Daedalus mega-ichnosite from the Muradal Mountain (Naturtejo Global Geopark, Central Portugal): Between the Agronomic Revolution and the Ordovician Radiation

Thick quartzite beds with large bedding planes exposures thoroughly bioturbated with Deadalus desglandi, that can be followed for several kilometers, were discovered in the Lower Ordovician (Floian) of the Armorican Quartzite Formation at Muradal Mountain, UNESCO Naturtejo Geopark (Central Portugal). The complex architecture of this form of Daedalus is discussed and the model of “subtidal pumping”, based on the presence of a draft-fill channel in each one of the burrows, is introduced to explain the feeding pattern in “clean” quartzites. Extremely crowded monoichnospecific levels show that they were strongly controlled by shifting substrate conditions but mostly limited in time, pointing to the colonization of event beds by soft-bodied populations in very shallow marine and very dynamic settings. The magnitude of the Daedalus ichnofabric fluctuations shows that sandflat substrate colonization by the Daedalus producer after each event was intense, mostly sequential, with an exclusive and almost total occupation of the emptied ecospace. The presence of only one preserved behavioral strategy, the substrate depth affected by these structures, the high density of burrows and passive patchiness rates are characteristics of rselected populations. Such large-scale and frequent disturbance events as storms, extreme on a gradient of disturbance intensities, were responsible for some of the earliest opportunistic behaviours in the fossil record. Daedalus was among the most resilient of them in the Early Paleozoic of this part of the world.