Two cryptospore-bearing land plants from the Lower Devonian (Lochkovian) of the Welsh Borderland

Abstract

Two examples of fragmentary, coalified plant fossils with cellular preservation and in situ spores are described using scanning and transmission electron microscopy (SEM and TEM), from a Lower Devonian (Lochkovian) locality in the Welsh Borderland. A sporophyte which produced dyads of ?Cymbohilates cf. horridus is unique in that stomata are numerous on the supporting axis, and are comparable with those described from contemporary vascular plant remains. The in situ dyads possess a bilayered exospore wall, with an outer exosporal envelope present over the distal faces. A fusiform sporangium, with externally smooth epidermis, contains specimens of the tetrad Velatitetras sp. Each tetrad is encompassed by a laevigate, folded, exosporal envelope of uniform thickness, which contains a layer of regular voids. Spores within the tetrads are ultrastructurally bilayered, with a complex, digitate outer margin presumably representing spore wall ornamentation. Neither the tetrads nor dyads reported in this paper are lamellate in ultrastructure. The combination of stomata, branching and dyads in the same sporophyte holds significance for the understanding of cryptospore affinities.