
Species | Pleuroacanthites polycycloides WÄHNER |
Author | WÄHNER |
Group | Psiloceraten |
Horizon | costosum-horizon |
Zone | calliphyllum-zone |
Stage | Lower Hettangian |
Description
This species of Pleuracanthites occurs first, only slightly above Psiloceras calliphyllum. The shell is involute, the coil cross-section almost round. The umbilical wall is steep and the umbilical edge rounded.
The sculpture is characteristic of this genre:
A distinct nodule stage is formed on 1-2 coils after the smooth initial stage. The nodules are very strong and merge into an apparently bulging, irregular ribbing. WÄHNER examined the shells intensively and noticed that the curved, bulge-like elevations and the depressions in front of them probably represent former mouth margins. This ribbing is found in all Pleuracanthites species, more or less pronounced. It is best observed in Pl. polycycloides. It is remarkable that the ridges do not correspond to the striation (accretion stripes) on the shell. WÄHNER remarks on p. 242: "We see here ammonites which are obviously in the process of transformation from smooth to sculpted forms, and it is evident that the regularly arranged ribs emerge from the former mouth margins".
* Description according to WÄHNER 1894, supplemented
Compare
Pleuracanthites polycycloides is very well distinguished from other species of this genus by the continuous bulging ribbing and the winding that increases little in thickness.
Pl. polycyclus, on the other hand, is almost unribbed, Pl. biformis and Pl. biformis evolutus are much thicker.
The specimen of Pl. polycyclus illustrated by LANGE, which is sculptured to the end, is therefore probably also to be placed here with Pl. polycycloides.
Dimension
Dimensions with D= 40mm:
NW % v. D: ca. 49%
WH % v. D: ca. 28%
WB % v. D: ca. 25%
Literature
- 1894 Pleuracanthites polycycloides n. f. – WÄHNER S. 252, Taf. 56. 8-9, 60.5
- 1952 Pleuracanthites polycyclus Wähner – LANGE S. 95, Taf. 11.5