262 



Origin of Biomes and Succession 



116). Many of the abundant dominant genera of freshwater stream 

 communities trace back in the fossil record to the Cretaceous ( Ross, 

 1956), and those of marine communities to the Paleozoic (Shrock 

 and Twenhoffel, 1953). 



(a) 



(b) 



(c) 



(d: 



;e) 



® 



(f) 



(h) 



()) 



(k) 



(1) 



:m) 



(n) 



5^ 



Fig. 116. Pollen grains of Upper Cretaceous plants in Minnesota and of 

 Recent species to which they bear a striking resemblance, (a), Cretaceous spe- 

 cies resembling (b), Menispcnniitn canadense. (d), Cretaceous species re- 

 sembling (c) , Platanus occident(dis. (e), (g). Cretaceous species resembling (f), 

 Lithocarpus densiflora. (i), Cretaceous species resembling (h), Hamamelis 

 vernalis. (j), (1), Cretaceous species resembling (k), Quercus laiirifolia. (m), 

 Cretaceous species resembling (n), Magnolia virginiana. (Photographs cour- 

 tesy of R. L. Pierce.) 



Marine organic reefs, because of the wonderfully rich fossil 

 record of their constituent species, afford an excellent example of 

 community change through time. As in the past, circulation of 

 warm marine waters across shallow, wave-agitated areas commonly 

 produces ecological conditions ideally suited to flourishing ben- 

 thonic communities of shelled organisms. Existing reefs have an 

 abundant fauna and flora which continuously contributes to the 

 reef deposits in the form of calcareous shells and skeletons. Wave 



