By Oligocene time, elephants showed the trends toward large size and had developed a long proboscis and large tusks, which are enlarged incisors. Most elephants developed tusks in the upper jaw only, but a few had them in both jaws, and one, the deinotheres, had only lower tusks.
The most familiar
elephants, other than living ones, are the extinct mastodons and mammoths.
Mastodons evolved in Africa, but from Miocene to Pleistocene time they
spread over the Northern Hemisphere continents and one genus even reached
South America. These large browsing animals died out only a few thousands
of years ago. During the Pliocene and Pleistocene, mammoths and living
elephants diverged. Most mammoths were no larger than elephants today,
but they had the largest tusks of any elephant. In fact, mam¬moth tusks
are common enough in Siberia that they have been and continue to be a source
of ivory. Until their extinction near the end of the Pleistocene, mammoths
lived on all Northern Hemisphere continents as well as in India and Africa.