| Species | Psiloceras naumanni NEUMAYR |
| Author | NEUMAYR |
| Group | Psiloceraten |
| Horizon | naumanni-horizon |
| Zone | calliphyllum-zone |
| Stage | Lower Hettangian |
Description
A very strong nodular stage is formed on the innermost visible 1-2 coils, followed by regular dense ribbing. The ribs start at the navel edge, slope slightly backwards to radial over the flanks and then bend slightly forward where they extinguish. They are still quite high at first, but flatten very quickly. Typical for Ps. naumanni is the weak, narrowly standing ribbing. In large, adult specimens, the ribbing may disappear on the living chamber or change into irregular ridges, which are oriented towards the attachment stripes and resemble mouth margins rather than ribs.
The cross-section of the windings is oval when young, becoming more rectangular with flattened flanks as the size increases. The flanks are not parallel, but slope towards the outer side, so that the largest Wb is measured at the umbilical edge. This phenomenon becomes more and more obvious with increasing size.
The coiling ratios change very strongly during ontogeny (e.g. Nw from 32% at a Dm= 11mm to Nw of 62% at a Dm= 178mm.). The width of the coils increases very little with age, which gives the shells a disc-shaped appearance. LANGE gives a comprehensive table of dimensions.
Psiloceras naumanni is very common and can grow very large. Specimens of 300mm diameter have already been found! The predominant size is about 100mm. It is a good leading fossil and characterises the next prominent ammonite horizon above Ps. calliphyllum in many profiles. Depending on condensation, this naumanni horizon may also be included in the so-called calliphyllum bed.
* Description according to LANGE 1952, supplemented
Compare
LANGE separates two subspecies: Ps. naumanni elatior (no flattened flanks, WQ always oval) and Ps. naumanni glabrum (only ribbed on the inner coils), but notes that they can be extremes of a wide range of variation. Ps. naumanni varies relatively strongly in number of ribs, Wh, Wb and age sculpture, but is clearly distinguishable from other forms by the dense ribbing and the flattened flanks.
At first glance somewhat similar is Psiloceras trochoeides, which however has a much larger umbilical width and less dense ribs. At second glance Ps. trochoeides resembles Ps. costosum or Ps. strongolum much more.
Dimension
Dimensions with D= approx. 100mm:
Nw % v. D: ca. 59%
Wh % v. D: ca. 22%
Wb % v. D: ca. 12%
Literature
- 1879 Aegoceras naumanni n. sp. – NEUMAYR S. 28, Taf. 4.1
- 1879 Aegoceras Naumanni Neum. – WÄHNER S. 145
- 1952 Psiloceras (Paraphylloceras) naumanni (Neumayr) – LANGE S. 120, Taf. 15.2-7, Abb. 49-53
