Axelrod: The Pliocene Verdi Flora of Western Nevada 121 



The floras having the largest numbers of species in common with the Verdi are 

 all characterized by a prominent Sierra-Cascade Component. Floras with few or 

 no species representing this group, which forms an important part of the Verdi 

 flora, even though of the same age, obviously could not be expected to have many 

 species in common with it. This explains why the Mio-Pliocene Remington Hill 

 (Condit, 1944a) and Table Mountain (Condit, 1944&) floras of the west Sierran 

 slope, though situated at no greater distance from Verdi than the Mio-Pliocene 

 floras of west-central Nevada, have so little in common with the Verdi: they repre- 

 sent different forests. Thus, as we shall see below, even though the Middle Pliocene 

 Deschutes, Alturas, Truckee, Mulholland, Mount Eden, and Oakdale floras each 

 have only 2 to 3 species in common with the Verdi, all of them are of the same 

 age ; there are few species common to these floras simply because they repre- 

 sent different types of vegetation. It is apparent that comparisons on a strictly 

 numerical basis may be quite misleading in terms of age assignment, particularly 

 in Late Tertiary time when localized climates had a profound effect on the dis- 

 tribution and composition of vegetation. 



As discussed in detail elsewhere (Axelrod, 1938, 1944c; 1948 ; 1949&; 1956), age 

 determination of Late Tertiary floras must be based on lines of evidence other than 

 the strictly stratigraphic ranges of individual species. By contrast, we must be con- 

 cerned with the evolution of floras in time and space. In the case of the Verdi flora, 

 data for age evaluation are provided by an analysis of (1) its geographic relations, 

 (2) its climatic implications, and (3) its cliseral aspects, all in terms of the tem- 

 poral succession of vegetation in this region. Similar floras in other areas are 

 homotaxial, not synchronous. 



Geographic occurrence. — A close relation exists between the age of a Tertiary 

 flora, the type of vegetation it represents, and the proximity of regions in which 

 living species similar to the fossil are found. These relations for western Nevada, 

 as outlined in table 7, show that the flora is not Early Tertiary, for there are no 

 plants in it that have relatives in tropical or subtropical regions. Nor can it be 

 Middle Tertiary, for floras of that age in this region are dominated by deciduous 

 hardwoods that are represented by close counterparts in eastern North America 

 and eastern Asia. The flora must also be younger than the Mio-Pliocene transition. 

 The Mio-Pliocene floras of the Carson Sink area 50 miles east have a prominent 

 forest group that has persisted in the Sierra Nevada, as well as species related to 

 those in the temperate parts of eastern North America and eastern Asia (Axelrod, 

 1956) . A flora of similar age in the Verdi area should, on the basis of its geographic 

 position, be more humid than the Carson Sink floras, and hence contain a larger 

 number of species allied to those now in the Sierra Nevada and in the eastern parts 

 of the northern continents. It is apparent that the Verdi must be post -Mio-Pliocene 

 (Early Clarendonian). 



Relatively little is known of the younger floras of this region. The Late Clar- 

 endonian Esmeralda flora has a few species with modern relatives in more remote 

 areas, as in the southwestern United States, but most occur on the lower slopes of 

 the southern Sierra Nevada and in southern California. Many of the species in the 

 small Middle Pliocene (Hemphillian) Truckee floras also have their closest rela- 

 tives in southern California and at low elevations on the west Sierran slope. These 



