316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



of a single sample by following one depth level horizontally across 

 the graph. Interpretation of a complex pollen or spore diagram is 

 facilitated by dividing the sequence into phases or zones, in order to 

 outline further the major features of the sequence. Definition of 

 zones can be based on any feature that seems pertinent, such as a 

 numerically dominant genus, or presence or absence of critical though 

 less abundant microspores. In our example, the zones entered on 

 the extreme left of figure 6 are labeled for convenience by alphabetical 

 symbols and are based on the dominant plant type or genus : T zones 

 for predominance of shrubs and herbs, A zones for spruce (Picea), B 

 for pine (Pinus), and C for hardwoods (here Quercus, oak). 



From one or preferably more than one such fossil samples or se- 

 quences, inferences can be drawn about the type of plants represented, 

 the general climate at the time of deposition, the environment of 

 deposition, and the approximate age of the sample. Examples of 

 such conclusions are discussed later in relation to paleoecological in- 

 terpretation and correlation by pollen and spores. It might be said 

 again, however, that the interpretation must recognize that the pollen 

 and/or spore rain represented in the sample does not define the exact 

 composition of the vegetation adjacent to the site of deposition. 

 Qualitative changes with time shown in pollen or spore sequences are 

 more meaningful than the composition of individual samples. 



RECONSTRUCTION OF PAST ENVIRONMENTS 



For paleoecological purposes, fossil pollen serves as a means of 

 determining the botanical relations of the plants in the assemblage. 

 Having identified the fossil pollen, the pollen analyst, under the as- 

 sumption that plants have not significantly changed their environ- 

 mental tolerances in time, can deduce that an environment like that 

 now required by these plants once existed in the vicinity of the fossil 

 locality. If a modern species is now limited in its distribution by 

 specific factors such as temperature or rainfall, then from a Pleisto- 

 cene occurrence of the plant one can infer rather precisely the cli- 

 mate at the time the plant grew. The validity of such conclusions 

 increases with the number of different plants on which they are based. 

 They are most exact for the Pleistocene and decrease in accuracy 

 with increasing age of the sample. 



Such precise climatic inferences cannot be made from plant as- 

 semblages of Tertiary age, but, because most modern genera (but not 

 species) have existed throughout the Cenozoic along with gradually 

 decreasing numbers of extinct forms, somewhat more general inter- 

 pretations are possible. 



Most of the plant genera of the older Mesozoic and Paleozoic are 

 now extinct. Hence ecological evaluation of these old assemblages is 



