Article: Silurian cryptospores and miospores from the type Llandovery area, south-west Wales
The oldest cryptospores and miospores have great significance in studies of the evolution of land plants: the former may represent the earliest direct evidence of such organisms and the latter may provide evidence for rhyniophytoid land plants as they have been recovered from the sporangia of Cooksonia pertoni Lang in the late Silurian. In the type Llandovery area, two distinct sporomorph assemblages are described from a composite section through uppermost Ordovician, Rhuddanian, Aeronian and basal Telychian strata. The older (latest Ordovician to late Aeronian) comprises eight genera and fourteen species of cryptospores (tetrads, pseudodyads, true dyads and monads). The younger (late Aeronian to Telychian) is dominated by smooth-walled trilete miospores of species of the genus Ambitisporites. Both assemblages have strong similarities to those described from similar horizons around the world. Specimens of Ambitisporites dilutus from the sedgwickii Graptolite Biozone of the Aeronian/Telychian type boundary section are the oldest known with unequivocal dating. They are used to define the base of the Ambitisporites dilutus-A. avitus Sporomorph Zone. Three new cryptospore genera (Rimosotetras, Segestrespora and Velatitetras) are erected and Tetrahedruletes is emended and synonomized with Nodospora. Six new cryptospore species (Velatitetras laevigata, V. reticulata, V. rugulata, Rimosotetra.i problematica, Segestrespora laevigata and S. rugosa), and two new varieties (T. medenensis vars medenensis and parvus) are described, and two combinations are made.