The fourth epoch of the Tertiary Period, from about 24 to 5 million years ago. During this time the climate was warmer than it had been in the Oligocene, and kelp forests and grasslands first developed. With the isolation of Antarctica, a circumpolar ocean current was established in the southern Hemisphere, reducing the amount of mixing of cold polar water and warm equatorial water and causing a buildup of ice sheets in Antarctica. The African-Arabian plate became connected to Asia, closing the seaway which had previously separated Africa from Asia. Mammalian diversity was at its peak. See Chart at
geologic time.