Among the earliest forms of fossils of suspected metazoan (multicelled animal) life forms are those found in the layers of rock just below the Precambrian-Cambrian boundary. These forms are collectively referred to as the Ediacaran Fauna. Although the affinities are an active subject for debate, these impression fossils, found generally in coarse quartzitic sandstone, leave little doubt that they were at one time alive. A web page illustrating several of these curious forms is here.
More recently, a fauna discovered in China consisting of microscopic cell clusters resembling embryos of metazoa, has been reported. These enigmatic fossils are found in phosphatic beds, which preserve delicate features of the cellular clusters. Some have been attributed to algae, and others to sponge embryos. At this time considerable effort is being directed to learn more about these fossil deposits, and the curious forms they hold.
2010 Arthur V. Chadwick, Ph.D.