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 nuclei - 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. Only about one-twentieth of the site's fossils have been excavated. The fossil beds are threatened by increasing intensity of
phosphate mining operations in the area. A workshop led in protest by local paleontologists resulted in a temporary halt to the mining in 2017. == Paleobiota ==