The Phanerozoic (occasionally Phanaerozoic) Eon is the current eon in the geologic timescale, an the one during which abundant animal life has existed. It covers roughly 545 million years and goes back to the time when diverse hard-shelled animals first appeared. Its name derives from the Greek meaning visible life, referring to the large size of organisms since the Cambrian explosion. The time previous to the start of the Phanerozoic was formerly called the Precambrian (now divided into the Hadean, Archaean and Proterozoic eons). 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 taxa of Proterozoic metazoa 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 archaeocyatha appear; at the first appearance of a complex feeding burrow called Trichophycus 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.