British Journal of Radiology (2004) 77, 420-425
© 2004 British Institute of Radiology
doi: 10.1259/bjr/27832269
X-ray imaging for palaeontology
P Hohenstein
Bundenbachfossilien, D-64686 Lautertal, Germany
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Abstract
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Foreword. While being dimly aware that X-rays are used in far more applications than I would ever be familiar with, I was, in my ignorance, startled when I first saw some of the beautiful fossil images obtained by Peter Hohenstein. Here certainly was a field of "radiology" of which I did not even know the existence. The images are certainly arresting but, other than in the pursuit of something aesthetically pleasing, the reasons why they were ever taken eluded me. My ignorance went deeper than this. How could successful fossil X-rays be taken at all, even of a vertebrate creature? The calcium of the bones would surely have been leached away; and might not slate itself be a good enough absorber of X-rays to "hide" almost anything within? What is "slate" precisely anyway, in chemical terms, and of course, what energy X-rays had been used? Peter Hohenstein explained these things patiently and I thought his beautiful work deserved a wider audience. There are, after all, precedents for the publication of non-clinical material. In a recent issue of Radiology, for example, readers were treated to a CT study of bowed stringed instruments [1]. Peter Hohenstein's work offers us an opportunity to step back some 400 million years and examine some of our ancestors.
P Dawson
Department of Radiology
University College London Hospitals
Mortimer Street
London WIT 3AA
Abstract. Few may be aware that X-ray imaging is used in palaeontology and has been used since as early as 1896. The X-raying, preparation and exposure of Hunsrück slate fossils are described. Hospital X-ray machines are used by the author in his work. An X-ray is vital to provide evidence that preparation of a slate is worthwhile as well as to facilitate preparation even if there is little external sign of what lies within. The beauty of the X-ray exposure is an added bonus.
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Introduction
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Everyone is familiar with the role of X-ray diagnostic imaging in human and even veterinary medicine. Generally speaking, however, most people, including radiologists, have only the vaguest idea of the many ways in which X-rays are also used in other fields such as the technology and engineering sectors and in materials science.
The medical profession and public at large may have had a glimpse, either in professional journals or in the popular press, of the application of imaging to our relatively recent archaeological past, revealing some of the secrets of the Egyptian mummies, for example, but few will be aware of its successful application to the study of artefacts from much more remote times, namely in the field of palaeontology. This little known field is the subject of this article.
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Beginnings
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The use of X-rays in palaeontology is not new. The first attempts to produce X-ray pictures of fossils were undertaken as early as 1896, by Brühl in Berlin and Lemoine in Paris, only some 9 months after Röntgen's initial discovery. Unfortunately, the physical evidence of these early attempts has been lost.
The first substantive work on fossil images was published in 1906 by Branco. Jaekel in 1921 and Mauz in 1929 sporadically X-rayed fossil-containing material. Lehmann was the first, from 1934 onwards, to use X-ray techniques systematically and widely in the investigation of the Hunsrück slate fossils. X-ray studies have also been performed on specimens from other fossil-rich areas, such as Messel and Solnhofen, but the author specializes in the preparation of fossils from Hunsrück slate and so, for that reason, only these will be discussed here.
Stürmer is considered to be the father of modern techniques for the study of the Hunsrück slate fossils [2, 3]. He was the first successfully to capture images of soft-bodied organisms and of internal organs of various fossils. Under his leadership, for several years during the 1970s, a mobile X-ray laboratory was established at the site of discovery of the Hunsrück slate fossils and a wide-angle X-ray technique for the examination of the fossil-containing materials was successfully developed. Now, purpose-built X-ray machines producing outstanding fossil X-rays are to be found in many institutions with an interest in palaeontology.
