Xogta ugu da'da weyn ee la yaqaan ee inta badan xayawaanka naasleyda ah ee casriga ah waxay u muuqdaan muddo gaaban gudahood Eocene hore. Bilowgii Eocene, dhowr kooxood oo naasley cusub ah ayaa yimid Waqooyiga Ameerika. Xayawaankaas casriga ah, sida artiodactyls, perissodactyls iyo primates, waxay lahaayeen astaamo sida lugaha dhaadheer, dhuuban, cagaha iyo gacmaha awood u leh qabashada, iyo sidoo kale ilkaha kala duwan ee loo habeeyay cunista. Noocyada dwarf ayaa xukumay. Dhammaan xubnaha xayawaanka cusub ee naasleyda ah waxay ahaayeen kuwo yaryar, oo ka yar 10 kg; iyadoo lagu saleynayo isbarbardhigga cabirka ilkaha, naasleyda Eocene waxay ahaayeen kaliya 60% cabirka naasleyda Palaeocene ee hore. Waxay sidoo kale ka yaraayeen naasleyda iyaga raacday. Waxaa la aaminsan yahay in heerkulka Eocene uu doorbiday xayawaanka yar yar yar ee awood u leh inay si fiican u maareeyaan kuleylka. Su
lugaha dhaadheer oo dhuuban