The sub-surface section of the Middle Ordovician Sueve Formation of the Cantabrian Zone, studied during the construction of a tunnel in the A-8 free highway of northern Spain, provided abundant fossiliferous horizons rich in trilobites of Darriwilian age. Fine-grained mudstones of the Cofiño Member delicately preserve a number of early stages of the calymenacean trilobite Prionocheilus mendax (Vaněk), from protaspides to meraspid degree 2, including in situ exuviae with disarticulated librigenae. From the upper part of the Bayo Member, small clusters of pyrite spheres ca. 0.5 mm in diameter are recorded, and may represent eggs of trilobites, being associated with the pliomerid Placoparia (Coplacoparia) tournemini. (Rouault) in the same beds. The sphaeroids may represent the pyrite infilling of the internal cavity of possible eggs, and the covering is replaced by phyllosilicates. Differences in shape and size from presumed olenid eggs may be caused by the particular environmental adaptations of the latter group. The relatively large size of both the calymenid larvae and the pyritized eggs suggests a lecithotrophy development related to higher latitudes and/or with scarce organic matter.