The Cordaitaceae was widely distributed throughout the Euramerican Flora and Cathaysian Flora in the Carboniferous and Permian of the Late Paleozoic. It consists of three natural genera: Mesoxylon, Shanxioxylon and Pennsylvanioxylon. All of them can be found in the Cathaysian Flora, While in Euramerican Flora only Mesoxylon and Pennsy lvanioxylon were discovered. The isolated organs of Mesosxylon have been identified as different organ genera: Stem: Mesoxylon Scott et Maslen 1910. leaf: Cordaites Unger 1850. male strobilus: Gothania Hirmer 1933. female strobilus: CordaitanthusFeistmantel 1876. ovule: Mitrospermum Arber 1910. pollen: Sullisaccites Millay et Taylor 1974, Felixipollenites Millay et Tayllor 1974. And those of Pennsylvanioxylon are: Stem: Pennsylvanioxylon Vogenellhner 1965. leaf: Cordaites Unger 1850. male and female strobilus: Cordaitanthus Feistmantel 1876. ovule: Cardiocarpus Brongniart 1881. pollen: Florinites sensu strictu Millay et Taylor 1974. The isolated organs of Shanxioxylon are put into the same genera as those of Pennsylvanioxylon respectively, except for the stem( named as Shanxioxylon Tian et Wang 1987). Since its stem, leaf, male strobilus and ovule are quite different from those of Mesoxylon and Pennsylvanioxylon, Shanxioxylon is considered as the third natural genus of the Cordaitaceae, which is found only in the Cathaysian Flora at present. The stem and reproductive organs of Mesoxylon are rather primitive. Its stem is similar to that of pteridophytes. The stem of Pennsylvanioxylon is rather advanced and quite similar to that of modern conifers, but its reproductive organs are quite primitive. The stem of Shanxioxylon lies between Mesoxylon and Pennsylvaniomylon, but its reproductive organs have advanced characteristics. It is supposed that Shanxioxylon is probably a kind of plant with great evolutionary plasticity, from which more advanced cordaites or even conifers might have been derived. In fact, stems of one species of Shanxioxylon are quite advanced. For example, it is of typical sympodial vasculature with sympodia undulate along the longitudinal direction, the same as that of modern conifers. Shanxioxylon is probably a kind of cordaites which possesses a great evolutionary potential and is a transitional type linking primitive cordaites and some primitive conifers. In the Euramerican Flora, the earliest fossil record of cordaites is probably at Tournaisian stage of the Early Carboniferous. The appearence of cordaites, especialy the anatomical features, in this time is not clear. The only known organ with internal amatomical structures is an ovule Mitrospermum bulbosum Long. It is inferred that the cordaites in this time probably belong to Mesoxylon. At the Westphalian stage, cordaites become common, but mainly Mesoxylon,such as M. sutcliffii, M. hultirame etc., in the Lower Coal Measures, England. Later, M. sp. (cf. M. multirame or M. sutcliffii ) also occurred in early Middle Pennsylvanian, America. Pennsylvanioxylon came to appear in this time. In the Middle Pennsylvanian, cordaites in North America reached its evolutionary peak. In addition to Pennsylvanioxylon, more advanced species of Mesoxylon (i. e. M. thompsonii )appeared. At the Stephanian stage(late Later Carboniferous) cordaites began to decrease. In the Permian, cordaites became rare except a few leaf impressions. An evolutionary line of cordaites in the Euramerican Flora is proposed based on the fossil record of the Late Paleozoic: Progymnosperm→primitive Mesoxylon (such as Mitrospermum bulbosum , Mesoxylon sutcliffii etc. )→more advanced Mesoxylon (such as M. thompsonii )→Pennsylvan- ioxylon→extinct. The earliest Cathaysian cordaites were living in the age of Early Carboniferous(Visean Stage). They were all impressions of leaves. The earlist anotomically preserved cordaitean fossils are the stem of Mesoxylon sp. in coal balls of the Hongtuwa Formation(early Late Carboniferous in age), Gansu Province. Lately, in coal balls of the Taiyuan Formation(early Early Permian in age), the advanced cordaites, such as Shanxioxylon and Pennsylvanioxylon, appeared. The fossils of cordaites in China are very abundant throughout the whole Permian beds, so it is convenient for us to study the late evolution of cordaites. Based on the available information, there probably existed two evolutionary lines in the Cathaysian Flora cordaites: 1. Progymnosperm→Mesoxylon→Shanxioxylon→Pennsylvanioxylon→extinct. 2. Progymnosperm→Mesoxylon→Shanxioxylon→extinct or evolved into somenew plants(such as early primitive conifers).
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