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Current practice
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Raw materials
The approximately 400 million-year-old Hunsrück slate fossils, which belong geologically to the "Lower Devonian" era, lend themselves particularly well to X-ray imaging. These fossils have generally been converted into pyrite or mineralized or silicated and covered with a layer of pyrite. Pyrite is a stable chemical compound of iron and sulphur (FeS2). Iron is of course an excellent absorber of X-rays in the usual clinical diagnostic imaging energy range.
The Hunsrück slate itself consists of layer upon layer of clay silt (calcium and magnesium silicates) with irregular inlays of layers of fine sand from a Lower Devonian shallow water lake (having a water depth of no more than 200 m). The sedimentary rocks first dried out and then in the course of many millions of years were fashioned into solid clay. The clay was then put under extreme pressure by massive movements of the Earth's crust and the high pressure and temperature converted it into "slate". In this manner sheets of mica (also silicates) were formed and uniformly organized in layers in the rock. These layers of mica allow the sedimentary rock to be cleaved into commercially useable pieces. The Hunsrück slate, which therefore consists in essence of calcium and magnesium silicates, absorbs X-rays to a considerably lesser degree than the pyrite-containing fossils held within. For this reason X-ray analysis is especially successful in this particular material.
X-rays and fossil preparation and exposure
The X-ray images are obtained not simply to generate something of aesthetic value (see Figures 1–9
) but in order to guide the exposure and preparation of the fossil. It seems equally reasonable from a scientific viewpoint to ask the converse, namely why fossils should be prepared and exposed at all, often at some considerable expense, if it is possible to capture the delicate internal structure of the organs so well on an X-ray image. However, notwithstanding the ability to obtain spectacular X-ray images, there are good reasons for preparing and exposing the fossils to direct view:
- Sometimes the X-ray may not reveal individual characteristics of the surface detail which is important for the precise classification of the fossil.
- Often, as a result of tectonic influences, the brittle parts of the fossil skeleton may sustain cracks that have come to be filled in with quartz. These are clearly visible on the X-ray image and simulate sutures or plate dividing lines which, in reality, do not exist.
- Very often, fossils which are heavily silicated do not display any detailed structure at all, merely outlines on the X-ray image. However, their surface can be successfully uncovered and after preparation, may show intricacies of the body structure and skeletal form.
- The skeletal and skin parts of fossils which are covered only by an extremely thin layer of pyrite, absorb X-rays so poorly that only very little contrast is apparent in X-ray images. The outer layers are therefore best revealed by careful physical exposure (Figure 1).
- Collections in museums, institutions and in private hands are not considered complete if consisting of X-ray images alone.
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Figure 1. Starfish Palasterina follmanni (a) X-ray/(b) surface picture; this fossil contains only a trivial amount of pyrite and absorbs few X-rays (a). The preparation shows more detail very clearly (b).
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Figure 2. Starfish Loriolaster mirabilis (a) surface picture of the unprepared plate/(b) X-ray; here the X-ray shows the details of the skeleton; even the delicate webbing is easily distinguished.
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Figure 3. Trilobite Chotecops sp; (a) in the un-X-rayed plate merely the head of the Trilobite and the apparently insignificant remainder can be assumed (see the surface picture). (b) The X-ray then, however, shows a complete animal with all body parts, excellently preserved and of rare pricelessness.
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Figure 4. Starfish Taeniaster beneckei with the arthropod Mimetaster hexagonalis (a) X-ray/(b) surface picture; the fine-limbed unusual arthropod would not have been found but for X-rays. The plate was only 3–4 mm thick, but gave no indication of what lay beneath.
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Figure 5. Three starfish Taeniaster beneckei: (a) surface picture of the partly prepared plate; (b) X-ray. The huge starfish overlaying another would not have been found without X-rays. The plate is about 8 mm thick and on the surface there is no hint of either fossil.
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Figure 6. Starfish Taeniaster beneckei (a) X-ray/(b) surface picture; one of the best preserved examples of its kind.
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Figure 7. Trilobite Chotecops sp. (X-ray); the body construction, as well as the numerous well-preserved small legs, are easily distinguished.
