Page "Phanerozoic" Paragraph 2

The Proterozoic-Phanerozoic boundary happened 540 million years ago.

In the 19 < sup > th </ sup > Century, the boundary was set at 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.

Most geologists and paleontologists would probably set the Proterozoic-Phanerozoic boundary either at the classic point where the first trilobites and reef building animals ( archaeocyatha ) such as corals and others appear ; at the first appearance of a complex feeding burrow called Treptichnus pedum ; or at the first appearance of a group of small, generally disarticulated, armored forms termed ' the small shelly fauna '.

The three different dividing points are within a few million years of each other.