Thursday, March 10, 2016

Just What Were the Noeggerathiales Plants?

Paleoecology of Noeggerathiales, an enigmatic, extinct plant group of Carboniferous and Permian times


Pfefferkorn et al


Noeggerathiales are late Paleozoic plants of uncertain systematic position. They may either be late survivors of the Devonian progymnosperms or represent a sister group to ferns and sphenopsids. Noeggerathiales carry cycad-like or fern-like leaves on small, unbranched stems. They produce microspores and megaspores in cone-like structures in which sporangia are attached adaxially to sporophylls. Noeggerathiales are rare in Carboniferous and Permian fossil floras in the Euramerican floral realm (Europe and North America). Most floras do not even contain a single specimen. In the Cathaysian floral realm (China) they are more common and some fossil floras contain Noeggerathiales in substantial or even large numbers. The paleoenvironmental and taphonomic interpretations of these occurrences allow us to deduce the paleoecological preferences of Noeggerathiales. Noeggerathiales were tropical plants as can be derived from the pattern of their occurrence and the fact that tuft-trees can survive only in frost free climates. The single growing point of a tuft tree would be endangered by frost and the investment in the large leaves is so high that they have to function for at least two to three years. In Euramerica Noeggerathiales are not present in wet tropical lowland floras on clastic substrates or on most peat-deposits. However, they occur at the margin of smaller basins and on peat that was fertilized by volcanic ash. In North America several occurrences of Noeggerathialean plants are known from the base of sedimentary sections, an indication that these Noeggerathiales were part of the extrabasinal or hinterland floras. They were present before sedimentation began and could therefore be preserved by the earliest sedimentation events. Hinterland or extrabasinal floras often grew in environments that experienced a wet–dry seasonality. In China Noeggerathiales occur in species-rich lowland assemblages preserved in sediments of wet lowlands indicating that these forms were adapted to wetter conditions. The pattern indicates a plant group that was adapted to tropical climate and needed nutrient rich soil, but could withstand wet–dry seasonality. By Permian times Noeggerathiales living on the North and South China microcontinents had adapted to ever-wet climates but still needed nutrient-rich environments.

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