Sedimentation in Ceratite shells
- Kategorie: English articles
- Veröffentlicht am Samstag, 18. August 2012 13:30
- Geschrieben von Stefan Wagner (translated by Thomas Billert)
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Update: In the course of a discussion on the sedimentation in ceratite shells held in the Steinkern.de forum, Thomas Billert has translated the present article. In order to make it accessible to a wider public we add the translation below. Many thanks to Thomas for this great effort.
Sedimentation in Ceratite shells
I was motivated to write this article, mainly targeted to the beginners among fossil collectors, by reading a paper (*) by Seilacher about the sedimentation in ammonite shells, a topic that I still find exciting. So I went through my collection for a few pieces that illustrate Seilacher's theory in a manner suitable for the Steinkern website. While doing so I got the insight that an ammonite that is incomplete is not necessarily uninteresting for the collection - it's rather quite the contrary, especially incompletely conserved pieces can teach us a lot about the processes that lead to fossilization. Particularly interesting for me is the Steinkern conservation, as this type of conservation is common among the Ceratites of the Upper Muschelkalk, and Ceratites form the main part of my collection.
Every collector knows them, incomplete Steinkern fossils that appear so much damaged that sometimes only the living chamber or part of the phragmocone is reasonably preserved, so you don't want to bend down for them at all in the field. But in fact these fossils are not damaged in a sense that parts of them got lost over time, they were rather only preserved in the incomplete way as we find them today.
The basic question connected to the Steinkern preservation of Ceratites is how several individual chambers, separated by thin walls, could be filled with sediment to achieve complete preservation of the entire fossil.
It's easy to imagine that mud at the sea ground was flushed into the living chamber after decomposition of the Ceratite's soft parts. But for the sediment to fill the phragmocone chambers behind the living chamber, it required two prerequisites that become obvious when looking at the general building plan of the Ceratite shell, while remembering school physics.
The general background is that the phragmocone chambers are not isolated from the living chamber, but connected by the Sipho, a thin tube of skin supplied with blood, situated - in case of the Ceratites - at the outer (median) side of the chambers. The Sipho allowed Ceratites to adjust salt concentration in the chambers applying the osmotic principle.
To have every chamber filled with sediment, the Sipho had to be leaky in each individual chamber, so mud was allowed to float in. On the other hand, the formation of a current requires a driving force, i.e. two openings of the shell, allowing water (with mud) to float in and out. Besides the opening of the living chamber this means that the shell needs to be damaged close to the navel, the centre of the inner windings, so that the water/mud mixture floating in via the living chamber can leave the shell there.
Only when both of these prerequisites were fulfilled and the Ceratite was exposed to ground current long enough, chances were given that the entire shell was filled with mud and - in the course of diagenesis - a pretty Steinkern as in Fig. 1 was formed.
Fig. 1: Ceratites compressus (7.5 cm) from Niesen. The Steinkern is almost flawlessly preserved.
If the Sipho was not damaged in some chambers, a Steinkern was formed as shown in Fig. 2. Here the living chamber and parts of the phragmocone are preserved, while some chambers were not filled with sediment at all. Therefore this Steinkern nicely shows the structure and form of the walls between the chambers, which display as suture lines on completely preserved Steinkerns.
Fig. 2: Ceratites spinosus (10.5 cm) from Vahlbruch. The Sipho stayed intact in some chambers, so these could not be filled with sediment.
If the Sipho was damaged in all chambers but the time was not sufficient for the mud stream to fill the entire shell, various stages of preservation can be observed.
Fig. 3 shows the first stadium. Here all chambers of the shell (which was lying on its side) were filled approximately to the level of the Sipho. Then sedimentation stopped, perhaps due to entombment of the shell, so a quite even level of filling was preserved and a Steinkern was formed that is rather contour-less on its upper side.
Fig. 3: Ceratites sp. (9 cm) from Gotha area. Incompletely filled Steinkern.
An interim stadium between Fig. 1 and Fig. 3 is shown in Fig. 4. Here some chambers of the phragmocone are completely filled, while other show an even level of filling at the niveau of the Sipho. This might give a tentative insight at the timely offset of the breakthroughs of the Sipho in the individual chambers.
Fig. 4: Ceratites sp. from Göttingen. The phragmocone chambers display different levels of sediment filling.
It's getting even more interesting if the shell was primarily filled to a higher level than that of the Sipho. From then on an even level of filling is no longer formed, as the water/mud mixture floating into the chambers moved upwards at the inner surface of the shell, so that the resulting Steinkern features a concave surface at the incompletely filled chambers. An example for this form of preservation is shown in Fig. 5.
Fig. 5: Ceratites compressus from Göttingen (7 cm). The sediment flowing into the shell exceeded the level of the Sipho and moved upwards at the inner shell surface.
At this point the question raises how the sediment was able to exceed the level of the Sipho at all. The cause for this lies in the particular properties of the flowing water/mud medium, which was swirled in the stream and oscillated in a sinus-shape between the chambers, so that the sediment was allowed to form downpour cones that exceeded Sipho level. One can already guess these cones partially in Fig. 5.
The regime of sedimentation becomes very clear when the shell was almost completely filled up with sediment. If this case, the outer side of the Steinkern sometimes displays a sinus-shaped oscillating line, which comprises the residue of the floating channel that only just stayed open. Such a channel can be observed in Fig. 6 and 7.
Fig. 6: Sinus-shaped streaming channel at the outer side of a Ceratites nodosus (14 cm) from Göttingen. The channel was presumably filled with yellow clay after disintegration of the shell.
Fig. 7: Sinus-shaped streaming channel at the side of a Ceratites compressus from Göttingen (7.5 cm).
Sedimentation of Nautilide shells, which can be found alongside of Ceratites in the entire Upper Muschelkalk, proceeded in a similar manner. In contrast to Ceratites, the Sipho of Nautilides is not situated at the outer side of the chambers but right in their center. Furthermore it formed a limy tube, so it was usually not damaged resulting in no preservation of the phragmocone chambers. In these cases the Steinkern only shows the inner cast of the stable, sediment-filled Sipho, which looks like a pearl chain then (Fig. 8).
Fig. 8: Nautilide Steinkern with "pearl chain" from Göttingen area (17 cm). Collection P. Osburg.
I will try to add more meaningful images displaying the individual stages of preservation. Further, I'm open to criticism and suggestions.
(*) A. Seilacher: Sedimentationsprozesse in Ammonitengehäusen, Akademie der Wissenschaften und der Literatur, Abhandlungen der mathematisch-naturwissenschaftlichen Klasse, Jahrgang 1967, Nummer 9.