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Preparation: Diademaproetus from the Tafilalt basin, Morocco

This report covers the preparation of a small Diademaproetus from the Talifalt basin. Notice the grey patches in the brownish-red matrix. These patches are notably harder and non-fossiliferous. It is likely we deal with a clastic deposit here. The left side of the cephalon has been exposed, and parts of the left eye and anterior cephalic rim are left in the counterpart. 

When fitting the part and counterpart, we can see there is some irreversible damage to the anterior cephalic margin and the distinct protuding 'tongue'-shaped preglabellar field, which is a pity because these are important features of this genus. The presence of the first thoracic segment, of which the upper part is just visible in the original piece, combined with the fact that both librigena are there and fit the cranidium tightly, are good indications that the trilobite is complete. The first image shows the piece before the preparation

The original piece.

The first step is to refit the missing parts, i.e. the eye and parts of the preglabellar field. A thin layer of shell is missing from the upper part of the glabella, but investigating the counterpart, it becomes clear that most of it is missing. Hence, no attempt is made to re-attach the remains of this shell.

We use a small diamond blade to cut out small blocks containing the missing parts. These are cleaned and carefully tried to see how they fit. Their contours are drawn on the matrix as a reference to ensure a quick and correct fit when gluing the pieces together. Because we'll need to work with the airtool on these small pieces, the adhesive bond must be very strong, so we apply an epoxy resin. An additional advantage here of using a resin, is that is fills the small gaps from the missing pieces of the preglabellar field. After attaching the missing pieces and a first rough prep using the airtool to find the left eye, the piece looks as follows: 

After attachment and partial prep of the missing parts.

Here, the eye is almost finished. 

The preservation of the trilobite is excellent; note the combination of terrace lines and tubercles on the anterior part of the glabella. A thin transparant mineral layer covering the shell forms whitish marks when it's damaged (see cephalic margin in the previous picture). As such, every accidental hit with the airtool, no matter how small, leaves a nasty mark. There is little room for error. Because it is unclear at this time whether or not this layer forms part of the actual shell, I decide to leave it untouched as much as possible, despite the fact that removing this layer would be not too difficult and would aesthetically improve the specimen. 

The pictures below show the next stages in this preparation process. At this point, the contours of the trilobite are exposed. The trilobite is partially enrolled, and the pygidium is tucked away under the cephalothorax. This means a lot of material needed to be removed. The first four thorx segments are completely freed of matrix, and end in a needle-shaped point at both sides. This makes the preparation of the following segments more difficult. Due to the curve, every next segment lies below the fragile point of the previous segment. It is also in this stage, that I decide to prepare the genal spines free-standing. The parial enrollment allows for sufficient matrix to remain as a sturdy base for the trilobite to rest on. Next, a nasty surprise when a large chuck of rock come off bearing about 1/3rd of the left genal spine. The cuck is immediately blown away by the airflow over the airtool tip. My first efforts to find the piece are futile, so I apply the clean & sieve routine: the whole surface of the working table is cleaned and the debris is sieved in a couple of different-sized fractions.  Then I select the fraction that I think corresponds most with the missing piece in terms of size and shape. After a mere 5minutes, I retrieve the spine: a sigh of relief. The spine is re-attached using epoxy. 

Because the chances are real that more parts of the spines come off during the next phases of the preparation, I decide to apply a thick strip of reversible glue to the upper part of the spines. The idea is that this glue prevents the pieces from being carried away by the airfloww in the likely event of them breaking. The cracks in the glabellar field are largely filled by epoxy. The remaining cavities are filled using an ultra-thin instant glue. 


The rough outlines of the trilobite are exposed. 

The next phase of the preparation is nerve-wrecking: using a powered-down fine airtool, two of the three faces of each librigenal spine are uncovered, along with most of the lateral pleurae. The working space here is very tight. Next, much of the remaining matrix is removed from the undersides of the genal spines. The left spine breaks again, but the glue on top holds the pieces together. Using the ultra-thin instant glue has advantages here, as it infiltrates the crack through capillary force. As such, there is not need to separate the part before re-attaching it. Convenient.

When almost half of the genal spines are standing free, it seems clear that it's better to switch to air abrasion to do remove the rest. I put this off to this point, because air abrasion may damage the speimen fast, and I have no other piece from this locality to try this technique. After some test runs on the matrix, I slowly work my way at low pressure from the tip to the base of the spine. The tip of the abrader is pointed away from the fossil as much as possible, in order to avoid unnecessary loss of detail. Frequent visual controls for quality loss reveal that there is almost none. We hit the nail on the head when choosing the medium and pressure. The undersides of the genal spine show pronounced terrace lines, a fascinating detail.

The fine work...

Now that the undersides of the genal spines is largely freed, there is more room to tackle the pleural spiny tips. Again we switch to the fine airtool. As pointed out earlier, the pleurae need to be finished one at a time, in order to avoid damage. The separation between shell and matrix is gooed, and the result is very nice, despite needing a magnifying glass to appreciate it.

 The thoracic spines are finished.

Before commencing the finishing of the genal spines (the glue layer still needs to be removed, along with some minor pieces of matrix), I do the finishing of the rock. A nasty surprise when the left genal spine breaks while I'm not working on the specimen. Odd. It's a clean crack so not all is lost. The spine tip is nicely tucked away in a micromount box to be re-attached at the end.

Then, the precarious job of removing the 'safety glue' covering the spines. This is mostly done using acetone, but also with an extremely sharp needle. All goes welluntil the right spine cracks, somewhere halfway, due to a sudden movement of my part. During the manipulation to get the spine tip into a micromount box, it breaks in half. A clear sign of fatigue, I should have called it a day half an hour before this.

The next day, the work is resumed with a calm hand. Preparing the pieces before gluing takes most of the time. I use a thick instant glue, and the pieces are manipulated using adapted toothpicks under binoculars. It goes smooth and once the pieces are in place, the glue hardens quickly. This is the end of this prep project. The end result is not only pleasant to the eye, but also yields a detailed sight on the underside ot the cephalic margin and spines, parts playing a significant role in locomotion and behaviour of this trilobite.

The end result.

 

Frederik Lerouge

 

 

 

 

 


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