Early post-embryonic trilobite stages and possible eggs from the `Túnel Ordovícico del Fabar` (Middle Ordovician, northwestern Spain)

Abstract

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 preserves a number of early stages of the calymenacean trilobite Prionocheilus mendax (Van¿k), 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 of ca. 0.5 mm in diameter are recorded, and may represent eggs of trilobites, being associated to 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 with 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 pyritised ?eggs suggest a lecithothrophy development related with higher latitudes and/or with scarce or stationed organic matter.