| Species | Angulaticeras marmoreum (OPPEL) |
| Author | OPPEL |
| Group | Schlotheimien |
| Horizon | pachygaster-horizon |
| Zone | marmoreum-zone |
| Stage | Upper Hettangian |
Description
A detailed discussion of the genus and species is given by BLOOS (1988).
The genus Angulaticeras shows the following characteristics up to about 20mm diameter according to BLOOS: "Loben line with high median saddle; strong translation of the ribs over the extern without stronger forward swing on the innermost convolutions; a very early stage of cleavage and switching ribs or alternating rib thickness; extern furrow on the stone core".
Angulaticeras marmoreum is large-growing. The cross-section of the coils is round when young, later highly oval. In young individuals a distinct umbilical edge is present, in large specimens the flank gradually slopes into the flat umbilicus. In general, according to BLOOS (1988), the species is very variable with regard to the shape of the shell. Thick, narrow-billed forms are opposed to thin and wide-billed ones. The dimensions should not be used for diagnosis.
The ribbing, on the other hand, is characteristic: the split ribs begin above the umbilical edge, stand radially on the inner coils up to a dm of about 20mm and are straight, then later become increasingly S-shaped, from a dm of about 60mm then straight with forward bending (sickle-shaped). Switching ribs occur. The ribs do not cross the external side, their endings are often not exactly opposite each other, but offset. Their angle to each other is clearly less than 180°. From about 40 mm the ribbing becomes increasingly weaker. In larger cases, the ribs end in front of the very tightly rounded external side, giving the impression of a keel.
The number of ribs is very variable.
* Description according to WÄHNER 1882 & BLOOS 1988, supplemented
Compare
A. marmoreum can be easily distinguished from other ammonites by its ribbing. Especially the "keel" is typical. Angulaticeras marmoreum is very common in the fauna of the upper Hettangian and gave its name to the "marmoreum crust". Only in localities where the faunal composition is very young, A. marmoreum may not occur at all (cf. biostratigraphy).
Dimension
NW % v. D.: ca. 30-32 %
WH % v. D.: ca. 38-42 %
WB % v. D.: ca. 20-26 %
Literature
- 1862 Ammonites marmoreus – OPPEL, S. 130
- 1882 Aegoceras marmoreum – WÄHNER, S. 180, Taf. 22, 1-5
- 1963 Schlotheimia marmorea – BLIND, S. 80, Taf. 2, 5
- 1988 Angulaticeras marmoreum – BLOOS, S. 6, Taf. 1-3, Fig. 7-8, Taf. 5-8, Fig. 1, Abb. 3-11
- Lit: BLOOS, G. (1988): Ammonites marmoreus OPPEL (Schlotheimiidae) im unteren Lias (angulata-Zone, depressa-Subzone) von Württemberg (Südwestdeutschland).- Stuttgarter Beitr. Naturk. B, 141, S. 1-47
