FIG. 21-1 North America dur- 

 ing the early Eocene, showing 

 the configuration of the con- 

 tinent (tinted area, superimposed 

 on an outline of the continent at 

 present) (Schuchert 1955), and 

 floral units (from information 

 given by Chaney 1947). The 

 Madro-tertiary flora was begin- 

 ning to differentiate in small 

 scattered areas within the general 

 area Indicated (Axelrod 1958). 



Madro-tertiary flora 



The Madro-tertiary flora had its center of 

 origin on the Me.xican plateau in the region of the 

 Sierra Madre, perhaps beginning in the Eocene in 

 scattered dry sites on the lee sides or rain-shadows 

 of high ridges and mountains. Its history previous 

 to the Miocene is obscure. The flora contained a 

 variety of small trees, shrubs, and probably some 

 grasses, although the fossil record of grasses is poor. 

 These species seem to have been derived principally 

 from the Neotropical-tertiary flora in response to 

 increasingly arid environments (Axelrod 1958). 



Minor elements of this flora extended into the Great 

 Basin area, but its main movement northward oc- 

 curred in the Miocene and Pliocene. During the 

 latter epochs, the flora came to occupy large areas in 

 southern California, the Great Basin, and the Great 

 Plains, areas which were being vacated by the other 

 two Tertiary floras because of the increasing aridity. 

 Derived in large part from this flora are the present- 

 day communities of woodland, chaparral, sagebrush, 

 subtropical scrub (thorn forest), desert, and arid 

 grassland. These communities are relatively young, 

 as distinct entities ; the North American desert biome, 

 for instance, is probably not older than Upper Plio- 

 cene (Axelrod 1950). 



Paleo-ecology 283 



