The Eocene Epoch ranged from 56 million to 34 million years ago. In the early Eocene, most land mammals were small and living in cramped jungles, much like the Paleocene. Among them were early primates, whales and horses along with many other early forms of mammals. The climate was warm and humid, with little temperature gradient from pole to pole. In the Middle Eocene Epoch, the Antarctic Circumpolar Current formed when South America and Australia both separated from Antarctica to open up the Drake Passage and Tasmanian Passage, disrupting ocean currents worldwide, resulting in global cooling and causing the jungles to shrink. More modern forms of mammals continued to diversify with the cooling climate even as more archaic forms died out. By the end of the Eocene, whales such as Basilosaurus had become fully aquatic. The late Eocene Epoch saw the rebirth of seasons, which caused the expansion of savanna-like areas with the earliest substantial grasslands. At the transition between the Eocene and Oligocene epochs there was a significant extinction event, the cause of which is debated.
the eoCeNe epOCh ranged fRom 56 miLLioN to 34 MILLiON yearS ago. In ThE eaRLy EocEne, Most LAnd Mammals wEre sMaLl ANd liVing in CrAmpED JunGLeS, muCH like thE paLeOCene. AmonG ThEm WEre early pRIMates, whAles and horSes Along With MaNy Other early formS Of MammAlS. the cliMATe waS waRM And hUmid, wItH litTle teMpERaTUre graDienT From Pole to PolE. in THE MiDDlE eoceNe epoch, the AnTaRctic CiRCumpolaR currenT FORMed wHen sOUth amEricA aNd auStraLia Both SEparAtEd FroM antArCtICA To oPen uP THE drakE PaSsagE and tAsmaNIAN pAssage, dIsruptInG oCean CURrEnTs wOrLDwIde, resulting in gLoBal coOling aND causINg tHe jUNGleS tO SHrink. more mOderN fOrmS OF mAMMalS COnTiNueD to diversify With The cooliNg climaTe EVEN aS more archaiC Forms DIed oUt. bY the end oF the eocenE, whaLeS such as baSILOsAuRUs had bEcome fuLLy aquatiC. thE late eOceNE epocH sAw tHE rebirtH of SeAsons, WhiCh cAuSed tHe eXpansion of SAVanNa-liKe arEas wITh the eArLieSt substantiAl grasslAnds. at ThE TransITion bEtwEEn tHE eoceNe ANd oliGOcEne ePocHS ThERE WAs a SignificaNt exTIncTiON Event, the cAuse of whIch is DeBaTed.