411 

 THE MESEOSERE. ^^^ 



tocene, they should prove to be cUseral costases, but investigation alone can 



^Triassic^succession— The scarcity of plants inTriassic formations until near 

 its close in the Rh^tic (Schuchert, 1914 : 280) indicates that the major portion 

 of this period was essentiaUy a continuation of the Permian, both as to chmate 

 and vegetation (ChamberUn and Salisbury, 1906: 3: 38). The latter part naarks 

 the return to warmer conditions, such as characterized the Jurassic, with tne 

 result that the upper Triassic and the Jurassic are sometunes to be distin- 

 guished only with the greatest difficulty. In both it seems plausible that 

 colder polar zones and arid interior regioiis contmued to exist, though doubt- 

 less more restricted than during the Permian. 



The serai development during the earUer Triassic must have been essentia,Uy 

 that sketched for the Permian. There is no evidence of cliseres since glaciation 

 was absent, but it is quite possible that there were subordinate movements of 

 the zones about the cold or arid regions. With the beginning of the Rhsetic, 

 the floral population had become greatly modified, changing the climaxes and 

 the course of development as a result. 



In the later Triassic (Knowlton, 1910 : 106)— 



"The dominant types of the Paleozoic have largely disappeared. The 

 Lepidodendrffi, Sigillarise, Calamites, Cordaites, Sphenophyllae, and Cycado- 

 filices, so far as ascertained have all gone, as well as a number of important 

 genera of ferns — Chdlanthites, Mariopteris, Megahpteris, etc. The most 

 notable survival from the Paleozoic is the so-called Glossopteris flora, which 

 has been found with a few associated forms in Rhsetic rocks. 



"The Triassic flora consists essentially of equisetums, ferns, cycads and 

 conifers of many genera. A few forms such as Ginkgo, CladophleMs, Thinnfeldia, 

 etc., had a small beginning in the Paleozoic and expanded in the Mesozoic 

 into large groups. But most of the flora is of distinctly Mesozoic and northern 

 origin. 



"It has often been said that the plants of the Triassic are depauperate and 

 pinched in aspect, indicating unfavorable climatic conditions. The paleo- 

 botanical facts do not altogether bear this out. In North Carolina, Virginia, 

 and Arizona, there are trunks of trees preserved, some of which are 8 feet in 

 diameter and at least 120 feet long, while himdreds are from 2 to 4 feet in 

 diameter. Many of the ferns are of large size, indicating luxuriant growth, 

 while Equisetum stems 4 to 5 inches in diameter are only approached by a 

 single living South American species. The cycads are not more depauperate 

 than those of subsequent horizons, nor do they compare unfavorably with the 

 living representatives. 



"The complete, or nearly complete, absence of rings in the tree trunks indi- 

 cates that there were no, or but slight, seasonal changes due to alternations of 

 hot and cold or wet and dry periods. The accumulations of coal — in the 

 Virginia area aggregating 30 to 40 feet in thickness — vindicate long-continued 

 swamp or marsh conditions, while the presence of ferns, some of them tree- 

 ferns, indicates on the whole a moist, warm, probably at least subtropical 

 climate." 



There appears to be good warrant for assuming the existence of at least 

 three great climaxes during the Triassic, namely, coniferous, cycadean, and 

 the Glossopteris flora, the latter not foimd in North America. In all proba- 

 bility there were mixed climaxes as well, and the assumption that the con^erous 

 and cycadean masses were differentiated into climaxes is at least plausible. 



