Several localities in the Cantabrian Mountains (Spain) yield data on the occurrence of the late fusulinellid/early schwagerinid genera Protriticites and Montiparus, but the Las Llacerias section near Covadonga (Asturias), offers the best opportunity for a better understanding of the evolution of these fusulinaceans. Nine levels in this undisturbed section were selected to describe the fusulinacean fauna. A new species, Triticites? leciae, is described from the top of the section. The vertical distribution of this fauna points to the presence of both the Protriticites and the Montiparus Zones which in the Russian Platform form the C 3 A Zone (Krevyakinsky and Khamovnichesky horizons). A level corresponding to the North American Desmoinesian/Missourian boundary lies either in the higher part of the Montiparus Zone, near the top of the section, or slightly lower, but still above the underlying Protriticites Zone. The former traditional Middle/Upper Carboniferous boundary of Eurasia could be a major biostratigraphic boundary that coincides with a level of important faunal change. It is at about this level that the late fusulinellid Protriticites (including species groups commonly assigned to Praeobsoletes and Obsoletes) evolved from different species groups of Fusulinella. However, the evolutionary change from Fusulinella to Protriticites, and then to the earliest schwagerinid Montiparus, was quite gradual and may have started in older strata containing distinctly Middle Carboniferous fusulinaceans. No rapid turnover of the fusulinellid/schwagerinid fauna occurs near that boundary. The minimal morphological divergence involved in the evolution from Fusulinella to the late fusulinellids does not justify the recognition of three genera (i.e. Protriticites, Obsoletes, and Praeobsoletes). For this reason, and also in view of the practical difficulties with regard to their distinction, the genus Protriticites is redefined here to include the other two genera.