International Geologiical Congress - Oslo 2008

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HPF-17 Trace fossils ? ichnological concepts and methods

 

Feeding traces of the Ediacaran animals

 

Andrey Ivantsov, Paleontological Institute RAS (Russian Federation)
 

 

There are five special types of low-relief impressions known in the Ediacaran (Vendian), which I interpret as the feeding traces of representatives of the Precambrian phylum Proarticulata Fedonkin. These extinct animals have a flat body consisting of two rows of "semi-segments" (isomers). These traces usually have large size and form extensive assemblages, occupying a large area, therefore their study and correct interpretation only became possible after the large-scale excavation work carried out for the first time in the Late Vendian deposits of the Archangelsk Region in 1996-2001. The traces of Proarticulata are represented by impressions on the bottoms of the fine-grained sandstone layers.

Sometimes the body of the trace producers occurs along with them. Trace impressions are characterized by low positive relief, whereas those left by buried bodies are negative and high-amplitude. The traces usually form the groups or consistent chains of elementary imprints (trace platforms) compared to the body imprints which are single. The special feature of the trace is that it usually lacks sharp border with the surrounding rock. Along the whole outline or in some parts of it the impression blurs, "pales", and fuses with the rock surface. Quite often only central (around the longitudinal axis) and posterior body areas are preserved.

Another special feature is the frequent intersection of the traces. The body impressions of Ediacaran organisms rarely overlap each other. Even the evidences of repulsion are known for the Proarticulata genus Dickinsonia. Among the intersected trace platforms the better pronounced is the last formed one. Some areas bear the same distinct relief of the both platforms. In this case the intersected impressions have well-pronounced reticulate pattern. The striking feature of Proarticulata traces is the complete absence of evidences of the body motion along the substrate; the impressions look like accurate die prints. We suggested in 2002 that the trace platform was probably formed as a result of eating through the feeding substrate surface (microbial mat) by the animal. The feeding was performed mainly in a mechanical way. It may be assumed that the ventral side of Proarticulata body was covered with cilia, which caught and transported particles of the food substrate into the special elongated pockets arranged on the borders between the isomers.

The digestion possibly occurred in the pockets, which were connected with the outside space along their whole length. The pockets as additional elongate forms are sometimes preserved on the Proarticulata fossils. All types of traces are referred to the one formal ichnogenus. For priority reasons, it is named Phyllozoon Jenkins & Gehling. Though Phyllozoon hanseni was described as a body impression (fragment of some large organism fallen into pieces), it predated Epibaion axiferus, the ?first' Proarticulata trace fossil.

 

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