Hard work breaks no bones: A bone from the Wealden facies gets revealed
Jens Lehmann (Germany)
The recent find of a big slab of Early Cretaceous lumachelle limestone of the Wealden facies containing a bone (Figs. 1 and 2) made for a time-consuming and technically ambitious preparation process. (Lumachelle limestone is a compact limestone or marble containing fragments of shells, encrinites and other fossils, which are sometimes iridescent, and display a variety of brilliant colours.)
The specimen looked disappointing at first sight, but the end result made the hard work worthwhile, as I discuss below. Indeed, the following is intended as an example of the technical aspects of palaeontology, which are too often forgotten or ignored.
The specimen was discovered in a loose, but very heavy slab on the beach. Therefore, efforts were made to reduce the size of the rock in the field to make it easier to carry, but, unfortunately, it broke into two pieces (Figs. 1 and 2).
The original surface of the bone was completely worn – with no details preserved (Fig. 3A) and therefore a transfer preparation had to be planned. On the other side, the cross section (Fig. 3B) gave me a pretty good idea of the shape of the bone before preparation and the exact thickness of the rock that would have to be removed (Fig. 3C). It also revealed the boundary between the finely grained limey siltstone surrounding the bone (light grey) and the lumachelle limestone (dark grey). (In the picture, the transitional zone between the two lithologies is marked by a pen.)
In the first stage, the two pieces were glued together and after that, the weight was reduced by cutting furrows around the fossil on the reverse side, using an angle grinder, and by chiselling the small ridges off (Fig. 3D-F).
Intermediate steps followed, in which a cast of the original surface of the worn bone was prepared. A neutral synthetic resin was used that can be coloured with pigments to get an almost perfect match in colour to the original specimen (Fig. 3G-L).
Reinforcement of the cast was achieved by using a glass fabric. However, a problem needs to be mentioned here. Parts of the bone and the sediment matrix remained on the surface of the cast, even though a layer of epoxy was placed on the specimen before the glass fabric was applied (see the red circle in Fig. 3K). It was wrong for me not to use a mould release agent, but I didn’t because the surface seemed to be pretty smooth. (Full details of casting with resin are given by Smith and Latimer, 1989.)
Sealing the bone and preparation
Before preparation of the bone was possible, the worn bone surface and its surrounding matrix needed to be sealed with synthetic resin (Fig. 4A-C). Again, a glass fabric was added to the first layer of epoxy (Fig. 4A). For this, I used an epoxy that shrinks less than the one used for the casting process and the borders of the mould were created with casting wax, which can be removed mechanically fairly easily, with any that is left being removed through melting by a hot-air drier.
After 48-hours hardening, I began to remove the rock matrix from the reverse side (Fig. 4D-F). A HW80 pneumatic chisel was used to remove most of the rock – it was robust enough to cut through even very hardest of material. (The matrix change from the darker lumachelle limestone to the fine limey siltstone enclosing the bone is marked in Fig. 4E.)
The detailed preparation of the bone itself (Fig. 4F) was carried out using a HW70 pneumatic chisel. Although all of the detailed preparation was done under a microscope, a few places show accidental penetrations into the underlying epoxy. These small pits were restored with a mixture of grinded matrix and a wax for furniture restoration during the final stage of preparation (see Fig. 4H; for details; and, for the pros and cons of this method, see Lehmann, 2013).
An assessment of the result of this complex fossil preparation shows a bone fragment that is likely to be from a dinosaur – based on its internal structure. However, this specimen cannot be identified and this is disappointing to some extent and some might say that the only thing you can do with it is to cut it into pieces and to use it for histology (the study of the microscopic anatomy of cells and tissues of plants and animals). Nevertheless, from an alternative point of view, the specimen represents a technically elaborate piece of preparation work.
Although it is scientifically irrelevant, it is also nice for display purposes, since the specimen is attractive and well-positioned in the rock matrix. In addition, it illustrates the often-forgotten technical part of palaeontology; and each single step of preparation was been documented and even a cast of the original condition is now available for study (Fig. 5).
Lehmann, J. 2013. Reversible Verfüllung von Rissen an Fossilien. Fossilien 30: 135-136.
Smith, J. & B. Latimer 1989. Casting with resins. In R. M. Feldmann, R. E. Chapman & J. T. Hannibal (eds): The Paleontological Society Special Publication, Vol. 4: 326-330, Paleontological Society.