Doushantuo Formation - Fossils

Fossils

Doushantuo fossils are all aquatic, microscopic, and preserved to a great degree of detail. The latter two characteristics mean that the structure of the organisms that made them can be studied at the cellular level, and considerable insight has been gained into the embryonic and larval stages of many early creatures. One contentious claim is that many of the fossils show signs of bilateral symmetry, a common feature in many modern-day animals which is usually assumed to have evolved later, during the Cambrian Explosion. A nearly microscopic fossil animal, Vernanimalcula ("springtime micro-animal") was announced in October 2005, with the claim that it was the oldest known bilateral animal. However, the absence of adult forms of almost all animal types in the Doushantuo (there are microscopic adult sponges and corals) makes these claims difficult to prove: some argue that their lack suggests these finds are not larval and embryonic forms at all; supporters contend that some unidentified process "filtered out" all but the smallest forms from fossilization. An alternative interpretation suggests that it was created by non-biological rock-forming processes. The team that discovered Vernanimalcula have defended their conclusion that it was an animal, pointing out that they found ten specimens (not illustrated) of the same size and configuration, and stating that non-biological processes would be very unlikely to produce so many specimens that were so alike.

The discovery was made when the rich phosphate deposits were being mined, and was first reported in 1998. The finds offer direct evidence that confirms expectations that major evolutionary diversification of animals already had occurred before the onset of the Cambrian period, with its apparent 'explosion' of metazoan life-forms and, therefore, that more remote ancestral forms of the phyla recognizable in Cambrian macrofossils must have existed previously.

The documented biota now includes phosphatized microfossils of algae, multicellular thallophytes (seaweeds), acritarchs, ciliates, and cyanophytes, besides adult sponges and adult cnidarians (coelenterates; these may be early forms of tabulate corals (tetracorallians)). There also seem to be what scientists cautiously report as bilateral animal embryos, termed Parapandorina, and eggs (Megasphaera). Some of the possible animal embryos are in an early stage of cellular division (that was first interpreted as spores or algal cells), including eggs and embryos which are most probably of sponges or cnidarians, as well as adult sponges and a variety of adult cnidarians.

An alternative possibility is that the "embryos" and "eggs" are in fact fossils of giant sulfur bacteria resembling Thiomargarita, a bacterium so large that it is visible to the naked eye. The interpretation would also provide a mechanism for phosphatic fossilization through microbially mediated phosphate precipitation by the bacteria, which has been observed in modern environments. If dark spots in the fossil transpire to be fossilised nucleii - an unlikely claim - this would refute the Thiomargarita hypothesis. That being said, recent comparisons of the Doushantuo fossils to modern decaying Thiomargarita and expired sea urchin embryos shows little similarity between the fossils and decaying bacterial cells.