GINKGOALES 21 



D 



5. History and distribution 



The leaves of Ginkgo are so characteristic that they are an unusually 

 trustworthy evidence of the existence of the group during previous 

 periods. Such leaves are found in abundance down to the Coal- 

 measures, and some of them at least must have belonged to Gink- 

 goales, though some of them may have belonged to other grouj)S. 

 In fact, Seward (28) is inclined to think that all of the so-called Gink- 

 goales of the Paleozoic are Cordaitales. The genus Whiltleseya from 

 the Lower Carboniferous, distinguished by its short petioles, seems 

 to be connected with Ginkgo itself by Ginkgodium, a recently described 

 genus from the Lower Oolite of Japan (52), 



The most important fossil leaf genus referred to Ginkgoales is 

 the mesozoic Baiera, which differs from the leaves of Ginkgo in the 

 shortness of the petiole and the repeatedly and deeply dichotomous 

 blades (fig. 214), resulting in narrow, ribbon-like, greatly elongated 

 lobes (14). Such characters are not constant, for even in the existing 

 Ginkgo the blade is quite variable (p. 189). Of great interest are the 

 paleozoic leaves referred to Ginkgo and Baiera, which occur especially 

 in the Permian. In reference to these, the general conclusion seems 

 to be that they cannot be disentangled from the foliage of certain 

 species of Cordaitales and of Cycadoiilicales, and perhaps of Fili- 

 cales. In any event, the evidence is convincing that Ginkgoales were 

 abundant and somewhat diversified during the Mesozoic, their great- 

 est extension occurring during the Jurassic ; and it is altogether prob- 

 able that they existed near the close of the Paleozoic, associated with 

 the Cordaitales and Cycadofilicales. The testimony of the mesozoic 

 leaves is confirmed by the discovery of staminate strobili, ovulate 

 strobili, and seeds. In a staminate strobilus {Antholithus Zeilleri) 

 recently described by Nathorst (49) and associated with Baiera, 

 the microsporophylls are very different in character from those of 

 Ginkgo (p. 193), but this need not exclude the strobilus from Ginkgo- 

 ales. " On the whole, the sum of fossil evidence is of sufficient weight 

 to prove the great antiquity of the gymnospermous family now 

 represented by the maidenhair tree" (27), a group distinctly fore- 

 shadowed in paleozoic vegetation. 



Our chief knowledge of the diversity and wide distribution of 

 Ginkgoales during the Mesozoic has come from the investigations of 



