144 



THE SPECIES PROBLEM WITH FOSSIL ANIMALS 



currents were effective in moving sediment. Figure 11 illustrates 

 the formation of sedimentary and paleontological hiatuses accord- 

 ing to this principle. 



Mere deposition of sedimentary layers does not safeguard their 

 preservation since large volumes of such rocks are destroyed by 

 uplift and erosion. Even if the sediments escape this fate, it does 

 not guarantee that even a small sample of skeleton-bearing or- 

 ganisms present in the original living community will become 

 collectable fossils. In order to be preserved a skeleton must escape 



Region A 



Region B 



t 



E 



, i ■ ,. ii 1 1 1 1 . 



Space 



Fig. 12. The effect of facies migration on the fossil record of evolving 

 lineages. Well-defined transient species occnr at horizons 1 and 3 in region 

 A. Different transient species occur at horizons 2 and 4 in region B. Even 

 if deposition and preservation have been continuous in this area, inaccessi- 

 bility of the rocks lying between regions A and B makes the available record 

 of this lineage incomplete. (After Newell, 1956.) 



chemical, biochemical, and mechanical destruction by agencies 

 acting before, during, and after burial. 



Figure 12 illustrates the combined effect of migration and in- 

 accessibility in reducing the number of collectable fossils. Stip- 

 pled and unstippled areas on this diagram represent three con- 

 trasting environments (as well as the geologic record of those 

 environments, or facies) which have continuously occupied the 

 area. Three environments, each with its associated organic com- 

 munity, are represented at any given instant. Under normal cir- 

 cumstances geological conditions responsible for the localization 

 oi physical environments change, and the associated communities 

 sln'll accordingly. Now consider the effect of this migration on the 



