| Species | Paracaloceras coregonensis (SOWERBY) |
| Author | SOWERBY |
| Group | Arieten |
| Horizon | atanatense-horizon |
| Zone | marmoreum-zone |
| Stage | Upper Hettangian |
Description
Paracaloceras coregonensis is an evolute species, the coils are almost always wider than high. Only with age, in adult specimens, the last turn can increase less in thickness and is correspondingly higher than wide. Especially on the innermost coils the WQ is highly trapezoidal, the outer side is strongly flattened, so that the transition flank-external side is formed as an edge.
To this end, the ribs on the innermost coils are very strong and high (WÄHNER calls them "combs"), separated by spaces wider than the ribs. The ribs run radially to slightly retroradiate, at first straight over the flanks, with increasing ontogeny they show a concave bend forward. In small specimens they increasingly increase in height from the inner bend and swell to knot-like formations at the outer bend, only to die out very quickly. This gives the coil - seen from the external side - the appearance of being bordered by a double row of spines.
The height of the ribs decreases in the course of ontogeny, but are always clearly marked. From a relatively small diameter (15-20mm) a keel may already be formed, with more or less distinct keel furrows. The ribs then run to the external side and end only at the keel furrow. The keel furrows may be particularly pronounced in adult specimens.
A very detailed description of the typical arietitid lobe line as well as the complete species is given by WÄHNER 1887.
It is remarkable that the growth ratios change in the course of ontogeny, a detailed table of measurements is available in KMENT 2000.
The thickness of the windings is very variable. There are all kinds of variants, so that no sub-types can be distinguished.
* Description according to Wähner (1888), supplemented
Compare
Paracaloceras centauroides is most similar, but clearly separated by the narrower umbilicus and the even thicker coils.
The typical ribbing on the inner coils and the characteristic course over the thick coils make Paracaloceras coregonensis very easy to recognise.
Dimension
Dimensions with D=35mm:
Nw % v. D: ca. 57%
Wh % v. D: ca. 24%
Wb % v. D: ca. 27-35%
—
Abmessungen bei D=100mm:
Nw % v. D: ca. 65%
Wh % v. D: ca. 19%
Wb % v. D: ca. 19%
Literature
- 1882 Aegoceras coregonense Sow. – CANAVARI S. 173, Taf. 19.12-15
- 1887-1888 Arietites coregonensis Sow. (Can.) –WÄHNER S. 311, Taf. 40.1-3, 41.1-4, 42.1-4, 43.1-6
- 2000 Paracaloceras coregonense Sowerby – KMENT S. 206
