A Mid Devonian seed-megaspore from East Greenland and the origin of the seed plants

TL;DR: It is demonstrated that S. allenii was contained within a sporangium exhibiting extreme anisospory, which is interpreted here as functionally bisexual, which provides a more likely mechanism for seed plant origin than the archaeopteridalean sporangia reduction model.

Abstract: A new species of late Mid Devonian seed-megaspore from East Greenland is described and named as Spermasporites allenii. The formerly monotypic genus Spermasporites is emended to accommodate this new species. Rare specimens of S. allenii, with near complete sporangial contents, occur together with specimens showing proximally adhering microspores. These demonstrate that S. allenii was contained within a sporangium exhibiting extreme anisospory, which is interpreted here as functionally bisexual. This is a key element in understanding its reproductive function. The megaspore was shed along with some attached microspores giving it potential for both cross- and self-fertilization. This confers the advantages of heterospory without recourse to separate micro- and megasporangia. This provides a more likely mechanism for seed plant origin than the archaeopteridalean sporangium reduction model. Ultrastructural study of the megaspore wall confirms the progymnosperm affinity of S. allenii. Comparable forms include the Frasnian Spermasporites devonicus and the Famennian seed-megaspores isolated from Archaeosperma arnoldii.