Because of the widespread prevalence of continental deposits during the Permian-Triassic transition, the boundary can often be established only on the basis of assemblages of land animals and plants. It has been the practice of many authorities to correlate the Paleophyte-Mesophyte plant boundary with that of the Permian-Triassic, as in Siberia, India, Korea and other regions. In Western Europe, on the other hand, the Paleophyte-Mesophyte floral change has been placed at the begining of the Late Permian. The author believes that neither of these correlations is valid, for the Zechstein flora of Western Europe is, in reality, not typically mesophytic in aspect. The main elements of this flora appeared in the Early Permian and many had disappeared before the beginning of the Triassic. In places other than Western Europe, where plant-bearing beds have been directly correlated with type sections, the change from paleophytic to mesophytic does not coincide with the Permian-Triassic boundary, occurring at different stratigraphic levels in different parts of the world. For example, several mesophytic elements, sometimes dominating whole communities, characterize the Kungurian of the Fore-Urals region and the whole Permian section in Lower Asia. Again, the Upper Tatarian flora (especially that of the Vjatsky Beds) in the type sections of the Russian Platform is typically mesophytic in appearance, as has been recently shown by the author. It consists of peltaspermaceous pteridosperms, conifers and ginkgophytes, the only paleophytic elements being protosphagnaceous mosses. According to Sadovnokov, the Early Korvuntchana flora of the Tunguss Basin is typically mesophytic, and appears to be of Late Tatarian age, consisting of ferns (dominant group), peltaspermaceous pteridosperms, ginkgophytes and cycadophytes. In India the relationship is reversed, with a typical paleophytic flora, rich in glossopterids, characterizing the Lystrosaurus zone of the Lower Triassic. Until quite recently this flora was regarded as Permian, based essentially upon its paleophytic appearance.

These observations, then, strongly indicate that the replacement of paleophytic floras by mesophytic floras was not synchronous in various regions of the world and therefore can not be used to accurately delimit the systemic boundary. This gradual change from paleophytic to mesophytic flora in fairly complete sections, such as the south Fore-Urals, is similar to changes in marine faunas described previously by Ruzhentsev

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and co-workers and ascribed by them to a vast regression at the end of the Permian. The regressive phase was probably concomitant with great volcanic activity in Northern Asia, as well as the removal of barriers which influenced plant migration and floral development during this time. However, the gradual and nonsynchronous nature of the floral changes across the boundary suggests that the rearrangement of barriers did not have a catastrophic effect on plant life.

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