In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan) fossils but several hundred groups (taxa) of metazoa of the earlier Proterozoic era have been identified since systematic study of those forms started in the 1950s.
The boundary was set at time of appearance of the first abundant animal (metazoan) fossils. This was in the 19th century. Several hundred groups (taxa) of metazoa of the earlier Proterozoic era have been identified since systematic study of those forms. This was in the 19th century. These forms are started in the 1950s.