![Ammonit - Schlotheimia donar (WÄHNER) (6) Ammonit - Schlotheimia donar (WÄHNER) (6)](https://www.hettangium.de/wp-content/uploads/2020/12/Ammonit-Schlotheimia-donar-WAeHNER-6-200x200.jpg)
Species | Schlotheimia donar (WÄHNER) |
Author | WÄHNER |
Group | Schlotheimien |
Horizon | seebachi-horizon |
Zone | megastoma-Zone |
Stage | Middle Hettangian |
Description
The cross-section of the coils is almost round on the innermost coils, becoming more oval to highly rectangular as they grow, the flanks being flattened with a tightly rounded, steep umbilical drop. The coils are very thick.
The ribs already start at the suture, run strongly retroradiate to the navel edge, there they bend so far that they only run over the flanks slightly inclined backwards. On the flanks the ribs are straight, but near the external side they bend forward and end externally in a thickening. The ribs do not meet those of the opposite side; on the external side a furrow remains free. The angle of the opposing ribs is 180 - 150° externally. The ribs appear concave towards the front because they have two bends. They are very close together, slender and high and rounded (not sharp). The spaces between the ribs are usually wider than the ribs; this is especially obvious on stone core specimens. The ribs are simple and regular; only in large specimens does a more or less regular rib splitting occur (diameter >100mm according to Wähner).
The winding cross-section changes and is less rectangular in larger casings: the umbilical edge is more broadly rounded, the flanks are no longer so flattened, but fall gradually towards the outer side. This appears rather sharpened. The ribs only begin at the navel edge and end before the closely rounded outer side, which remains smooth. A furrow can therefore no longer be observed.
The ribbing becomes weaker and weaker in the course of ontogeny, but this is the case with all large ammonites in the Hettangian.
* Description according to WÄHNER (1884), supplemented
Compare
Schlotheimia donar and Schlotheimia donar pachygaster are clearly distinguishable from other Schlotheimiids by their thick coils and very dense ribbing. The species can grow quite large. Wähner estimates a diameter of up to 500mm. It is very difficult to distinguish them from each other. A distinction according to the horizon of discovery is possible to a limited extent. It seems that S. donar is restricted to the uppermost horizon of the middle Hettangian, while S. donar pachygaster only occurs in or above the marmoreum crust in the upper Hettangian. This is problematic in so far as older ammonites are very often found in the marmoreum horizon. So both species can occur there together. It is certain that already under the marmoreum crust with the typical marmoreum fauna Schlotheimia donar is occasionally found. However, the findings are sparse, either because the form is very rare, or because it really only occurs immediately before the marmoreum crust. In the large specimen illustrated, the very late rib fission, as described by Wähner, can be demonstrated.
S. donar pachygaster differs from S. donar by "smaller thickness, more pronounced forward bending of the ribs on the external side, greater thickness of the ribs compared to their height and the width of the interspaces (especially on the external side) and by the earlier onset of rib fission" (cited by Wähner 1884).
Dimension
Dimensions at D= 55mm (according to Wähner):
Nw % v. D: ca. 33%
Wh % v. D: ca. 39%
Wb % v. D: ca. 32%
—
Abmessungen bei D= 250mm (nach Wähner):
Nw % v. D: ca. 40%
Wh % v. D: ca. 33%
Wb % v. D: ca. 18%
Literature
- 1884 Aegoceras donar n.f. – WÄHNER S. 172, Taf. 19.4, 21.1-2
- 1963 Schlotheimia (Encyloceras) donar Wähner – Blind S. 82, Taf. 3.4