Extensive outcrops of fusuline-rich limestone strata of Pennsylvanian age occur in the Eastern or Ándara Massif of the Picos de Europa mountains in Northern Spain. The stratigraphic section is composed of the upper part of the Picos de Europa Formation, a carbonate succession deposited on a broad platform top in a distal foreland-basin margin setting, and the unconformable overlying Las Llacerias Formation, which represents carbonate-ramp strata deposited during the deformation of the underlying Picos de Europa platform. The tectonic setting allowed the accumulation of a very thick stratigraphic succession during the late Pennsylvanian, when the basin was located at paleoequatorial latitudes in the gulf of the western Palaeo-Tethys Ocean, adjacent to the eastern coast of Pangea.

An in-depth stratigraphic and micropaleontological study involving six sections and several isolated localities revealed an interval spanning the uppermost Moscovian to the lower-middle Kasimovian. This study has allowed us to establish a composite section, more than 400 m thick, recording the uppermost part of the Fusulinella Zone (F. alvaradoi subzone), the Protriticites Zone and the Montiparus Zone. Forty-one fusuline taxa are described, including four new species, Protriticites grajalensis, P. quasiovoidalis, P. benshae, and Pseudotriticites cantabricus.

The composition of the fusuline assemblages differs from those of the Carboniferous in key areas of Eastern Europe, and reveals their affinity to Central Asian assemblages as part of a separate biogeographic province. In spite of these paleobiogeographic differences, the observations on the fusuline evolutionary lineages, as well as the presence of a few common forms, made it possible to establish an approximate correlation between the Ándara and the Eastern European successions. We also report a previously unknown thrust fault, refine the stratigraphy and age of the successions outcropping in the Ándara Massif, and determine the diachronism of the base of the unconformable Las Llacerias Formation.

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