(Material presented below is currently in press as a paper in the Journal of Sedimentary Research)
Experiments were conducted in order to reconstruct conditions in which certain trace fossils, referred to as "mantle and swirl" burrows observed in the Chattanooga Shale were formed. Various rubber bait worms and live earth worms were used in the experiments. Special attention was given to:
a. viscosity of the media (liquid mixture of plaster of Paris)
b. morphology of the worms (their length as well as presence, or absence of appendages, or fins)
c. mode of the motion of worms through the mixture (straight forward motion vs. peristaltic).
Blocks of plaster were sliced in a vertical plane and produced traces examined. To simulate compaction slices of plaster with traces were scanned into a graphics program and virtually "compacted". Trace fossils found in rocks are believed to be produced by vermiform organisms with circular in cross section bodies. In order to compare experimental traces with those found in the Chattanooga Shale, pictures of actual burrows from rock samples were scanned and "decompacted" by using the same graphics program.
Click on pictures to enlarge them.
|All of the rubber worms used in experiments. Notice different body morphologies.|
|A rubber worm is pulled through two different colored layers of watery mixture of plaster of Paris.|
|"Virtual compaction" of traces produced by a rubber worm shown below. Viscosity of the mixture at the moment of pull: buttermilk at room temperature. Percentages show simulated water content of the substrate.|
|Rubber worm which produced traces shown above.|
|A bait worm with legs and a tail. The arrow shows the trace it produced. Viscosity of the mixture at the moment of pull: buttermilk at room temperature.|
|Two rubber worms and traces they produced. "Compacted" plaster simulates substrate water content of 60%. Viscosity of the mixture at the moment of pull: buttermilk at room temperature.|
|Traces produced by three rubber worms (shown) in plaster mixture similar in viscosity to that of lithium grease at room temperature.|
|Group scan of live earthworms which participated in the experiments (against their will).|
|Illustration showing earthworms moving to the surface of the plaster mixture.|
|Features produced by earthworms in plaster. In the central portion of the picture is a horizontally oriented earthworm trace that shows how peristalsis affects burrow diameter. The worm was moving from left to right, the consistency was that of buttermilk, and we see the development of swirl like features. The dark circle in the top right half of the picture is a "mantle"-type burrow produced by another worm that crossed this volume of plaster a little later when it had reached the viscosity of lithium grease. The hole in the bottom right half of the picture is due to a worm that got stuck and then decayed. Decay processes produced the fuzzy gray halo in the plaster adjacent to the worm. The image is 120mm wide.|
|"Compaction" of traces produced by earthworms. Percentages indicate simulated water content of the substrate.|
|"Virtual decompaction" of a real trace fossil from the Chattanooga Shale. Colors of the scan were reversed for better contrast. Percentages indicate calculated water content of the shale. "Decompaction" was halted once the trace reached a circular shape.|
|"Virtual decompaction" of trace fossils from the Chattanooga Shale based on the circular burrow found in the upper right corner of the picture indicated by an arrow. Percentages indicate calculated water content of the shale. "Decompaction" was halted once the trace reached a circular shape.|
|"Virtual decompaction" of trace fossils from the Chattanooga Shale based on the circular burrow found in the upper left corner of the picture indicated by an arrow. Percentages indicate calculated water content of the shale. "Decompaction" was halted once the trace reached a circular shape.|
|Comparison of a burrow found in a rock sample ("decompacted") and a trace produced in an experiment by the worm shown below.|
Biogenic structures found in the Chattanooga Shale and termed "mantle and swirl" traces, are essentially identical to experimentally generated structures that formed when worms moved through a liquid to semi-liquid substrate. From comparison with experiments it appears that the organisms that produced the structures in the Chattanooga Shale were worm-like, lacked appendages (such as legs or fins), and "swam" through soupy muds with a water content of about 70 percent. The "mantle and swirl" traces in the Chattanooga Shale probably fall in the category of biodeformational structures.
Experimental structures were influenced by the consistency of the substrate, the morphology of the worms, and the type of locomotion. At low viscosities fluid mixing and "swirl"-like structures were prevalent, whereas at larger viscosities laminar flow and "mantle"-like features were more common. Appendages, such as fins and legs, tend to interfere with fluid movement around the worm and produced irregularities in the developing "mantle and swirl" traces. Simple forward motion produces burrows/structures with essentially uniform diameter, whereas peristalsis leads to highly variable burrow diameters. The more or less uniform burrow diameter in the Chattanooga Shale suggest that the burrowers moved in a smooth, continuous fashion.
"Mantle and swirl" traces of the type described from the Chattanooga Shale are easily misidentified as Planolites or Paleophycus burrows or also as halo burrows. Because such burrows are typically constructed in a firm substrate, the misidentification will lead to a mistaken assessment of substrate conditions. It will also lead to erroneous assumptions about environmental conditions, because there are implicit behavioral adaptations associated with Planolites or Paleophycus. Examination of photos in the trace fossil literature suggests that burrows of the "swirl and halo" type are probably widespread in the sedimentary record, and likely have been misidentified in previous studies.
"Virtual" decompaction of mudrocks as described in this paper can be used to quantify the initial water content of muddy sediments and to shed light on the morphology and life habits of the burrowers. It should become a useful tool in the study of trace fossils in shales.
Back to Pictures on Pyrite Ooids: Enigmatic Particles of Uncertain Origin
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More Pictures on Miscellaneous Sedimentary Features
More Pictures on Silica Filled Cysts in the Chattanooga Shale
More Pictures on Sculpting of Muddy Bottoms, Erosion Surfaces and Ripples
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© Jürgen Schieber, UTA Department of Geology
Last updated: March 25, 2000.