This paper describes some outstanding ichnofossils recorded from a roofing slate quarry in the Arouca region, 50 km SE from Oporto (Portugal). Corresponding beds are of Middle Darriwilian age and represent temporary dysoxic episodes within the Valongo  formation. The low-­‐oxygen conditions on the substrate are harnessed by opportunistic and systematic deposit-­‐feeders that generate, in the horizontal plane, some giant concentric traces (Aroucaichnus) as well as flabellated burrows (Phycodes). The new Aroucaichnus valerioensis igen. et isp. nov. represents an ellipsoidal trace formed by 9–19 endichnial ribbons inclined to the centre of the structure, showing an obliquely-­‐laminated active infill. The structure occurs in association with large longicone nautiloids, and the trace is formed when the trace maker harvested the decaying bacterial colonies developed around these remains. Endocerid nautiloids were also the most probably producers of the bite marks recorded on a number of trilobite fragments and exuviae. These are described as Bicrescomanducator serratus, being Mandibulichnus proposed as a younger synonym of the ichnogenus Bicrescomanducator. The record of Phycodes noha Mikuláš (Lower Ordovician of Bohemia) is enlarged to the Iberian Peninsula, where it occurs as composite forms with Tomaculum problematicum Groom in the Middle Ordovician.