Complex sentence: 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.
Split into simpler sentences: 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.