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Preparation: Crotalocephalina association from the Pragian of Morocco

This report descibes a preparation project with some twists. The piece originates from the 'Couche Rouge' at the base of Jebel Issoumour, and was acquired on the spot in 2012. The unprepped specimen shows a rare sight, with a large part of the trilobite neatly exposed. 

A promising piece, not in the least because the preservation seems to be excellent. The trilobite is quite large, with an estimated length of about 7.5 cm (3 inches). Embedded in the same rock are a damaged Odontochile pygidium, and what seems to be a complete Reedops, safe for some pieces of shell missing on the glabella. A nice association of 3 spieces. 

The piece as acquired. 

A first moment of disappointment early in the preparation process, what seems to be the leftovers of some pockets of air in the glabella. The shell is locally converted to powdery limonite, and all detail is lost. A large part of the glabella shows this problem, and together they form a flat surface. An interesting feature, because now it becomes clear what was up and what was down at the moment of burial. The left librigena is there, in good condition, and forms a tight fit with the cranidium. That is the good news. 

A series of air pockets in the glabella?

A second pleasant surprise follows shortly after, when prepping the pygidium. The pygidial spines are slender and more elongated than expected. This seems to be no ordinary C. gibbus, put perhaps something else. This phenomenon is well known. The preservation of the pygidium is near perfect. Despite the damaged cephalon, this turns out to be a nice and above all, interesting specimen.

Note the missing part of the middle pygidial spine in the picture. This is caused by a missing fragment that was lost when opening the rock. The cavity has been filled with an epoxy resin, and I used the negative imprint of the spine as a mold. The result is the epoxy acting as a bridge between the base and tip of the spine, in a shape that closely resembles the original spine. I have no interest in painting it, and this 'issue' remains clearly visible. 

Progress. Note how the damage on the cephalon is markedly level. 

Detail of the pygidial spines. The epoxy bridge is clearly visible.

As I'm making progress, the overall preservation of the piece turns out to be really good. The association of species is fascinating, too, and especially how they are located relative to each other. The Crotalocephalina covers part of the Reedops with its cephalon. The latter shows dissociation of some thoracic segments at that spot. The missing part of the glabellar shell of the Reedops is probably lost when opeing the rock. The exposed edges of the shell are fortified using a thin instant glue. The Odontochile pygidium is a nice additional touch to the piece. Three species in a row.

The Crotalocephalina seems to push the Reedops down.

These pictures show the downright exquisite preservation, down to every pore, tubercle and lens

Because both librigena tightly fit the cranidium, and I need to remove a lot of material in front of the Crotalocephalina, I decide to try and locate the hypostome. When probing about a millimeter past the edge of the cranidium, it seems to be missing at first. A little later, I find the edge of the hypostome. Halfway of exposing it, it becomes clear why: there was obviously some pressure exerciced on the anterior part, because it has cracked and has been pushed inwards. The shift is slightly more than a millimeter - quite a lot when working with high levels of magnification. 

The hypostome, with the anterior part shifted inwards.

When the preparation is finished, I try to summarize the story behind this piece. First, the association of three different species on a palm-sized rock is quite remarkable. The Crotalocephalina eventually turns out to measure 6.83 cm (2.7 inches), the initial guess was a slight overestimation. 

The Crotalocephaline was almost certainly buried alive (hence the tight fit between cranidium and librigena, and the presence of the hypostome), and I believe the Reedops was too. The sediment seems to have settled with some forced at or shortly after burial: the Crotalocephalina cephalon is slightly dissociated and tilted, and halfway the thorax two segments were pushed apart. The pressure of the Crotalocephalina on the Reedops seemed lagre enough to push the latter apart. 

The head of the Crotalocephalina lies nearly horizontal on the bedding plane. We can tell this because of the air pockets under the glabella, causing limonite to be formed, associated with a bad preservation and loss of detail. Otherwise, the preservation is exquisite. Also notable is the presence of many grey patches in the rock, likely the remains of pebbles. It seems the bedding plane had a layer of small stones. During preparation, these grey spots are noticable harder and devoid of fossil remains.

Another interesting feature: something has nibbled at this trilobite, at least on two occasions, so it seems. Parts of several thoracic segments at the left hand side are missing. They show no or minimal healing and were likely inflicted shortly before the death of the animal. The extreme left pygidial spine was also shortened, but seems to have healed (although it remained short). An old wound. 

The Odontochile pygidium was not entirely prepped free, and this was done on purpose. The remaining piece of matrix holds a couple of fragile spines, which have been prepeed on one side. It seems they are hollow. In any case, they are lost easily during the prep, and this is a way to leave the remaining spines intact for the time being, without investing too much time in an otherwise damaged specimen. 

 

 The finished piece.

 

By Frederik Lerouge

 


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