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Strange and rare: Ceratites
Ceratites are a strange group of ammonoids which are characterized by a rather unique suture consisting of simply curved “saddles” and complicatedly zigzagging lobes (see figure). The species of this groups belong almost all to a single, variable genus, Ceratites, and they are almost entirely restricted to occurrences in the Central European Basin (also known as “Germanic Basin”) and to an amazingly short stratigraphic interval in the Ladinian of the mid-Triassic. The Central European Basin experienced a short marine episode, the rocks and time (!) of which are known as “Muschelkalk”. This marine period features only the flooding of a relatively small epicontinental basin. However, the limited exchange with the Tethys sea served for a restricted and characteristic fauna with a number of unique species.
The upper part of the Muschelkalk only lasted for ca. 3.5 million years, and only the last ca. 2.8 million years of this period cover the development and existence of the ceratites. Nevertheless, this episode was enough to let more than 20 different species to evolve in the basin, in a rock succession of only ca. 60 meters thickness. The short existence of the single species enabled to reconstruct one of the most precise biostratigraphy so that the identification of a species of Ceratites allows to recognize from which strata the species is originating (see figure).
It should be mentioned that the group is almost, but not entirely restricted to the Central European Basin, but its spatial distribution contributes to the odd nature of these ammoinoids. Apart from very few scattered ceratite genera and species in mid-Triassic rocks of the Alpine-Dalmatian part of the Tethys ocean, the only other species of ceratite ammonoids that occur with some frequency are known from Afghanistan.
Finally, even the preservation of the “Germanic” Ceratites specimens is unusual: Due to the chemistry of the basin, almost none of the Ceratites specimens was even found with the shell preserved. The normal preservation is as steinkerns, which allows the view to the spectacular suture. However, the specimens are fairly frequently found with shells of the unusual oyster Placunopsis ostracina sticking on them. How is this possible? Well, first of all, the oysters usually grew on the ammonoids partly already during life time, but also after the dead ammonoid had sunken to the ground. The shell substance of the ammonoids was aragonitic, and the relatively aggressive water rapidly dissolved the shells. By contrast, the shells of the oysters consists of calcite, which is more stable and does not easily dissolve so that it is still preserved. Consequently, it may seem that the oysters grew on the steinkerns, which is of course not possible.
Another strange feature on “Germanic” Ceratites specimens is that the steinkern (or internal mould) is usually incomplete. The reason for this feature is not a “bad” preservation, but lies in the nature of the animals. Most of the specimen consists of the phragmocone, which is a part of the conch built by gas-filled chambers with which the living animal was able to control buoyancy. When the dead animal sank to ground, the small amount of body tissue inside the chambers began to decay, but the chambers under normal circumstances would be sealed and not filled by sediment. Sediment filling would only take place through the small openings between the chambers. However, the shell of the ammoinoid regularly would crack so that the fine sediment somehow would enter the chambers. Nevertheless, if the chambers were more or less stay intact, they would not be filled. This is often recognizable at least partly, when the gas stayed in the upper part of the chambers so that the steinkern for this reason remained incomplete.
