Preparation of a Scabriscutellum from the Devonian of Morocco

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Morocco keeps coming up with impressive, dramatically spiny trilobites. Their preparation can be really elaborate and needs a lot of patience and caution. More common and morphologically less exciting trilobites need to be carefully uncovered as well in order to reveal their full beauty. The preparation of one of those trilobites shall be presented here step by step.

The trilobite in question is a Scabriscutellum hammadi CHATTERTON et al. 2006 from the Upper Emsian of the Timrhanrhat formation from Jbel Gara el Zquilma. It lies perfectly in the middle of the stone and seems to be unbroken at a first glance.

 

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Fig. 1: Scabriscutellum before the preparation: On top you see the pygidium in the stone, on the bottom the cephalon.

 

Looking closer, three small pieces of shell can be seen still “stuck” in the negative. This is the case quite often and nearly impossible to prevent as the stones need to be cracked open to determine if there are fossils inside.

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Fig. 2 (on the left): You can see three small gaps in the shell. There is a fine calcite vein running from top to bottom through the stone.

Fig. 3 (to the right): The missing pieces of shell are still in the negative and can be transferred.

 

These pieces of shell were carefully cut out of the negative, grinded preferably small and transferred precisely and fit with super glue to the correct spot. The cutting out of small pieces like that can be done even without a wet saw, using either a small angle grinder or diamond saw blades on a rotary tool.

Super glue, which can be bought in different viscosities, is best suited for the glueing of small counterparts. In this case inviscid glue was a good choice to keep the gaps small.

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Fig. 4: The pieces have been glued together and the glue can harden over night.

 

The next step was to uncover the trilobite. The preparation was done with several different air scribes to enable a slow and careful operation .

As the transferred pieces of shell should not split off again, the cubes were cut as flatly as possible and the overhanging material was carefully chiseled down. It’s important to work from top to bottom and not the other way around, because otherwise the affixed pieces could break loose easily. Using this method it’s possible to save even tiny pieces. The next step was to figure out if the trilobite was complete. Therefore it was necessary to acquaint myself with the morphology of the trilobite. The best method is to look at pictures on the internet or in different books and to compare them to the trilobite during the preparation to make sure you’re working the right way. The next step was to check if the left librigena was in place – and it was perfectly articulated.

 

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Fig. 5: The piece of shell at the cephalon has been transferred and I spotted the left librigena. If you look closely you can see a fine dark line under the cap of the eye in the right bottom picture. It marks the transition from cranidium to librigena.

 

Then I wanted to make sure that the pygidium was also in place. With success! It was present and a little angled to the bottom, as I had assumed beforehand because of the position of the trilobite.

 

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Fig. 6: The pygidium has been found!

 

After that I searched for the right librigena, which was also in place.

 

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Fig. 7: The cephalon is complete. Lightly moistened you can see the transition from cranidium to librigena even better.

 

The next step was to continue uncovering the left side of the trilobite. I needed to work really carefully, because the sabre-like pleurae are really thin and delicate and tend to overlap and to break easily. In this case if you prepare from back to front you risk to damage the tips of the pleurae without even noticing them beforehand.

The left librigena was completely uncovered and held a suprise that may have bothered many collectors, but it has a kind of aesthetic from my point of view. The shell was broken into small pieces that are still compounded. This aggravated the preparation, but contributes to making the piece unique.

 

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Fig. 8: The left half of the body is nearly free and shows the beautiful mosaic of the librigena.

 

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Fig. 9: Comparing sizes with the hands of the preparator. For a better contrast the trilobite has been blown upon in the dark workshop.

 

The pygidium was uncovered millimetre for millimetre. I needed to remove more material, because the pygidium was sunken deep within the stone (figure 10).

 

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Fig. 10: On the side you can see the overlapping pleurae as well as some remnants of glue on the spine that will be removed later.

 

It was the same thing with the cephalon. I needed to reduce matrix to reveal it step by step. Thereby I started to create a round spot to surround the trilobite later and give it emphasis. Small pieces of stone within gaps and folds between the segments of the thorax were carefully cleaned away later.

 

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Fig. 11: The cephalon is uncovered and about 23 millimetre wide.

 

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Fig. 12: The piece from the top. The pygidium has a right angle to the bottom.

 

The right half of the trilobite was now completely uncovered and the spot extended further. It was important to me to preserve as much natural stone as possible. Therefore I used an air hammer to split off larger pieces of stone at one time without leaving scribe marks on the stone.


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Fig. 13: The stone gets it´s shape.

 

I proceeded on in the same way with the pygidium to complete its preparation. It was important not to go too deeply into the stone next to the pygidium, because it ends very thin and can easily break.

 

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Fig. 14: Still a lot of material needs to get removed.

 

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Fig. 15: The pygidium of the trilobite is uncovered!

 

Most of the work was done now and the trilobite was uncovered. I continued with the final smoothing of the stone around the trilobite. There was no need to create a cut basis as the stone offered a natural surface that fitted perfectly for display.

 

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Fig. 16: The trilobite right before completion. It’s approximately 43 millimetre long.

 

The fine preparation was a bit harder. To remove the last remnants of stone and glue I thought about using air abrasives, but because the separation was rather good and since I had not needed to use air abrasives up to this point, I also wanted to remove the last bits by hand. Instead of this I used a very fine air scribe along with a sharpened needle which was used exerting a minimum of pressure, otherwise the shell could have been damaged. Additionally the spaces between the pleurae, all the segments and the shards of the left librigena were cleaned with a needle in order to bring out a better contrast. Fortunately I was able to remove even the thin traces of glue. For that purpose I coated the spaces a bit with acetone and was then able to split off the layer of glue with a thin scalpel without damaging the shell. A coating of diluted PVA to bring out the nice brown colour even better was then applied. The preparation of the Scabriscutellum was completed after 12 hours of work. There will follow different views of the finished specimen.

 

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Fig. 17: The finished piece from the front. Enlarge photo.

 

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Fig. 18: The stone from the topview. Enlarge photo.

 

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Fig. 19: Overview with pygidium. Enlarge photo.

 

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Fig. 20: Detailed view of cephalon and thorax. Enlarge photo.

 

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Fig. 21: Detailed view from the top. Enlarge photo.

 

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Fig. 22: Detail of the pygidium. Enlarge photo.

 

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Fig. 23: Detailed view from the front. Enlarge photo.

 

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Fig. 24: Detailed view right of the undamaged librigena. Enlarge photo.

 

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Fig. 25: Detailed view of the right eye. Enlarge photo.

 

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Fig. 26: Detailed view of the left librigena.

 

Acknowledgement

Many thanks to Julia Schulz, who translated my article into English and also to Roger Furze for his kind revision of the text at hand.

 

Paul Freitag for Steinkern.de