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Figure 8. Crinoid Parisangulocrinus schmidti (surface picture); the crown is broadened out and shows the digestive tract next to the filigreed brachiolae; a second animal is partially hidden.
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Figure 9. Crinoid Codiacrinus schultzei (X-ray), extraordinarily well preserved. A preparation of this lovely creature is unfortunately impossible because of the glassy hardness of the surrounding sediment rock.
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Without the aid of X-ray images the author's preparation work would be much more difficult. The X-rays represent essential signposts for the preparation work and make possible quicker and safer exposure and conservation of the fossil (Figure 2
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Practical X-ray methodology
The handful of dedicated, specialist institutional X-ray machines are only rarely at the author's disposal for his own work. A good practical alternative has proved to be the use of a hospital X-ray unit. The highest quality achievable in X-raying fossils with specialist institutional machines cannot quite be matched, but excellent images can be produced quickly and cost-effectively. The dedicated institutional machines use a much higher radiation dose and a considerably longer exposure time which is not possible with clinical units without significant modification.
Plates of slate of a thickness up to around 35 mm may be X-rayed with this facility (Siemens MULTIX TOP ACSS; Siemens, Erlangen, Germany; high frequency generator POLYDOROS LX 30/50 Lite with 2.0 kW image performance). The very best images can be achieved with a plate thickness of around 10 mm. There is a considerable variation in X-ray absorption between plates, a variation only partly explained by variations in thickness. Radiation exposures are to a large extent based on experience; it is an art, rather than science, so to speak. The exposure parameters chosen for an average 7 mm thick plate are 46 kV and 5.6 mAs. For this, an X-ray film with a speed of 200 is used. For a similar plate the specialist scientific institutions might use exposure factors of, say, 80 kV and 20 mAs and would use a high-resolution X-ray film, e.g. Agfa–Structurix (Agfa, Leverkusen, Germany).
Choice of plates to be X-rayed?
Almost all the most recently discovered Hunsrück slate fossils originate from the commercial roof slate production of the last working slate mine, (Johann and Backes Co. of Bundenbach) in which fossil-containing material represents only a very small proportion of the total aggregate of worked sediment rock.
Often, small elevations on the outer layer of the slate suggest there may be a fossil hidden within. An X-ray will quickly and unequivocally provide an indication as to whether the painstaking work of preparation is likely to be worthwhile or whether there is nothing or only an insignificant fossil remnant (Figure 3).
Now and then, on X-raying a plate, a fossil will be found accidentally where there had been little or no external sign. In such cases these are usually small, very poorly developed fossils but, occasionally, are extremely rare and important examples (Figure 4). Relatively large fossils are once in a while found in such specimens (Figure 5).
During rock formation the stratum of the sediment and the alignment of the slate may be inclined at some angle, one to the other. This often results in the fossils being broken when the rock splits; and they must then be glued together before being prepared. X-rays of both parts of the broken plate provide useful information as to which side of the plate (post-gluing) the preparation may be optimally carried out and as to which is the more interesting side of the fossil. X-ray pictures provide valuable documentation of the restoration process and the use of artificial materials within the prepared finished product. Such documentation needs to be provided with each prepared fossil.
Preparation of the fossils following X-ray exposure
After X-raying the slate plate, the fossil's surface is revealed by scraping away slate with a scalpel. It is a very time-consuming and exacting procedure. It must be done only with the greatest of caution if exceptionally fine and extremely thin parts are to be safeguarded.
Using a binocular microscope the remaining thin slate coating is then removed gently from the fossil with an air abrasive machine-sanding tool. Iron powder and granulated starch flour and, occasionally, glass pearls with a maximum size of 60 µm are used in these machines. The jet is appropriately adjusted according to the degree of hardness of the surrounding stone and the resistance of the fossil material.
