The Dawn of Experimental Taphonomy

Charles Walcott’s Jellyfish Experiments

At the end of November, I had the pleasure of spending several days in Denver, working with my colleague Whitey Hagadorn toward an improved understanding of the jellyfish fossil record. During that time I ended up re-reading most of Charles Walcott’s 1898 monograph Fossil Medusae, and rediscovered my admiration for this scientific pioneer.

Those who work in other fields sometimes denigrate paleontology because it is largely a descriptive science, lacking the experimental underpinnings of other disciplines. This criticism no longer holds the sort of strength it once did; paleontology has become much more of a quantitative, analytical subject in the past few decades as it has incorporated approaches from geochemistry and statistics.

A century ago, it was almost unheard of for paleontologists to carry out experiments, but there was the occasional exception as individuals attempted to find out just how their fossils might have been preserved. The discipline of experimental taphonomy (the study of decay and fossilization) actually goes back a long way, and one of its earliest practitioners was the great Charles Doolittle Walcott of the United States Geological Survey. I am tempted to say that Walcott was the earliest to do this sort of work, but undoubtedly there are other examples I am not aware of.

Haeckel’s diagram of the Solnhofen fossil Semaeostomites (re-published in Walcott, 1898, fig. 17)

A couple of decades before his 1909 discovery of the Burgess Shale, Walcott became interested in the idea of jellyfish fossilization. In part, this was because he was trying to understand some fossils he had received, collected from the Cambrian of Alabama.  This interest also came from his reading of the work of Ernst Haeckel, who had published a series of papers on the remarkable fossil jellyfish from the Jurassic Solnhofen limestone of Bavaria (Haeckel 1866, 1869, 1874).

The modern jellyfish Aurelia. Walcott wondered how a fossil like Semaeostomites could have come from medusae such as these.

Walcott wondered how a jellyfish could fossilize; how could the fossils described by Haeckel have come to be? He spent a considerable amount of time contemplating the living conditions of modern jellyfish such as Aurelia and Cassiopea, reading descriptions, and observing them directly. Fortunately, he periodically visited coasts where jellyfish occur:

“While experimenting, in 1895, with some living specimens of Aurelia flavidula in the Indian River, on the coast of Florida, a large specimen was thrown into the quiet, shallow water at my feet, where it remained for some little time, when I observed that it was lying on its back and that the arms had dried and shrunken in the bright sunlight, and the body had swollen so that the genital openings and the mouth were completely lost. On picking up the medusa, I found it to be firm and hard, and when tossed on the wharf it did not break or tear. The shrunken arms were tough, and it required considerable force to pull them apart. This condition might possibly explain how a medusa, when killed by being overwhelmed by a sudden incursion of muddy sediment into the water in which it was living, might retain its shape a sufficient length of time to have the sediment settle closely about it and its cavities and to solidify, so as to make a mold of its exterior form.” (Walcott, 1898, p. 5)

Dead Aurelia on the shore of the Baltic Sea, Estonia

Following up on this observation, and thinking that any jellyfish that was fossilized must have been buried in quick-setting mud, Walcott decided to experiment with fresh medusae and plaster:

“I found no difficulty in securing plaster casts of Aurelia. A bed of soft plaster was prepared and a living medusa taken directly from the water and laid thereon and at once covered by pouring a thin mixture of plaster over it so as to completely bury it in the mass. When the plaster had set, the water in both plaster and medusa wa removed by evaporation and an opening made into the cast. It was then found that the plaster had penetrated the genital openings and cavities and the mouth and gastric cavity (so far as the latter was open), and that the cast showed the details of the form of the animal in a very beautiful manner.” (Walcott, 1898, p. 5)

One of Walcott’s beautiful photographs of an Aurelia cast in plaster (Walcott, 1898, pl. 31)

Walcott incorporated this work, including his stunning photographs, into his 1898 monograph on fossil medusae. Not only did he produce “fossils” of Aurelia for the purpose of comparison with ancient medusoids, but he experimented with producing traces by dragging Aurelia‘s oral arms across wet plaster. These, he similarly compared with various strand-like fossils that he thought might have been produced by the movement of ancient jellyfish.

In the course of developing his monograph, Walcott made significant strides toward the understanding of how jellyfish might have been preserved as fossils. Unfortunately, that did not prevent him from including in his work some phenomena that are clearly not fossilized medusae – a modern assessment indicates that quite a few of these are burrows (trace fossils) or sedimentary structures. And these non-jellyfish include the Cambrian forms from Alabama that had started him onto this line of inquiry.

Drag marks produced by moving the oral arms of Aurelia across the surface of wet plaster (Walcott, 1898, pl.33)

Of course, like other scientists of the time, Walcott had a poor understanding of sedimentation in marine environments, and of decomposition of organisms. But by carrying out those simple taphonomic experiments, by contemplating jellyfish fossils as the partial remnants of three-dimensional bodies, and by actively considering where and how medusae could have been preserved, he showed a very modern approach to inquiry. It would be impossible to complete the understanding of something so complicated within a single work, but Walcott’s study was a strong first step.

References

Haeckel, E., 1866. Über zwei neue fossile medusen aus der familie der Rhizostomiden. Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 1866, 257-282.

Haeckel, E., 1869. Ueber die fossilen medusen der Jura-Zeit. Zeitschrift für Wissenschaftliche Zoologie 19, 538-562.

Haeckel, E., 1874. Ueber eine sechszählige fossile Semaeostomee. Jennaische Zeitschrift für Naturwissenschaft 8, 308-330.

Walcott, C.D., 1898. Fossil Medusae. Monographs of the United States Geological Survey, Washington 30, 201 pp.

© Graham Young, 2012