Ploughing, tunnelling and biting in the Middle Ordovician of northern Portugal

Abstract

Middle Ordovician shales from the Valongo and Moncorvo formations recorded temporary dysoxic environments, inhabited by opportunistic faunas that also include highly specialized deposit feeders. Resulting ichnofossils include Phycodes noa Mikuláš, a horizontally-ramified burrow complex, so far only known from older beds in the Prague Basin, as well as a large spiral grazing trace close to Rotundusichnium zumayensis Gómez de Llarena, which is widely distributed in Alpine flysch deposits. Phycodes noa from the Valongo Formation occurs as quite large, horizontally flabellate structure with up to 10 diverging passages. Specimens from the Canelas quarry (Arouca) are commonly infilled by pellets (Tomaculum), and are preserved flattened and tectonically expanded. Giant Rotundusichnium-like forms are exclusive from the Canelas quarry and consist of ?concentric to tightly spiral traces with a large elliptical outline (major axis up to 130 cm long), showing endichnial ribbons inclined to the centre of the structure. These are associated with imploded fragmocones of nautiloids preserved under them, and the trace maker harvested the microbial proliferation around them in tight centrifugal coils. The third remarkable trace corresponds to bite marks preserved in large trilobite carapaces. They have a consistent acute V-shaped outline and represent marginal breakage, usually placed in the right pygidial pleural border of asaphids, in diverse margins of calymenaceans or in semi-infaunal molluscs. These triangular bite marks are attributed to large nautiloid predators or scavengers, sometimes attacking freshly-molted giant trilobites.