Lack of early animal fossils: insights from taphonomic experiments on placozoans and their traces

Abstract

Placozoa are simple two-layered multicellular animals. Can such a simple animal preserve in the fossil record? The results of our taphonomic experiments showed that it is chemically possible; however, the chances of the presevation are negligible for two reasons. Firstly, the resistance of living Trichoplax to lethal factors turned out to be quite high. Secondly, post-mortem changes in the body of Trichoplax preclude preservation. We observed these changes for the first time: their bodies immediately disintegrated into a cloud of individual dead cells, apparently unrecognizable in the fossil record. We hypothesized that the absence of a basement membrane, which unites each cell layer into the epithelia, is a key factor in this dramatic disintegration. This hypothesis adds an option to the explainations of the gap between the time of emergence of metazoan body fossils and the reconstructed time of metazoan divergence from a single-celled ancestor. However, it is theoretically possible to find ichnofossils of Trichoplax-like locomotion. For the first time, we observed traces of Trichoplax on a soft substrate and identified a previously unknown type of traces. They were 3D chains of compacted round lumps of the substrate, similar in some features to the Precambrian meniscus ichnofossils.