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In search of the earliest seed plants
18/12/2004 00:00 | Durée 00:10:40

Intervenants : John Marshall , School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, UK .  

The appearance of seed plants in the Late Devonian is a critical episode in earth history. How seeds evolved is still an open question. Recently, in Palaeontology, Marshall and Hemsley described Spermasporites allenii from a Middle Devonian lake on Ella Ø, East Greenland. S. allenii is a seed megaspore bearing at its apex not only three smaller aborted spores of the original tetrad, but also a cluster of microspores. This suggested a new path to seed evolution, where the seed megaspore is most likely to be fertilized by microspores from within the same sporangium but also allows for fertilization by microspores from another sporangium or individual. In order to determine the taxonomic affinity of the seed-meagaspore bearing plant, CMB, JEAM, and AH mounted a NERC-funded expedition to Ella Ø (see poster) under the umbrella of CASP. Excavation of the layers in which the seed megaspores were concentrated yielded numerous partial specimens of the plant. Single megaspores are snugly contained in sporangia which are inserted erectly, singly and terminally on the end of an isodichotomous branching system. Association suggests that these were carried laterally on small sparsely dichotomously and trichotomously branching naked stems. This suggests morphology more similar to that of latest Devonian seed plants than to that of the contemporaneous advanced progymnosperm Archaeopteris/Svalbardia found elsewhere in the same lake, which may otherwise be an obvious candidate for ancestry of seed plants.