The beginning of the phanerozoic is marked by what occurrence

The Phanerozoic beginning point is usually set around 542 or 543 million years ago, but, the exact time of the end of the Precambrian era and the beginning of the Phanerozoic era is slightly uncertain. During the nineteenth century, the boundary was set at the first appearance of metazoan fossils. However, since then, several hundred species of Precambrian metazoa have been identified due to the systematic study of those forms which begun in the 1950s. Most paleontologists would probably agree that the Precambrian-Phanerozoic boundary is either at the classic point where the first trilobites and archaeocyatha appear; at the first appearance of Trichophycus pedum; or at the first appearance of a group of small, armored forms termed, "the small shelly fauna". The three different dividing points are within a few million years of each other.