Phanerozoic
The exact time of the boundary between the Phanerozoic and the Proterozoic is slightly uncertain. In the 19th century, the boundary was set at the first abundant metazoan fossils. But several hundred kinds of Proterozoic metazoa have been discovered in the Ediacaran. Study of the Ediacaran biota started in the 1950s. There are three dividing points at the Proterozoic-Phanerozoic boundary. It might be where the first trilobites and archaeocyatha appear; or at the first appearance of signs of burrowing; or at the first appearance of the 'small shelly fauna'. The three different dividing points are within a few million years of each other.