Permanent dyads in sporangia and spore masses from the Lower Devonian of the Welsh Borderland

Abstract

Dyads, interpreted as cryptospore permanent dyads, are reported from sporangia and spore masses recovered from fluviatile deposits of Lochkovian (Early Devonian) age from the Welsh Borderland. The morphology, anatomy and ultrastructure of the specimens have been analysed using LM, SEM and TEM. Two specimens consist of a single sporangium terminating an axis: one is cup-shaped and attached to an unbranched axis and namedCulullitheca richardsoniigen.etsp. nov., the other comprises an isotomously branching axis with one sporangium preserved and is namedFusitheca fanningiaegen.etsp. nov.Two specimens show dyads adhering to cuticular fragments presumably derived from sporangia. In one the cuticle is of irregular shape, in the other it is elliptical. The final specimen comprises an elongate spore mass. In all cases ultrastructural analysis reveals that the spore walls are essentially homogeneous. Our findings suggest that cryptospore permanent dyads were produced by rhyniophytoid plants of small stature. The affinities of these plants remain conjectural, as is the relationship of the cryptospore permanent dyads with other sporomorph morphotypes (cryptospores and trilete spores). However, the production of dyads by a plant with a bifurcating sporophytic axis is evidence against affinity with extant bryophytes. The mode of formation, adaptive significance, affinities, phylogenetic relationships and stratigraphical history (including demise) of cryptospore permanent dyads is discussed.