suit of later,- rigorous climatic zonation, and a more 

 extensive physiographic diversification over the con- 

 tinent than prevailed in the early Tertiary. Instead 

 of evolving new tolerance limits, species, with some 

 exceptions, dispersed into those regions that con- 

 tinued favorable to them and became extinct in re- 

 gions that became unfavorable. Species with similar 

 ranges of tolerance thus came into association and, 

 as interdependent coactions became established, into 

 closely knit communities. 



Neotropical-tertiary flora 



This flora is known from several Paleocene, 

 Eocene, and Oligocene deposits (Berry 1937). It 

 was composed of tropical and subtropical plants now 

 limited largely to southern Florida, Mexico, and the 

 tropics. Its counterpart, the Paleotropical-tertiary 

 flora, occurred in western Europe. Trees character- 

 istically had broad, thick, evergreen leaves. The 

 laurel familly, Lauraceae, was particularly well devel- 

 oped in North America, and some modern descend- 

 ants are found in the temperate flora. The Neotrop- 

 ical-tertiary flora extended from the tropics as far 

 North as at least 49° latitude in the West and 37° 

 latitude in the East. With the drying and chilling of 

 the continent that began in late Oligocene and Mio- 

 cene, this flora retreated southward and eastward to 

 the localities in which it is found today. 



Arcto-tertiary flora 



This flora completely encircled the North Pole, 

 except for the Atlantic Ocean, with little variation 

 in composition or character. Plants migrated freely 

 across the Bering land bridge between North Amer- 

 ica, Asia, and Europe. Probably no land bridge ex- 

 isted directly between North America and Europe 

 during the Tertiary (Lindroth 1957). On the Arctic 

 islands and in northern Siberia, the flora reached 

 North to within 8° latitude of the North Pole. How 

 these species tolerated the long seasons of darkness 

 is a problem ; possibly, daily photoperiods, as well as 

 temperature, were different then than now. An eco- 

 tone with the Neotropical-tertiary flora began at 57° 

 latitude on the Pacific coast and 51° in southern 

 British Columbia, Alberta, and Saskatchewan. In 

 Asia, an ecotone began at 42° latitude in Manchuria ; 

 in central Europe, at 48°-50°. 



Much of the present similarity between eastern 

 North American and Eurasian floras and faunas may 

 be traced to the continuous and extensive distribution 

 of the Arcto-tertiary flora during the early Tertiary. 

 Types that occur prominently in eastern North 



America and eastern Asia, for instance, are, among 

 the plants, tuliptrees, magnolias, sweetgums, sassa- 

 frasses, witch-hazels, and partridge-berries ; among 

 the animals, paddlefish, alligators, fresh-water tur- 

 tles, lizards {Leiolopisma, Eutneces). snakes (Na- 

 trix, Opheodrys. Elaphc, Agkistrodon) , hellbender 

 (Schmidt 1946); as well as various birds and mam- 

 mals. 



The chief differentiation of the Arcto-tertiary 

 flora was latitudinal into boreal and temperate units. 

 The boreal unit contained such trees and shrubs as 

 the Mctaseqiioia, baldcypress, pines, spruces, willows, 

 birches, and hazels. The temperate unit included 

 maples, alders, birches, hornbeams, chestnuts, dog- 

 woods, hawthorns, beeches, ashes, walnuts, pines, 

 sycamores, poplars, oaks, willows, baldcypress, bass- 

 woods, elms, and Mctaseqiioia. Although there was 

 some mixture, deciduous species predominated in the 

 temperate unit : the boreal unit contained relatively 

 more coniferous species. These same latitudinal rela- 

 tions obtain at the present time. The climate of the 

 temperate unit was probably humid, with summer 

 rainfall, and with moderate temperatures not regu- 

 larly falling below freezing (Chaney 1948). 



With the elevation of the western mountains in 

 the Miocene and Pliocene and the drying and cooling 

 of the climate, the Arcto-tertiary flora retreated from 

 the far North, and the American portion lost its con- 

 tact with Asia. In North America, the main move- 

 ment of the temperate unit was to the South and 

 East, but a secondary movement of broad-leaved 

 evergreen and deciduous trees and shrubs proceeded 

 southward along the moister mountain slopes, val- 

 leys, and Pacific coast. Beech, basswood, elm, and 

 hornbeam disappeared from the West, probably be- 

 cause of the lack of summer rain there, but were 

 prominent in the movement to the Southeast. Meta- 

 sequoia. on the other hand, became limited to the 

 Pacific coast in North America, and evolved into 

 our present day redwood forests. Living Mctase- 

 qiioia are still to be found in central China. The 

 boreal unit followed closely behind the temperate 

 unit ; and in the higher elevations of the mountains, 

 it extended as far southward as the temperate. There 

 were changes in the taxonomic composition of the 

 Tertiary forest during this long period. Some genera 

 were lost entirely, some new ones were added, others 

 were changed by evolution ; but all these changes 

 were conservative, and the present-day mixed meso- 

 phytic forests of the Cumberland Mountains of east- 

 ern Tennessee have nearly the same composition and 

 appearance as did the ancient forests. A related 

 Antarctic-tertiary flora, derived from the Cretaceous 

 flora and containing both conifers and deciduous 

 trees, occurred in the Southern Hemisphere, but its 

 history during the Tertiary was entirely independent 

 of the Arcto-tertiary flora. 



282 Geographic distribution of communities 



