64, fig. 26/3.

2005 Pleydellia (Pleydellia) aalensis ( Zieten, 1830) ; Seyed−Emami et al. 2005: 360, fig. 6C, D, F.

Material.—Several specimens from Kuhe Bashm (050511−6).

Dimensions (in mm)

Specimen D U H W

050511−6/3 33 30 45 26

Description and discussion.—See Seyed−Emami and Nabavi (1985: 266) and Seyed−Emami et al. (2005).

Stratigraphic and geographic range.—Kuhe Bashm, level VI. Late Toarcian Pleydella aalensis Zone.

Pleydellia (Pleydellia) buckmani Maubeuge, 1947 Fig. 5F, G.

1947 Pleydellia Buckmani ; Maubeuge 1947: 76, pl. 2 (upper figures). 2005 Pleydellia (Pleydellia) buckmani Maubeuge, 1947 ; Seyed−Emami et al. 2005: 362, fig. 6A, B, G.

Material.— Seven specimens from Kuhe Bashm (050511−5, 050511−6).

Dimensions (in mm)

Specimen D U H W

050511−6/2 27 26 47 26

050511−6/3 34 25 45 25

Description and discussion.—See Seyed−Emami and Nabavi (1985: 265) and Seyed−Emami et al. (2005).

Stratigraphic and geographic range.—Kuhe Bashm, level VI, Late Toarcian, Pleydella aalensis Zone.

Pleydellia ( Pleydellia ?) ruttneri Seyed−Emami sp. nov. Fig. 5L, M.

Type material: The holotype (050511−6/9) is a nearly complete shell. Paratype: A fragmentary specimen (050511−6/10).

Type horizon: Shemshak Group , Upper Toarcian, Pleydella aalensis Zone, Kuhe Bashm section level VI at 270 m ( Fig. 2) .

Type locality: Kuhe Bashm−e−Dehsufian, 13.5 km east of Shahmirzad , about 25 km north of Semnan .

Derivation of the name: In honour of the Late Dr. Anton Ruttner (1911–2006), who contributed much to the geology of east−central Iran.

Material.— Three specimens from Kuhe Bashm (050511−6).

Dimensions (in mm)

Specimen D U H W

050511−6/9 (holotype) 31 ~34 ~39 ~25 (at the end of phragmocone) 38 ~35 ~37 ~24 (at the end of body whorl)

Description.—The holotype ( Fig. 5M 1 , M 2) is a slightly crushed microconch with rather short lappets and parts of the original shell. The body whorl begins at a diameter of 40 mm and occupies about 1/2 of the last whorl. It is moderately evolute, with nearly parallel flanks and a rectangular to ovate whorl cross−section. The venter is distinctly tectiform, with a narrow and sharp keel, which persists to the end of the body chamber. The umbilicus is shallow, with a nearly vertical wall. The ribs are fairly fine, dense and sinuous. They start at the seam and project strongly on the venter forming an angle and end at the keel. Some ribs are irregularly paired on the body whorl. Towards the end of the body chamber the ribbing becomes slightly finer and denser.

Discussion.—The new species can be considered as a microconch belonging to the P. aalensis group. The short lappets look very much like those of P. aalensis figured by Buckman (1890: pl. 32: 3), but the ribbing is different. The species also differs from the microconchs of P. aalensis figured by Henrique and Ureta (2002: 150, fig. 93). The parallel flanks and the fastigate venter with rather fine and partly pair−wise bundled ribs is reminiscent of Cotteswoldia and Walkeria . However, it cannot be assigned to any known species of these subgenera.

Stratigraphic and geographic range.—The new species was found on a slab, together with Pleydellia (P.) buckmani , from the (uppermost?) Late Toarcian, Pleydella aalensis Zone.

Figures

Fig. 1. Distribution of the Shemshak Group in the central and eastern Alborz, northern Iran. 1: Position of the Shahmirzad and Kuhe Bashm−e−Dehsufian sections; 2: Position of the Sharif Abad section.