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AMERICAN JOURNAL OF BOTANY 
[Vol. 8 
Early Cretaceous Climate of the Cycadeoid Habitat 
In no case is it necessary to assume a Saharan heat, or even a markedly 
warm temperate climate, for the North American cycadeoid belt. As 
above pointed out, Cycadeoidea had the habitus to resist dry cold, while 
the associated conifer forests might well have been as able to withstand 
zero temperatures [-15° to -20° C] as the Chilo-Argentine Araucaria 
imhricata forest with its lower limit undergrowth of monocarpic bamboos. 
Nor is it improbable that a closer scanning of the contemporaneous floras 
of the Lower Cretaceous "Rim" of the Black Hills as recorded from both 
the Morrison and Lakota, and yet destined to great extension, will compel 
much revision of our views of all-tropic Mesozoic climate. Even a brief 
notice of what is already known here may be illuminating. 
In the 19th Annual Report of the U. S. Geological Survey, Fontaine 
gives an account of the plants below the Dakota — that is from the Lower 
Cretaceous ''Rim" of the Black Hills, immediately in, above, or slightly 
below the great series of cycadeoids from Minnekahta and the Piedmont- 
Black Hawk localities. Of dicotyls there are four, called Ulmiphyllum, 
Ficophyllum, Quercophyllum, and Sapindopsis. Two of these occur in 
the Potomac of Maryland, and all are small-leaf types. It is only by a kind 
of elaborate argument carrying these generalized types, only known within 
family limits, into other zones, that they can be made to appear to have lived 
solely in tropic climates. And the same is true of a few species each of 
more or less closely associated ginkgos, conifers, and araucaroids — all small- 
leaf species that by themselves would easily fit into dry, to even cool climates. 
While three small-leaf plants of presumably cycadeoid affinity find their 
nearest relatives in Oregon and Greenland. Once more it is only by cumula- 
tion of disconnected inferences based on series which require much further 
study that these plants can be brought into the all-tropic scheme. Of the 
fern assemblage — all small of leaf — little more is known than that according 
to previous interpretation they easily fit the tropic category because re- 
sembling their Jurassic antecedents. 
It is early to attempt a revised, well founded estimate of the full climatic 
significance of the Lakotan plants. This would in fact involve a further 
study of all Lower Cretaceous floras. Though it is evident that too much 
weight has been given to the superficial resemblances in extinct floras. 
These must always have a certain prevailing cast due to the average course 
of geologic change continental in magnitude. True, if a widespread tropic 
plant facies marks the early Mesozoic, the identical or closely related 
species must have a wider north-south range than later on; hut always, the 
similar elements of given floras, which are a reasonably sure indication of 
synchroneity, are only the unsafe criteria of climatic equivalence. 
