MESOZOIC ERA: THE AGE OF REPTILES 565 



The probable relationships of the mammals and other vertebrates 

 are shown in the table (Fig. 525). 



REFERENCES FOR MAMMALS 



Scott, W. B., — A History of Land Mammals in the Western Hemisphere, pp. 642-644. 

 Woodward, A. S., — Vertebrate Paleontology, pp. 246-260. 



Plants 



The vegetation of the Mesozoic is of great interest, since it was 

 during this period of world history that the now dominant types of 

 plants were introduced. Mesozoic plant life, as indeed does the plant 

 life of all geological ages, affords a reliable clue to the climatic and 

 physical conditions which prevailed during the several periods, and 

 incidentally offers, to some degree, an explanation of the striking 

 changes which took place in the animal life. 



In discussing the vegetation of the Mesozoic, a division into Lower 

 and Upper should perhaps be made, because of the introduction of 

 modern plants (angiosperms) in the Lower Cretaceous and the sub- 

 ordination of the typical early Mesozoic plants in the Upper Creta- 

 ceous. 



Owing to considerations, physical and otherwise, concerning which 

 there is not complete agreement, the lower part of the Triassic affords 

 but scant remains, and it is not until we come to the upper part 

 (Rhaetic) that the plant remains can be really dignified as a flora. In 

 North America there are less than 150 species, and the entire Triassic 

 flora of the world probably does not exceed 300 or 400 forms. 



Horsetails. — The horsetails, which entirely replaced the cala- 

 mites of the Carboniferous, do not appear to have differed markedly 

 from those now living, except that they were often of larger size, 

 some having been reported that are from five to eight inches in diam- 

 eter. It is presumed that they formed dense growths, like canebrakes, 

 in or along swamps, marshes, or lakes, as do certain of their living 

 representatives to-day, the largest of which — a South American 

 species — is an inch in diameter and 20 or 30 feet in height. 



Cycads. — Among the most characteristic and abundant plants of 

 the Triassic and Jurassic was the great group of cycads (using the term 

 in the broad sense to include the Bennettitales and Cycadales). They 

 were similar in general appearance to those of the present, but differed 

 in some important characters. Fossil cycad trunks (Fig. 526) are 



