Species Gonioptychoceras sebanum (PICHLER)
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Group
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Stage
Description

Both Neumayr (1879) and Wähner (1889) have described ammonites from the Alpine Hettangian whose sculpture consists of high, forward-drawn ribs that extinguish outwards and strongly resemble Storthoceras. The ribs are highest near the external side. A unique feature of these forms, however, caused both authors to distinguish the two species Aegoceras sebanum (Pichler) and Aegoceras gonioptychum (Wähner) from the forms later named "Storthoceras": a keel. The juvenile stages are still keel-less, then the outer bend of the coils is canted. Finally, in the adult stage, a distinct keel is present. Lange (1941) introduced the genus Gonioptychoceras for this and placed it in the subfamily Proarietitinae.

From the naumanni/johnstoni horizon a specimen is known that has to be identified as Gonioptychoceras sebanum (for a long time only the (sub)species described as progastor and G. gonioptychum were available): The coils are circular when young, then increasingly oval, on the last coil there is a clear keel in the form of a sharpening of the external side.
After a strong nodule stage, high, curved ribs set in, which are retroradiate and bend strongly forward near the external side. The ribs reach their greatest height at the point of bending, then weaken and end in the canted external side.

* Description according to Lange (1952), supplemented

Compare

The species is distinguished from Megastomoceras & Storthoceras by the clearly keeled external side and the ribs, which are strongest on the outside.
G. progastor has a much larger Wb, G. gonioptychum a larger umbilical width and less high ribs.

Dimension

Dimensions with D= 70mm (according to Neumayr)
Nw % v. D: ca. 48%
Wh % v. D: ca. 29%
Wb % v. D: ca. 25%

Literature
  • 1879 Aegoceras sebanum Pichler – Neumayr S. 34, Taf, 4.2-4
  • 1952 Gonioptychoceras sebanum Pichler – Lange, S. 148
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