A detailed reexamination of a key Wenlock–Ludlow transition section in the Valle syncline region, Spain, and the incorportation of data from a coeval section in the Barrancos area, Portugal, both within the Ossa Morena Zone, reveal seven biostratigraphic units. These are Monograptus belophorus? and Cyrtograptus lundgreni−Monograptus testis biozones, representing the upper Sheinwoodian and the preextinction, lower Homerian faunas, respectively; the Pristiograptus dubius "parvus" – Gothograptus "nassa" Concurrent Range Biozone, Colonograptus? praedeubeli – Colonograptus? deubeli Biozone, and Colonograptus? ludensis Biozone, representing the upper Homerian postextinction faunas; and the Ludlow Neodiversograptus nilssoni and Saetograptus fritschi linearis biozones. The extinction beds between the lower Homerian lundgreni−testis Biozone and the lowest upper Homerian dubius "parvus"−"nassa" Interval Biozone make up a several centimetre thick, soft, orange-weathered, almost barren, volcanic? bed.