Preparation of a Hallucigenia from the Cambrian of China
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- Veröffentlicht: Sonntag, 25. Oktober 2020 09:50
- Geschrieben von Paul Freitag
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The Lower Cambrian Chengjiang fauna of the Yunnan Province of China is widely known for its richness of species and thereby provides a fascinating insight into Cambrian radiation. Extremely remarkable is the soft tissue preservation of the fossils from the Maotianshan Shale: antennas, legs or digestive systems can be observed on well preserved specimens. These soft bodies are present as thin film-like aluminium silicate layers and often show finest details. In the best case the fossils are found as "lucky splits" or they only need to be prepared from just one rock. However, a large proportion of the finds are recovered torn on positive and negative plates and require precise transfer preparation. The following example of a preparation of a Hallucigenia shows how this can be done.
Hallucigenia represents a problematic fossil that is assigned to the lobopods. In the Yuanshan Member the genus occurs with the species H. fortis. The second so far known species H. sparsa originates from the Burgess Shale of Canada.
Characteristic is a worm-like body with two rows of spine-like extensions on the upper side and two rows of tentacle-like extremities on the lower side.
The present specimen shows only one row of appendages on the upper and lower side, whereas the extremities of the lower side are only vaguely preserved. The fossil was found on a positive and a negative plate, that both yielded fossil substance (see fig. 1-3).
Fig. 1: Positive plate before prepration, width approximately 16.5 cm. Enlarge photo.
Fig. 2: Detailed view of the fossil on the positive side, length approximately 2 cm.
Fig. 3: Detailed view of the fossil on the negative side.
Before starting the preparation, I looked very closely at both parts under the microscope. Especially with valuable fossils, which show a special state of preservation and whose preparation is difficult, a precise planning of the procedure is important to avoid mistakes. In this case it had to be clarified in advance in which areas a transfer of fossil substance can be realised and which areas are rather simple to expose. Fortunately, there were only two areas that had to be transferred from the negative (see Fig. 4, 5).
Fig. 4: The marked areas needed to be transferred. Enlarge photo.
Fig. 5: The areas where substance had to be transferred from the negative are marked in black; fossil substance still hidden under a thin layer of rock is dashed in red. Enlarge photo.
Easier said than done: after all, the fossil consists only of a thin layer of silicate and the parts to be transferred are only a few millimetres in size. Therefore, very precise cutting was necessary (see fig. 6).
Fig. 6: A tiny part of fossil substance, marked by the black triangle in the photo, needed also to be transferred.
The bonding had to be done just as precise, because errors can hardly be corrected in the Maotianshan slate. In this case, dissolving an unsuccessful bond in acetone would have lead to the disintegration of the slabs, as the slate becomes soaked with liquid and then swells and breaks apart. Short-term wetting or wiping with alcohol or similar is usually possible without problems, but insertion for a longer period of time should be avoided in any case!
After the shards had been placed several times to find the fitting position, they were then glued in place - fortunately without any incidents (see Fig. 7).
Fig. 7: Both parts were taken from the negative and transferred to the positive with a perfect fit. The still hidden part of the Hallucigenia was prepared at a length of a few millimetres.
The next step was to reveal the fossil. I started by uncovering a few millimetres of the still hidden end and then checked the transferred areas (see fig. 8).
Fig. 8: The exposure of the transferred areas shows that the transfer succeeded.
In addition to needle and scalpel, fine air-chisels can also be used here, as long as the hand guidance is precise and "wobble-free".
Remarkably, the "head" turned out to be completely preserved, including the eye-spot, which stands out in contrast – a remarkable exception (see Fig. 9)!
Fig. 9: After advanced preparation the head end including the eye spot was revealed.
Now the task was to smooth the surrounding matrix in order to lift the Hallucigenia from the rock in a contrasting way and to create a harmonious overall look without disturbing preparation traces. In addition, the plate was reduced in size to achieve a more balanced relation of fossil to rock size (see Fig. 10).
Fig. 10: General view of the completed specimen on matrix. Enlarge photo.
Figures 11 and 12 show the finished specimen in close-up without and with alcohol to enhance the contrast. In addition to the excellently preserved eye spot, the digestive tract should be mentioned, which can be observed especially at the head end and in the middle part.
Fig. 11: Detailed view of the fossil without alcohol wetting, length approx. 3 cm. Enlarge photo.
Fig. 12: Detailed view with alcohol wetting. Enlarge photo.
About the fossil:
Species: Hallucigenia fortis
Location: Anning, Yunnan Province, China
Formation: Maotianshan Slate, Yuanshan Member, Heilinpu Formation
Length of the fossil: 3 cm
Time required for the preparation: 12 hours
Preparation: Paul Freitag
Collection: Yuxi Wang
Paul Freitag for Steinkern.de
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