The Lower Cambrian (520 million year old) Emu Bay Shale on Kangaroo Island is well known for producing large trilobites and other fossils, all preserved as a distinctive red coloured calcium carbonate. One particular fossil however is different. Instead of the red calcium carbonate preservation, Myoscolex is most commonly preserved as grey calcium phosphate. The calcium phosphate actually preserves muscle tissues.
During the decay of labile tissues such as muscle, phosphate and fatty acids are produced which decrease the pH of the surrounding microenvironment. In the presence of oxygen, this process lasts a few days to a week. However, where oxygen is excluded, or at very low levels, this decay process can last for several weeks. Laboratory work has shown that if the pH level can be lowered for more than a few weeks, calcium phosphate is deposited around decaying tissues. Once decay has ceased, the pH of the microenvironment start to rise back to normal. The calcium carbonate which entered the pore fluids at low pH now starts to precipitate around the calcium phosphate and what is left of the organism.
In the case of Myoscolex, the decay process appears to have taken place in the presence of very low levels of oxygen. Here, decay lasted long enough to allow phosphatization of the muscle tissues before decay ceased. In all other fossils, the lack of phosphatization is due either to the fossils either being moults - in which there was insufficient labile tissues, or the oxygen level of the environment was too high. In both cases, conditions would not promote prolonged low pH conditions, and so they are preserved as calcium carbonate.
The presence of phosphatized muscle tissue in the Emu Bay Shale is the oldest such occurrence yet reported from the fossil record.