Ammonites of Hokkaido: Preparation of an Eubostrychoceras japonicum

Japan has become a favourite travel destination for me and my partner in recent years. The vibrant cities, the peaceful temples and shrines, the delicious food and the Japanese culture fascinate us again and again. Of course, the fossils are also very important to me, and Japan has a lot to offer in this regard too. The ammonites from the Upper Cretaceous of Hokkaido are particularly fascinating and well-known among experts. Especially the bizarre-looking heteromorph ammonites are admired by collectors around the world. Since I most enjoy the preparation of fossils as part of our beloved hobby, I had the desire to prepare a few of these ammonites myself. Thanks to my friends Ito Takashi and Mikami Tomoyuki, this wish was actually fulfilled in the spring of last year: I was able to exchange some raw material. In this article, I would like to report on the preparation of the first of these ammonites, an Eubostrychoceras japonicum.

If you ever have the opportunity to visit Tokyo and are interested in the ammonites of the Japanese Upper Cretaceous, a visit to the National Museum of Nature and Science is recommended. The museum has a website (https://www.kahaku.go.jp/english/#), where you can explore an overview of some of the exhibitions, including those of the ammonites, via the "Permanent Exhibitions" tab item "History of the Japanese Islands" ---> "Sea of ​​Ammonites". In one of the numerous exhibition rooms, an impressive overview of the entire ammonite fauna is given: It comprises rather simple, plane-spiral rolled ammonites such as Gaudryceras and impressive giant Puzosias as well as heteromorph forms such
as Scaphites, Polyptychoceras, Scalarites, Hyphantoceras and Eubostrychoceras, including some of the most bizarre species like Nipponites, Muramotoceras and the extremely rare Hypoturrilites, represented by some well-preserved specimen.

Fascinated by the diversity of the Japanese ammonites, I immediately wanted to take action and start the preparation of my material. What I noticed when looking at the concretions and after the first preparation-"tests": the rocks are quite hard and therefore most of the concretions cannot be prepared properly by using air abrasives. Also, the rock separates poorly from the fossil shell, especially in case of strongly ribbed forms.
Only a few geodes are a bit softer and can actually be abraded, which makes the work considerably easier and significantly improves the result that can be achieved.
I started with the Eubostrychoceras presented here. On the one hand because I really wanted to work on a heteromorph ammonite, and on the other hand because it was one of the comparatively soft geodes, which promised a relatively "relaxed" preparation.

 

The preparation

The unprepared specimen immediately caught my eye among the fossils I had brought home with me. It was an intact piece of a larger concretion without further breaks or cracks. In addition, the ammonite was already visible on the worn out surface of the geode, which made it possible to roughly estimate the position and orientation of the whorls (Fig. 1).

 

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Fig. 1: The unprepared geode, width approx. 14 cm. Enlarge photo.

 

At these worn out areas I started uncovering and first carefully blasted the first 2-3 millimeters to familiarise myself with the nature of the rock. The weathered surface is usually easy to work with, but the rock beneath it becomes significantly harder towards the centre of the concretion. I then chiseled small areas of the matrix leaving a few millimetres of rock around the fossil in order to then expose the shell using a fine sand-blast unit. In this way, a relatively riskless approach is possible, in which damage to the shell surface is avoided. This approach and the constant alternation between airscribes and air abrasives are necessary, since the position of the whorls cannot be exactly predicted. Fortunately, the first exposed areas showed a very well preserved shell (Fig. 2 and 3).

 

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Fig. 2: Gradual exposure of the part of the ammonites phragmocone. The fact that this area belongs to the phragmocone can be seen from the suture patterns and the calcite filling in the worn area. Enlarge photos.

 

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Fig. 3: Bit by bit, the last turn that was still present also appeared, which already belongs to the living chamber, as can be seen from the sediment filling. Enlarge photos.

