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Geological age of the Ptilophyllum flora; a critical reassessment


Geological age of the Ptilophyllum flora; a critical reassessment



Memoirs of the Geological Survey of India 46: 245-256



ISSN/ISBN: 0369-0024

Ptilophyllum Flora is one of the three major floras that populated the Indian sub-continent during the Early Permian-Early Cretaceous time interval. Till recently this set of floras was also known as the Gondwana Flora. On the basis of stratigraphy and palaeobotany, it has now been shown that the Indian Gondwana did not extend beyond Rhaetian. The Ptilophyllum Flora therefore, is now excluded from the purview of Gondwana. The sediments containing elements of the Ptilophyllum Flora are distributed over most of the peninsula, particularly in the Rajmahal, South Rewa, Satpura, Jaisalmer, Kutch, Saurashtra, Palar, Cauvery, Krishna-Godavari, Pranhita and Mahanadi basins. Besides Ptilophyllum which is the dominant and characteristic element, megafossils representing pteridophytic families Polypodiaceae, Marattiaceae, Osmundacea, Gleicheniaceae, Matoniaceae, Dipteridaceae and Weichseliaceae, members of the Bennettitales, Cycadales, Coniferales, Pentoxylales and some Ginkgoales have been recorded in this flora. The dominant palynofossils are Araucariacites and Callialasporites; other important elements are Impardecispora, Foraminisporis, Contignisporites, Cicatricosisporites, Appendicisporites, Densoisporites, Aequitriradites, Crybelosporites, Cooksonites, Coronatispora, Microcachryidites Podosporites, and Podocarpidites. While the palynological data unequivocally indicate an Early Cretaceous age for the Ptilophyllum Flora in India, except for that recorded from some parts of the marine sediments of Jaisalmer and Kutch basins, the megafossil data have been interpreted to indicate an Early Jurassic to Early Cretaceous age for this flora. This has been mostly due to comparison with the distant Yorkshire Flora of England, and on the assumption that the taxa Onychiopsis and Weichselia are the only indicators of an Early Cretaceous age. Now, these elements represent a particular ecological niche and hence their absence only indicates a different environment, and not a relatively older age. Very often the megaflora are poor in composition, e.g., that from the Hartala, Dubrajpur and Kota Formations (South Rewa Gondwana, Rajmahal and Pranhita-Godavari basins respectively) and the taxa are fairly long ranging, and thus should not be considered for age determination. It is therefore desirable that the age of the sedimentary sequence containing elements of the Ptilophyllum Flora be decided not only on the basis of contained plant megafossils and palynofossils, but the data obtained from animal fossils and absolute age determinations be also taken into consideration. A comprehensive analysis of all the available data on these parameters indicates an Early Cretaceous age, and shows that there, in fact, is no discrepancy in age inferred from mega- and micro-floral and faunal evidences.

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