The systems of the
Phanerozoic were defined during the 19th century, beginning with the
Cretaceous (by Belgian geologist
Jean d'Omalius d'Halloy in the
Paris Basin) and the
Carboniferous (by British geologists
William Conybeare and
William Phillips in 1822). The
Paleozoic and
Mesozoic were divided into the currently used systems before the second half of the 19th century, except for a minor revision when the
Ordovician system was added in 1879. The
Cenozoic has seen more recent revisions by the
International Commission on Stratigraphy. It has been divided into three systems with the
Paleogene and
Neogene replacing the former
Tertiary System though the succeeding
Quaternary remains. The one-time system names of
Paleocene,
Eocene,
Oligocene,
Miocene and
Pliocene are now
series within the Paleogene and Neogene. Another recent development is the official division of the
Proterozoic into systems, which was decided in 2004. ==References==