 

The whorls were tectonically broken in some places and slightly shifted, which made it somewhat difficult to progress. In interaction with calcite veins, which run through the concretion, there was a risk of confusion between the veins and the ammonite shell at some points during preparation. In addition, Eubostrychoceras has strong ribbing with irregularly appearing higher ribs. In order not to damage the bulged ribs with the airscribe, it was necessary to abrade a few minutes longer. In this way, it was much easier to follow the fossil into the rock (Fig. 4).

 

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Fig. 4: At the front end of the phragmocone there was an offset in two places and several calcite veins, which made the work more difficult. Special care also had to be taken when preparing the beaded ribs, which clearly protrude over the rest of the ribbing. Enlarge photos.

 

A piece of shell emerged near the largest whorl, and I was initially unsure whether the shell was broken and displaced or whether it was just debris. Such a fragment could originate from the living chamber of the same individual or from another specimen (Fig. 5).

 

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Fig. 5: At the top right you can see the shell piece in question, which is either a broken and drifted part of the living chamber of the Eubostrychoceras or a fragment of another ammonite.

 

My main concern was that this section of the ammonite would be fragmentary. But it was equally possible that the fossil was only covered by this piece of shell and would continue underneath, so there was only one thing to be done: keep going and to look for the connection!
For this I exposed the shell piece a little further and also followed the whorl underneath. I dug a bit deeper behind it, at the area where the whorl should normally be located. And I was lucky, the ammonite continued without any disarticulation below of the shell fragment (Fig. 6)!

 

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Fig. 6: The whorl continues as hoped under the shell piece, so that it could be removed without hesitation during the further preparation.

 

Now I was relieved and could continue the preparation without any worries. The shell debris was removed and I started to roughly expose the areas between the whorls (Fig. 7).

 

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Fig. 7: Side view of the Eubostrychoceras: there are at least three whorls of the ammonite in the stone. As my Japanese collector friends said, in most cases only small fragments are found, so this state of preservation is definitely a reason to be happy. Enlarge photo.

 

Now it was time to shape the matrix. I had already roughly considered in advance what the finished specimen should look like at the end: I wanted to preserve the geode as far as possible and only smooth the area around the fossil in order to achieve a good contrast to the whorls of the ammonite. For this, however, the whorls had to be completely cleaned from the inside as well (Fig. 8).

 

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Fig. 8: Interim status shortly after the "half time" of the preparation: the rough work was done and enough orientation was given to be able to tackle the further exposure of the turns in a targeted manner.

 

Due to the very steep angles and the increasing hardness of the rock, it became more and more difficult to clean the shell from the matrix on the inside. But the problem almost resolved itself when cracks appeared along two calcite veins running through the ammonite. Instead of glueing it with thin superglue, I took off the two halves of the whorl. This made it much easier to clean the inside (Fig. 9).

 

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Fig. 9: The two extracted whorls with last remnants of rock.

 

The three half turns remaining on the geode could now also be easily exposed due to the improved accessibility. All that was left now was to clear out the gaps and smooth the surrounding matrix (Fig. 10).

 

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Fig. 10: Shortly before completion: a bit of matrix had to be removed before the two winding fragments could be attached again.

 

In the last step, the two winding fragments were glued on again, which, thanks to the clean break along the calcite veins, did not require any fillings. The preparation was completed after a good 20 working hours.


Figures 11 to 13 show different views of the preparation result.

 

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Fig. 11: Overall view of the finished preparation. Enlarge photo.

 

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Fig. 12: Side view. Enlarge photo.

 

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Fig. 13: The wide winding whorls of the Eubostrychoceras in focus. Enlarge photo.

 

Thanks to:
At this point I would like to thank my good friends Ito Takashi and Mikami Tomoyuki again for their hospitality, the awesome time spent together and of course for the fossils that now enrich my collection. I hope to see you again soon!

 

General information

Fossil: Eubostrychoceras japonicum

Locality: Haboro region, Hokkaido, Japan

Formation: Upper Cretaceous, Turonian

Length of the fossil: 8 cm

Work required: 20 hours

Preparation and collection: Paul Freitag

 

 

Photos and report: Paul Freitag, Steinkern.de

 


 

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