Ultrastructure of laevigate hilate spores in sporangia and spore masses from the Upper Silurian and Lower Devonian of the Welsh Borderland

Spore masses and isolated sporangia, containing laevigate hilate cryptospor es attributable to the dispersed taxon Laevolancis divellomedia sensu late, have been recovered on bulk maceration of Upper Silurian (Pridoli) and Low er Devonian (Lochkovian) deposits from the Welsh Borderland. Detailed morph ological, anatomical and ultrastructural analysis, using light microscope, scanning electron microscope and transmission electron microscope technique s, reveals subtle differences between the specimens and they can be grouped into five distinct types. The different groups are distinguished principal ly by using sporangia-spore mass characteristics, presence or absence of ex tra-exosporal material and nature of spore wall ultrastructure. Of the grou ps, one has a uniformly homogeneous exospore and the other four groups have a bilayered exospore. In the former the spores lack extra-exosporal materi al and occur in a discoidal sporangium. Of the bilayered groups, two have e xospores of homogeneous composition but with the two layers differing in el ectron density. They occur in discoidal sporangia and spore masses and are distinguished on the presence or absence of extra-exosporal material and di fferences in the widths of the two layers. Finally, two bilayered groups po ssess a lamellate inner layer, but vary in presumed sporangial shape. Elong ate sporangia have spores with concentric continuous lamellae, lacking furt her ultrastructure. In contrast, spores From a discoidal spore mass have wh ite-line-centred, presumably tripartite, lamellae which are laterally disco ntinuous, overlapping and irregularly spaced. These findings, which suggest that morphologically similar spares were produced by a number of plant tax a, have important implications regarding the assessment of early land-plant diversity. The affinities of hilate cryptospore-producing plants are unkno wn and problematic, particularly as no extant non-angiosperm plants produce dyads, other than through meiotic irregularity, and spore-sporangial chara cters have no exact counterpart in coeval plants. Studies of specimens with ill situ hilate cryptospores suggest that they derive from rhyniophytoids, i.e. plants that resemble the simplest of vascular plants but lack evidenc e of vascular tissue, although hilate cryptospore-containing examples show no axial branching. It might be argued, based on evidence from spore wall u ltrastructure, that some of the plants have more in common with lycopsids a nd filicopsids than bryophytes, a surprising finding bearing in mind the st ratigraphic distribution of hilate cryptospores-dyads and inferences that t he producers were bryophyte-like. Detailed studies of wall structure in the hilate cryptospores permit consideration of spore wall development. It is suggested that extra-exosparal material derives from a tapetum and is thus produced by the diploid sporophyte. The white-line-centred lamellae in a si ngle specimen provide the earliest evidence for the presence of such struct ures in early land-plant spores and provide further evidence that sporopoll enin deposition on such structures is the most primitive mode of sporopolle nin deposition among land plants.