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Abiostratigraphic method based on a quantification of fossil tracheophyte characters - its application to the Lower Devonian Posongchong flora (Yunnan Province, China)

The biostratigraphic method used here is based on a quantification of individual plant characters and not on determination of whole fossils. The method is as follows: (i) each biocharacter found at a given locality receives a score according to whether it is primitive or derived, so that the biocharacter score increases with increasing derivation; (ii) the biostratigraphic coefficients of well-dated localities are calculated, the biostratigraphic coefficient of a given plant locality being the mean of all the biocharacter scores; and (iii) the scores of these well-dated localities are used to build a reference scale to which localities under investigation will be compared. A detailed example is developed for Early Devonian times and the updated quantification table of Targrove (Wales) is presented. An application of the method to the Posongchong flora (Yunnan Province, China), including its quantification table, is also demonstrated. It suggests that the mean age of this flora might late Pragian (Lower Devonian). This is the first direct biostratigraphical dating of the Posongchong flora.