Dilemma of late Palaeozoic mixed floras in Gondwana

摘要

During Carboniferous and Permian Periods the plant fossil assemblages of northern and southern hemispheres were distributed in four floral provinces and each flora had its own characteristics. Pteridosperms were dominant in the Euramerian floras of Europe and America; cordaitales and equisetales along with Angaridium/. Angaropteris represent the Angara flora of Russia, Russian federation and Far East provinces; Gigantopteris flora characterizes the Cathaysian province of China, Japan and south-east Asian countries; and Glossopteris flora are present in Southern Hemispheric land masses of Gondwana which includes India, Australia, Africa, South America and Antarctica. Well defined floral provinces quite often show the intermixing of different floristic elements exemplifying the presence of mixed floras. The occurrence of mixed floras suggests that they have two modes of distribution. One distribution pattern shows mixed floras along the marginal basins of the Tethyan region during the Middle and Late Permian. Another pattern is evident in the Gondwana floras where Glossopteris floras contain a number of extra-Gondwanic elements. Likewise, some Euramerian, Cathaysian and Angaran floras show the possible presence of glossopterid related elements having comparative morphological features as part of the flora. Mixed floral elements of northern and southern floras have been examined in detail and it is suggested that in all probability the so-called elements of the mixed floras had their ancestry or affiliation with Carboniferous floras, and during the early Permian they duped due to severity of climatic conditions but developed again during Late Permian when climate became more hospitable for their development. The Cathaysian and Euramerian forms discovered with Gondwanan forms in the mixed floras of Tethyan region, i.e. Hazro, Saudi Arabia, New Guinea, Oman, Jordan, Kashmir, central Tibet and southern Tibet, belong to Middle and Upper Permian sequences and during this time the warm humid and temperate climate helped to revive the northern hemispheric forms which had their endurance in a relatively similar climate of the Late Carboniferous. The morphological conservatism/flexibility, radiation, relationship and analogous features of mixed floral elements have been examined and attempts are made to sort out their disputed presence in different floral provinces.