<Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2017) (Learn how and when to remove this template message) </Td> </Tr> <P> The Proterozoic - Phanerozoic boundary is at 541 million years ago . 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 the 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 . </P> <P> In the older literature, the term Phanerozoic is generally used as a label for the time period of interest to paleontologists, but that use of the term seems to be falling into disuse in more modern literature . </P> <P> The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, which are further subdivided into 12 periods . The Paleozoic features the rise of fish, amphibians and reptiles . The Mesozoic is ruled by the reptiles, and features the evolution of mammals, birds and more famously, dinosaurs . The Cenozoic is the time of the mammals, and more recently, humans . </P>

Why does the phanerozoic eon have more divisions than the archean eon