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Are These the Oldest Metazoan Trace Fossils?

Erle G. Kauffman and James R. Steidtmann
Journal of Paleontology
Vol. 55, No. 5 (Sep., 1981), pp. 923-947
Stable URL: http://www.jstor.org/stable/1304516
Page Count: 25
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Are These the Oldest Metazoan Trace Fossils?
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Abstract

The Precambrian Medicine Peak Quartzite of southeastern Wyoming is part of a largely marine sedimentary sequence which lies unconformably on gneissic basement rocks 2.41 ± 0.5 BY old and which is cut by 2.0 BY old granite. The Medicine Peak is almost pure quartzite with primary sedimentary structures which collectively suggest intertidal to shallow subtidal paleoenvironments. Among the primary structures are nine distinct morphological types of sediment-filled tubes distinguished by: a, difference in grain size between the tube and the surrounding matrix; b, concentrations of mica, iron and/or distinct grain sizes at the tube boundary; and c, by sharp color contrast, the tubes being darker than the matrix because of limonite staining or mica concentrations. None of these tubes closely resemble metamorphic segregations, gas and water escape passages, or larger dewatering structures. All tube morphologies have broadly similar Cambrian and/or younger analogs accepted as burrows of invertebrate metazoan organisms; yet there is no unequivocal evidence that these are trace fossils. They are dubiofossils whose morphology, size-frequency, orientation, and distribution relative to bedding suggest a biologic origin. However, the known record of Precambrian life, and widely accepted interpretations of biologic, oceanic, and atmospheric evolution ±2 BYBP strongly argue against a metazoan origin. The theory of "explosive" Late Precambrian metazoan evolution requires that these tubes be abiotic structures. The less popular theory of gradual Precambrian evolution makes metazoan affinities of the tubes possible. Are these the oldest metazoan trace fossils?

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