| Species | Angulaticeras trapezoidale (SOWERBY) |
| Author | SOWERBY |
| Group | Schlotheimien |
| Horizon | atanatense-horizon |
| Zone | marmoreum-zone |
| Stage | Upper Hettangian |
Description
The shells are quite involute. The WQ is oval and quite broad. The flanks are strongly convex, the outer side broadly rounded.
The ribs start slightly above the seam, run more or less radially over the flanks and bend forward hardly noticeably near the outer side. They are low and strong. They are split only on the second turn, sometimes already on the inner turns. The cleavage can often only be recognised on close inspection; the ribs arise from a common stem near the navel.
Before the ribs go out externally, they form a kind of thickening so that the external side appears flattened. The ribs on both sides face each other at almost a 180° angle, leaving a central furrow.
There is no noticeable weakening in old age.
* Description according to Wähner (1884), supplemented
Compare
The coils of Angulaticeras trapezoidale increase in thickness very quickly, roughly comparable to Schlotheimia donar and S. pachygaster. However, the ribs are split earlier, which distinguishes the species from the others. In addition, the ribs bend forward less and meet externally at about a 180° angle. This feature distinguishes A. trapezoidale from all other Schlotheimiids of this zone.
Angulaticeras charmassei D'Orb. is very similarly ribbed, but has a more highly oval cross-section and would only occur in the next younger ammonite zone. The form has not yet been found in the Alps.
Dimension
Dimensions with D= 50mm (according to Wähner):
Nw % v. D: ca. 30%
Wh % v. D: ca. 41%
Wb % v. D: ca. 35%
Literature
- 1884 Aegoceras trapezoidale Sowerby – Wähner S. 185, Taf, 21.6, 23.1-4
- 1963 Schlotheimia (Encyloceras) trapezoidalis (SOW) – Blind S. 81, Taf. 1.21
