Bioturbation is a recent development in Earth history. It was not until early in the Cambrian Period some 500 million years ago that seafloor organisms began living and feeding in the mud. And until the Silurian some 100 million years later, no animal of any sort lived on land. After that time, the soil and the seafloor mud almost always has been subjected to bioturbation. In detailed fossil studies in mudstone, especially of the marine microfossils so important to climate researchers and the oil industry, a certain amount of blurred boundaries must be expected and accounted for.
In rocks younger than Cambrian, the absence of bioturbation is a signal of dire times. After the great Permian-Triassic mass extinction there was none for millions of years.