Finally comes the step which always gives the author particular pleasure, giving as it does an opportunity to add a little artistry to the preparation technique. The slate adjacent to the fossils is so levelled and smoothed that the contrast between the grey-black plate and the "old-brass"-coloured fossils is enhanced and these appear to stand out.
In this manner an optimal aesthetic effect is achieved in the finished exhibit, an effect enhanced by a sense of being a witness to evolution. Some of these carefully prepared fossils are in considerable demand from museums and collectors.
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Discussion
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It is accepted that the Lower Devonian Hunsrück salt lake was a shallow water region with a water depth of up to 200 m, whose banks already supported sparse vegetation.
Fossilized sponges, jellyfish, conulariids, corals, molluscs (snails, bivalves, nautiloids), brachiopods, worms, arthropods, echinoderms and vertebrates have all been found in unbelievably diverse varieties. In by far the greatest numbers are the arthropods (trilobites, crustaceans, sea spiders and so on) and the echinoderms (starfish, crinoids, homalozoans and cystoids; and more rarely, holothurians and echinoids).
In the Lower Devonian era there were not yet any land-dwelling vertebrates. The rare fossil vertebrate finds from the time consist of various kinds of armoured fish.
Found equally rarely are fossil fragments of simple plants which, in the Lower Devonian era, are taken to be inhabitants of the lake bank area. They already possess a small "vascular" system for the supply of water to all parts of the plant, but they do not yet have leaves and were probably appropriately developed for the environment close to water.
Well-preserved fossils are rare in Hunsrück slate but when found are often in outstandingly good condition with their detailed structure fully preserved. The palaeontological discovery sites and the uniqueness of the information has achieved world-wide recognition because microscopic surface details, minute body appendages such as small legs, antennae, the trachea, spines, fine hair, skin and skeletal parts are all preserved with such extraordinary fidelity (Figures 6–9). The role of X-rays in finding and guiding their exposure and preparation is extremely valuable.
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Conclusions
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In order to achieve the best of fossil exposure, preparation and presentation, it is crucial to have at the outset a good quality X-ray image at one's disposal. Only then is it possible to complete the preparation without running into the danger of removing the plate surface and impairing or ruining the fossil. The fact that the X-ray images are themselves aesthetically pleasing is in itself a welcome bonus. Yet another field, far removed from medicine, owes a debt of gratitude to Wilhelm Conrad Röntgen.
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Collection
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Krista and Peter Hohenstein, Bundenbachfossilien, D-64686 Lautertal, Germany email: fossilien{at}bundenbachfossilien.de website: www.bundenbachfossilien.de
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Acknowledgments
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My special thanks to Professor and Mrs Peter Dawson, who prompted this paper, read it critically and translated it. I am grateful to the management of Johann and Backes Co., Bundenbach, who tolerated my fossil hunting with good humour; in addition I thank the slate miners Hans Vogtel, Werner Stilz, Joachim Seeger, Hans Theis and Alois Backes, who have provided me with such interesting fossil plates and who over the years have become my friends. Special thanks go also to the team of the Imaging Department of the Heilig-Geist-Hospital of Bensheim who have happily helped prepare numerous images for my archive and equally, to the hospital administration who put up with it all. I further thank my friend Gunnar Düppenbecker for the outstanding surface photographs of the fossil plates.
Received for publication March 27, 2003.
Revision received September 2, 2003.
Accepted for publication October 15, 2003.
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References
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- Sirr SA, Waddle JR. CT analysis of bowed stringed instruments. Radiology 1997;203:801–5.[Abstract/Free Full Text]
- Stürmer W, Schaarschmidt F, Mittmeyer HG. Versteinertes Leben im Röntgenlicht. Frankfurt am Main: Kleine Senckenberg-Reihe 11, 1998.
- Bartels C, Briggs DEG, Brassel G. The fossils of the Hunsrück Slate-marine life in the Devonian. Cambridge: Cambridge University Press, 1998.
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Abstract
Introduction
