Oligocene

61:2.1This pe­riod was char­ac­ter­ized by the fur­ther and rapid evo­lu­tion of pla­cen­tal mam­mals, the more pro­gres­sive forms of mam­malian life de­vel­op­ing dur­ing these times.

61:2.2Al­though the early pla­cen­tal mam­mals sprang from car­niv­o­rous an­ces­tors, very soon her­biv­o­rous branches de­vel­oped, and, ere­long, om­niv­o­rous mam­malian fam­i­lies also sprang up. The an­giosperms were the prin­ci­pal food of the rapidly in­creas­ing mam­mals, the mod­ern land flora, in­clud­ing the ma­jor­ity of pres­ent-day plants and trees, hav­ing ap­peared dur­ing ear­lier pe­ri­ods.

61:2.335,000,000 years ago marks the be­gin­ning of the age of pla­cen­tal-mam­malian world dom­i­na­tion. The south­ern land bridge was ex­ten­sive, re­con­nect­ing the then enor­mous Antarc­tic con­ti­nent with South Amer­ica, South Africa, and Aus­tralia. In spite of the mass­ing of land in high lat­i­tudes, the world cli­mate re­mained rel­a­tively mild be­cause of the enor­mous in­crease in the size of the tropic seas, nor was the land el­e­vated suf­fi­ciently to pro­duce glaciers. Ex­ten­sive lava flows oc­curred in Green­land and Ice­land, some coal be­ing de­posited be­tween these lay­ers.

61:2.4Marked changes were tak­ing place in the fauna of the planet. The sea life was un­der­go­ing great mod­i­fi­ca­tion; most of the pres­ent-day or­ders of ma­rine life were in ex­is­tence, and foraminifers con­tin­ued to play an im­por­tant role. The in­sect life was much like that of the pre­vi­ous era. The Floris­sant fos­sil beds of Col­orado be­long to the later years of these far-dis­tant times. Most of the liv­ing in­sect fam­i­lies go back to this pe­riod, but many then in ex­is­tence are now ex­tinct, though their fos­sils re­main.

61:2.5On land this was pre-em­i­nently the age of mam­malian ren­o­va­tion and ex­pan­sion. Of the ear­lier and more prim­i­tive mam­mals, over one hun­dred species were ex­tinct be­fore this pe­riod ended. Even the mam­mals of large size and small brain soon per­ished. Brains and agility had re­placed ar­mor and size in the prog­ress of an­i­mal sur­vival. And with the di­no­saur fam­ily on the de­cline, the mam­mals slowly as­sumed dom­i­na­tion of the earth, speed­ily and com­pletely de­stroy­ing the re­main­der of their rep­til­ian an­ces­tors.

61:2.6Along with the dis­ap­pear­ance of the di­no­saurs, other and great changes oc­curred in the var­i­ous branches of the saurian fam­ily. The sur­viv­ing mem­bers of the early rep­til­ian fam­i­lies are tur­tles, snakes, and croc­o­diles, to­gether with the ven­er­a­ble frog, the only re­main­ing group rep­re­sen­ta­tive of man’s ear­lier an­ces­tors.

61:2.7Var­i­ous groups of mam­mals had their ori­gin in a unique an­i­mal now ex­tinct. This car­niv­o­rous crea­ture was some­thing of a cross be­tween a cat and a seal; it could live on land or in wa­ter and was highly in­tel­li­gent and very ac­tive. In Eu­rope the an­ces­tor of the ca­nine fam­ily evolved, soon giv­ing rise to many species of small dogs. About the same time the gnaw­ing ro­dents, in­clud­ing beavers, squir­rels, go­phers, mice, and rab­bits, ap­peared and soon be­came a no­table form of life, very lit­tle change hav­ing since oc­curred in this fam­ily. The later de­posits of this pe­riod con­tain the fos­sil re­mains of dogs, cats, coons, and weasels in an­ces­tral form.

61:2.830,000,000 years ago the mod­ern types of mam­mals be­gan to make their ap­pear­ance. For­merly the mam­mals had lived for the greater part in the hills, be­ing of the moun­tain­ous types; sud­denly there be­gan the evo­lu­tion of the plains or hoofed type, the graz­ing species, as dif­fer­en­ti­ated from the clawed flesh eaters. These graz­ers sprang from an un­dif­fer­en­ti­ated an­ces­tor hav­ing five toes and forty-four teeth, which per­ished be­fore the end of the age. Toe evo­lu­tion did not pro­gress be­yond the three-toed stage through­out this pe­riod.

61:2.9The horse, an out­stand­ing ex­am­ple of evo­lu­tion, lived dur­ing these times in both North Amer­ica and Eu­rope, though his de­vel­op­ment was not fully com­pleted un­til the later ice age. While the rhinoceros fam­ily ap­peared at the close of this pe­riod, it un­der­went its great­est ex­pan­sion sub­se­quently. A small hog­like crea­ture also de­vel­oped which be­came the an­ces­tor of the many species of swine, pec­ca­ries, and hip­popota­muses. Camels and lla­mas had their ori­gin in North Amer­ica about the mid­dle of this pe­riod and over­ran the west­ern plains. Later, the lla­mas mi­grated to South Amer­ica, the camels to Eu­rope, and soon both were ex­tinct in North Amer­ica, though a few camels sur­vived up to the ice age.

61:2.10About this time a no­table thing oc­curred in west­ern North Amer­ica: The early an­ces­tors of the an­cient lemurs first made their ap­pear­ance. While this fam­ily can­not be re­garded as true lemurs, their com­ing marked the es­tab­lish­ment of the line from which the true lemurs sub­se­quently sprang.

61:2.11Like the land ser­pents of a pre­vi­ous age which be­took them­selves to the seas, now a whole tribe of pla­cen­tal mam­mals de­serted the land and took up their res­i­dence in the oceans. And they have ever since re­mained in the sea, yield­ing the mod­ern whales, dol­phins, por­poises, seals, and sea li­ons.

61:2.12The bird life of the planet con­tin­ued to de­velop, but with few im­por­tant evo­lu­tion­ary changes. The ma­jor­ity of mod­ern birds were ex­is­tent, in­clud­ing gulls, herons, flamin­goes, buz­zards, fal­cons, ea­gles, owls, quails, and os­triches.

61:2.13By the close of this Oligocene pe­riod, cov­er­ing ten mil­lion years, the plant life, to­gether with the ma­rine life and the land an­i­mals, had very largely evolved and was pres­ent on earth much as to­day. Con­sid­er­able spe­cial­iza­tion has sub­se­quently ap­peared, but the an­ces­tral forms of most liv­ing things were then alive.