One of the early explanations for the appearance of the SSFs – and the evolution of mineralized skeletons – is a sudden increase in the ocean's concentration of calcium. However, many SSFs are made of other minerals, such as silica. The first SSFs appear around the same time as organisms first started burrowing to avoid predation, so it is likely that they represent early steps in an evolutionary arms race between predators and increasingly well-defended prey. Nevertheless it is still true that the animals used minerals that were most easily accessible.