New plant taxa from the Lower Devonian (Lochkovian) of the Welsh Borderland, with a hypothesis on the relationship between hilate and trilete spore producers

A detailed investigation of charcoalified Lochkovian mesofossils, undertaken to circumscribe diversity and disparity in early land vegetation, has revealed a number of discoidal spore masses, enveloped by a featureless cuticular layer. The specimens are interpreted as incompletely preserved terminal sporangia. Based on haptotypic features of the spores, two groups of sporangia are recognised: the first contains alete sculptured monads, the separated products of dyads that are assigned to the dispersed cryptospore genus Cymbohilates Richardson; the second contains laevigate and apiculate trilete monads, the products of tetrads that are assigned to the dispersed genera Ambitisporites Hoffmeister, Streelispora (Chaloner and Streel) Richardson and Lister, and Aneurospora Streel. A new genus, Lenticulatheca, with four new species, L. magna, L. mesodeca, L. variabilis, and L. allenii, has been erected for the cryptospore producers, distinction being based on species and varieties of Cymbohilates. The trilete spore producers, which were far more numerous, have been assigned to a second new genus, Paracooksonia, to emphasise similarities with Cooksonia Lang, both in gross morphology of the spore mass and in situ spore genera. Paracooksonia apiculispora is erected for specimens with in situ apiculate spores and Paracooksonia ambitispora is erected for laevigate in situ spores. Similarities in the bilayered exospore and sculpture, together with those of gross morphology have led to the hypothesis that the two new genera are closely related, the major distinction arising from changes in the timing of cytokinesis during meiosis that resulted in either dyads or tetrads. Information on missing vegetative features is essential to elucidating unequivocally the proximity to the tracheophytes, but similarities with the Cooksonia complex suggest that the two new genera should be considered rhyniophytoids.