Morpho-evolutionary biohorizon stratigraphy and cladistics in saccate pollen through Gondwana Sequence of India

Abstract

The morphographic characters of fossil saccate pollen and their evolutionary sequence through the span of Gondwana formations play a vital role in the biohorizon stratigraphy. Based on evolutionary changes in the morphology of the pollen group, their FADs (First Appearance Datums) and LADs (Last Appearance Datum) and the cluster levels, eleven biohorizons and ten inter-biohorizon Zones (Interval-zones) have been identified. The model of alliance among the prime morphographies has been acquired through simple character state analysis. By extrapolation of lineages, four unique monosaccate and one simple disaccate organizations could be identified to have their origin in the early Upper Carboniferous stock of Australia, which, in turn, were related with the Devonian progymnospermopsida complex. The five outgroup stocks continued as such into the late Early Asselian Talchir Formation of India and sprouted in the subsequent Permian time. A gap in the sequence has been identified during Upper Carboniferous of Australia and the lowermost Permian in India. The sixth major lineage of striate pollen branched off from the simple disaccate pollen lineage to appear in the Late Asselian. Cladistic analysis supports the applicability of the proposed biohorizons as important stratigraphic parameter. The congruence of derived and ancestral states of internally compatible and persimonically harmonious characters in pollen organizations has been used in drawing relationship trees. Thus, the cladograms and stralograms (nested diagrams of stratigraphic occurrences) together depict the maximum diversification at the lower Upper Talchir and the P/Tr boundary. The major extinction of monosaccates at the Lower Permian-Upper Permian boundary and that of the striate-disaccates in the basal Triassic have also been brought to light. The reasons of such a behaviour is attributed to environmental stresses caused by massive glaciations during Early Permian Talchir Formation, climatic changes towards warmer-drier situation at Lower/Upper Permian passage and global cooling and regression at the permo-Triassic level. Pollen features, like striations, taeniae, girdling monosaccus, clefts and remnant of germinal marks, etc. of the Permian and Triassic have perished completely by the end of Triassic. Most of the Jurassic palyno-assemblages contain the fag-end organizations of simple disaccate lineages; the experimentation seems to have attained a more simple state in the morphography of saccate pollen which continued thereafter.