by A. Seilacher,
The Royal Tyrell Museum of Palaeontology, Drumheller, Canada 1997, 64 pages, $10.00 (US).
Reviewed by David J. Bottjer,
Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA.
Art and science. To a lot of people they don't sound like they should mix. And, yet, we know that to many people fossils have aesthetic qualities that cause them to be prized and collected. In a blend of art and science, Fossil Art is the book which Dolf Seilacher has produced to accompany his traveling exhibit of the same name.
Every once in a while an individual comes along who is able to bridge the gap that typically exists between art and science. Audubon, that great nineteenth century popularizer of American birds, was able to portray the aesthetic qualities of birds in his art so successfully that today many images from his great double-elephant folio are among the most prized of natural history prints. His modern legacy is as one of America's greatest artists. However, as pointed out by Gerald Weissmann in a recent essay entitled "Darwin's Audubon" (in Weissmann 1998), Audubon coupled his art with scientific observations that in his lifetime made him a member of many learned societies, and many of Audubon's observations were used by Darwin in his writings on evolution.
Those who know Dolf Seilacher have always been aware that he is interested in design and the aesthetic values of fossils. His monumental contributions to the understanding of functional morphology - the design of organisms - are testament to these interests. He is a visual thinker, as anyone discovers who has studied the wonderful illustrations in his papers. And, we have always known that he was drawn towards deciphering the clues left by the behavior of organisms in sediment - trace fossils.
So, it comes as no surprise that Seilacher has spent some of his Crafoord Prize money to try and cross that no-man's land between art and science. This has resulted in his development of a traveling exhibit displaying primarily the artworks of earth's invertebrates, as left in their record of trace fossils, and as selected and interpreted by Dolf for their aesthetic qualities.
Like Audubon, Seilacher has also mixed a scientific message in with his artistic display. This message concerns much of his vision of what life was like at the Proterozoic-Phanerozoic transition, as metazoans were initially evolving. In Fossil Art he outlines these ideas in a clearly written Introduction. Further elaborations are made in the excellent line drawings he provides to accompany many of the photographs of the casts of large bedding surfaces which form the core of the exhibit. The main point of his message is encompassed in his illustration entitled "Agronomic Revolution" (Figure 1). Seilacher has been a champion of the view that in the Proterozoic, before extensive bioturbation, most subtidal environments were covered with microbial mats that provided a layered structure to the sedimentary component of ecospace in which early metazoans evolved. Thus, these earlier sea-floor environments he terms "matgrounds" were transformed to "mixgrounds" in the early Phanerozoic with the evolution and development of bioturbat organisms (Figure 1).
In contrast to carbonates and stromatolites, it has always been tough to demonstrate that microbial mats typically covered siliciclastic sea floors during the Proterozoic and the Proterozoic-Phanerozoic transition. Seilacher, and his colleagues such as Friedrich Pflüger, have decided to tackle this problem through a complex unraveling of what seem to be ordinary sedimentary structures. An example of this approach is the analysis of palimpsest ripples (Figure 2): where two sedimentation events have created ripples leaving evidence that the sediment surface after the first event was coated with a microbial mat or film. Palimpsest ripples are named after medieval murals in which new pictures were laid over old ones so that now the earlier images can be reconstructed through removal of the later ones. In the top example (Figure 2) the starved secondary ripples overlie the perfectly preserved primary ripples, indicating that an erosion-resistant surface (such as a microbial mat) covered the primary ripples. In the middle example (Figure 2), Seilacher has deciphered the following sequence:
- a secondary sedimentary event completely buried the underlying primary ripples;
- subsequent formation of ripples in the overlying bed led to erosion in the secondary ripple troughs of the underlying ripple crests; which
- left erosional scars (seen here as casts), with a scoop-shape indicative of an erosion-resistant surface (microbial mat) covering the primary ripples.
In the bottom example (Figure 2), Seilacher postulates that the presence of a slimy microbial film on the lower primary ripples led to a distinctive pattern of load-casting when the secondary sedimentary layer was deposited.
Surely if siliciclastic sea floors were covered with microbial mats, early metazoans must have evolved strategies to exploit this food resource. One such strategy is presented on another slab in the exhibit (Figure 3) which shows arcuate sets of scratches on a Neoproterozoic sediment surface, interpreted to have been made by the radula of a large slug-like mollusc. Seilacher suggests that this mollusc was likely similar to Kimberella (Figure 3), recently described (Fedonkin and Waggoner 1997) from similarly-aged strata in the White Sea area of Russia. Not only do the scratch marks indicate a concentrated food resource (such as a microbial mat) at the sediment surface, but the absence of a trail by their slug-like producer indicates that some feature, such as a mat, was firm enough to allow the body to cross this surface without leaving a trace.
Mats not only provide a concentrated source of food on the sediment surface, but the presence of decomposing mats below the surface would also create a potentially enticing layered trophic resource. Seilacher has postulated that the mining by deposit-feeders of these decomposing layers, a strategy that he terms "undermat miners," was a common one when sea floors were characteristically matgrounds. In another example from the exhibit (Figure 4) we see a slab covered with the trace fossil Oldhamia, created in an Early Cambrian deep-sea setting by undermat miners. As shown in Figure 4, this strategy persisted in the deep-sea for some time into the Paleozoic. This occurred because the deep sea served as an environmental refuge for matgrounds. Ultimately, however, more extensive bioturbation and mixgrounds became the predominant mode in deep-sea sedimentary settings, as it already had become in shallower environments. This non-actualistic approach to analyzing organism-sediment relationships during this time period is only just coming into its own. A variety of other geobiologists are also delving into this field.
The examples I have discussed constitute only a small part of the thirty-six cast bedding planes from the exhibit which are described in Fossil Art. Seilacher presents each of these additional bedding planes both as artwork and as the basis for an intriguing scientific story. Many more of the slabs relate to early metazoan environments and lifestyles, but there are also a number of examples from younger intervals in the geologic column. Each of the slabs and their analysis highlights the uniquely "Seilacherian" approach of taking an anomalous individual sample or specimen, teasing-apart the reasons for those anomalous features, and using the results to build more general statements about the ancient Earth and its life. Like Audubon, the message truly is one of both art and science.
The exhibit was originally created by Seilacher and his preparator, Hans Luginsland, at the forcer's long-term home, the Geologic Institute of Tübingen University in Germany. It has recently been touring North America under the auspices of The Royal Tyrrell Museum of Palaeontology, Fossil Art exhibit, in Drumheller, Alberta. Unfortunately, this beautifully-produced "Fossil Art" book is not widely available, but it can also be purchased from the Tyrrell Museum by contacting Janice Leonhardt.
Copyright: Palaeontologia Electronica, 15 March 1999