Ordovician spore 'thalli' and the evolution of the plant sporophyte

Cryptospores from the Dapingian-Darriwilian Kanosh Shale at Fossil Mountain, Utah, USA, occur as tetrads, dyads, irregular clusters, or planar sheets of spore dyads. These spore 'thalli' are placed into the new taxon Grododowon orthogonalis gen. et sp. nov. based on the nature of division patterning and gross overall shape. The antithetic hypothesis of embryophyte origins dictates that spores evolved first and that the vegetative sporophyte evolved later via mitotic cell divisions that preceded meiosis and spore formation. We interpret the planar spore sheets of Grododowon gen. nov. to have formed via the co-option of a prior vegetative gametophytic developmental pattern which was expressed in the zygote, resulting in a two-dimensional, thalloid bauplan. However, the ploidy of the resultant ` spores' as haploid is necessarily conjectural, and this pattern of growth is clearly not the ancestral condition in the streptophyte lineage that gave rise to the first axial plant sporophyte.