Monday Musings: Why are there so many perfectly preserved soft-bodied animals found in the Cambrian?
There are a number of ways to get the perfect preservations needed to fossilize soft parts but none of them are particularly common. On the other hand, most of them require water and there was quite a lot of it 518 million years ago.
Phosphatization occurs when large quantities of phosphate are present, either in sea water or from the tissues of a decaying organism. In some cases, microbes that fed on the tissue control the phosphatization. Many soft tissues are preserved this way in the Burgess Shale. The phosphate comes from the tissue itself and when pH is low and oxygen is absent, it becomes the primary method of fossilization.
Silicification is one of the most common ways to fossilize something because silicates is the most common rock forming minerals in the crust. Silica often replaces other minerals that have dissolved out such as calcite shells. This is usually seen to preserve things like trilobites. It doesn't often fossilize soft tissue.
Another form of preservation found at least in the Burgess Shale is carbonaceous film. This occurs when something is buried under several layers of sediment and diagenetically altered (in this case by heat and pressure brought on by compaction) and the animal lacks a hard skeleton or shell.
When we look at quarry locations on a paleo map,
and examine the rocks, we see that they lived and died in the right place at the right time (if the taphonomic and preservation bias don't lead us astray).
The Burgess Shale beds were deposited at the base of a cliff of calcareous reefs below the depth agitated by waves during storms. The most widely accepted hypothesis for burial is that part if the reef became detached, slumped and transported rock and debris several kilometers and quickly burying anything in its path.
On the other hand, the Maotianshan Shale was probably buried periodically under turbidity currents, basically an underwater mass wasting event. This is why we don't build our homes on old landslide deposits kids.
The Sirius Passet lagerstatte of Greenland was yet a different environment close to the boundary of an oxygen minimum zone according to geochemical analysis. It is thought that the original preservation was phosphatization that was later altered to silica by low grade metamorphism during the Devonian Period mountain-building events.
Finally, we have the Sinsk Biota of Siberia which inhabited an open marine basin where storms created back currents that sent many animals off into the oxygen depleted depths below. Anoxic conditions prevent growth of microbes that would normally decay flesh allowing soft tissues to preserve.
Now, it is also important to note that oftentimes, parts labeled soft tissue are not necessarily as soft as you believe. Take keratin for example which makes up nails, hair, feathers and sheaths over horns. It's not really that soft in some cases but it is softer than bone which makes it harder to preserve.
Whether we mean keratin or chitin (a natural polymer used to strengthen fungi and invertebrates) or internal organs which really are soft tissues, the Cambrian lagerstatten really are something else.
