414 MESEOSERE AND PALEOSERE. 



themselves. This is indicated by Wieland (J. c, 52), in connection with Cyca- 

 della, when he says that "the ramentum borne by the peduncles and young 

 strobUi droops down over the surface of the trunk. The general appearance 

 may have been not unlike that of an 'old man' cactus, and, taken with the 

 small size of this group, may mean that the several species referred to grew 

 in some drier and less-favored situation than did such huge trunks as are 

 found in the eastern and southern Black Hills region." A similar suggestion 

 IS contained in the statement that" Zamia floridana is often very abundant in 

 the great reaches of pine woods of the southern ha]f of Florida. In its habitat 

 this plant closely parallels the manner in which many widely distributed cyca- 

 dean species of the Mesozoic grew in open dells of the great coniferous forests 

 of that age." (192) 



Finally, Wieland's account (1. c, 23-25) of the deposition of the cycadean 

 beds furnishes a suggestive picture of the ecological relations at that time: 



"In the Black Hills Rim, there are two clearly distinct cycad-bearing 

 horizons. The lower of these horizons is doubtless equivalent to that of the 

 Freezeout Hills of Carbon County, Wyoming, and lies near the base of the 

 shales and sandstones of the freshwater Jura or Atlantosaurus beds of Mai;sh. 

 So far as yet determined, the plants of the Atlantosaurus beds, includiug large 

 logs and cycads, were brought by streams into the oxbows and estuaries about 

 a large, and as has been commonly supposed, a deep freshwater lake (23). 

 In some cases, the trunks are embedded in the sandstone layer as interspersed 

 with clay seams, but actually rest on the heavy clay layer beneath. Evidently 

 this clay formed the bottom of a freshwater lake, along the shores of which 

 patches of cycads and great groves of Araucarias were prominent in the forest 

 facies; and with some change in estuarine conditions the clay bottom was in 

 some places thickly strewn with cycad trunks and a few Araucarian logs, in 

 others with numerous logs and occasional cycads which were quickly covered 

 over by an inrush of sand, carrying with it further scattering cycads and a few 

 tree trunks, all of which were doubtless early siUcified (24). In the Black 

 Hawk locality, 60 miles northeast of Minnekahta, the conditions are very 

 much the same as at Mionekahta, except that there is less clay in and near 

 the cycad horizon. The conditions of siUcification in the Freezeout Hills of 

 central Wyoming varied but little from those of the lower cycad-bearing 

 horizon of the Black Hills. This much is certain from the great similarity in 

 general surroundings, as evidenced by both the character of the strata in which 

 the cycads are embedded and the other fossils present." 



Comanchean succession. — ^WhUe the Comanchean was ushered in by the 

 general cooling which occurred at the close of the Jiurassic, its climate was 

 probably little cooler than that of the latter. It is generally agreed that it 

 was warm temperate in character, and without great diversity. Elms, oaks, 

 magnolias, maples, etc., grew as far north as 68° to 72° in Greenland and Alaska. 

 Beyond this warm temperate zone must have existed boreal zones, for the fossil 

 trees of Spitzbergen at 75° to 80° N. showed pronounced annual rings, while 

 EiKopean fossils generally afford evidence of the existence of zones. Thus, 

 the differentiation of climate must have been rather greater than during the 

 Jurassic, and there must have been a corresponding division of vegetation 

 into climax zones and alternes. The dominant cycads and conifers continued 

 practically throughout the Comanchean, and their climax and serai relations 

 must have been almost identical with those already noted for the Jurassic. 



