The Phanerozoic Eon represents geologic time from the end of Precambrian time, approximately 544 to 570 million years ago (mya), until the present day. As such, the Phanerozoic Eon includes the Paleozoic Era , the Mesozoic Era , and the current Cenozoic Era . The Phanerozoic Eon and constituent eras are then further divided into 12 geologic periods.
The Phanerozoic Eon derives its name from phaneros, meaning visible or evident, and zoon, meaning life. Although early life existed in Precambrian time—including prokaryotes (e.g., bacteria) and eukaryotes (organisms with a true nucleus containing DNA)—the onset of the Phanerozoic Eon marks the start of complex life (e.g., invertebrates) found in the Cambrian Period .
In terms of the fossil record , the Phanerozoic Eon represents not the origin of life , but of life capable of leaving extensive fossil remains (e.g., organisms with shells, etc.). Fossilization refers to the series of postmortem (after death) changes that lead to replacement of minerals in the original hard parts (shell, skeleton, teeth, horn, scale) with different minerals, a process known as remineralization. Infrequently, soft parts may also be mineralized and preserved as fossils . Fossils of soft bodied Precambrian time fossils have been found but, as expected, they are rare and present an incomplete evolutionary record.
See also Archean; Cretaceous Period; Dating methods; Devonian Period; Eocene Epoch; Evolution, evidence of; Fossil record; Fossils and fossilization; Historical geology; Holocene Epoch; Jurassic Period; Miocene Epoch; Mississippian Period; Oligocene Epoch; Ordovician Period; Paleocene Epoch; Pennsylvanian Period; Pleistocene Epoch; Pliocene Epoch; Proterozoic Era; Quaternary Period; Silurian Period; Tertiary Period; Triassic Period