<Tr> <Td> Rupelian </Td> <Td> 33.9 </Td> </Tr> <Tr> <Td> Eocene </Td> <Td> Priabonian </Td> <Td> Moderate, cooling climate . Archaic mammals (e.g. Creodonts, Condylarths, Uintatheres, etc .) flourish and continue to develop during the epoch . Appearance of several "modern" mammal families . Primitive whales diversify . First grasses . Reglaciation of Antarctica and formation of its ice cap; Azolla event triggers ice age, and the Icehouse Earth climate that would follow it to this day, from the settlement and decay of seafloor algae drawing in massive amounts of atmospheric carbon dioxide, lowering it from 3800 ppmv down to 650 ppmv . End of Laramide and Sevier Orogenies of the Rocky Mountains in North America . Orogeny of the Alps in Europe begins . Hellenic Orogeny begins in Greece and Aegean Sea . </Td> <Td> 37.8 </Td> </Tr> <Tr> <Td> Bartonian </Td> <Td> 41.2 </Td> </Tr> <Tr> <Td> Lutetian </Td> <Td> 47.8 </Td> </Tr>

The longest unit of time in the geologic time scale is