PLANTS OF THE PAST 



■31 



[Chap. LII 



The cycad plants were either staminate or carpellate, while the Ben- 

 nettitales had both staminate and carpellate, or even mixed cones on 

 the same plant. Both of these plant assemblages seem to have been 

 derived from the seed ferns, and to have been the forerunners of the 

 modern gymnosperms and angiosperms. 



The land plants of the early Cretaceous period in America were 

 dominated by conifers, ferns, and cycads. During this period the land 

 surface became lowered and much of it was submerged for long inter- 

 vals. The sedimentary rocks formed at this time contain a large array 



Fig. 364. Leaf prints in rocks of Upper Cretaceous age in Alaska. Represented 

 here are (a) dogwood, (b) alder, (c-d) two species of Gingko, (e) redwood, and 

 (f) oak. From Professional Paper No. 159. United States Geological Survey. 



of fossil angiospemis with genera similar or identical to those of present- 

 day vegetation (Fig. 364). Apparently angiosperms had been evolving 

 on the earth for many thousands of years previous to this time. Ferns 

 and cycads declined rapidly; but the conifers remained abundant, with 

 species belonging to genera now extinct, but also including species of 

 pine and Sequoia. 



The amazing part of the upper Cretaceous fossil record is the rapid 

 diversification of the angiosperms and their spread over all the northern 

 hemisphere, some even reaching Africa and South America. In the 

 fossil floras studied there is abundant evidence of the presence of 

 familiar genera such as Magnolia (10 species), Liriodendron (5 sp.), 



