This paper is the first example of how Middle Ordovician to Lower Silurian land plant microfossils, commonly present in otherwise unfossiliferous or poorly fossiliferous, nonmarine, or nearshore facies, may be used for biostratigraphic purposes. This new information permits the recognition of a broad regional unconformity extending from the Paraná Basin to Arabia. Conclusive evidence for the presence of Silurian rocks in the Paraná Basin of southeastern Brazil is developed. The age is based on organic microfossils, including tetrahedral tetrads of land-plant spores and phytoplankton (“acritarchs” and prasinophytes) that were recovered from body- and trace-fossil-bearing rocks of the northeastern Paraná Basin. Both the plant spores and phytoplankton indicate an Early Silurian (early Llandoverian) age, and support the previous permissive Silurian age assignment based on the trace fossil Arthrophycus and invertebrate body fossils.