Microbial-related biogenic structures from the Middle Ordovician slates of Canelas (northern Portugal)

A review with the description of the low diverse ichnoassemblages from the slates of Valongo Formation is made for the first time. Among the organic-rich mining, simple trace fossils dominated by Planolites-like worm burrows and very rare Chondrites developed in short-termed dysoxic intervals, we describe Phycodes canelensis nov. isp. as an adaptation to store faecal pellets poorer in nutrients away from the feeding area or for cultivating bacteria. The most remarkable are very large-size, concentrically ringed, pyritized structures, developed in relation with the decomposition of orthoconic cephalopods and dissolution of their shells, which are reinterpreted now as microbialites. This study documents a dysoxic-dominated ecosystem in dependence of bacterial metabolism, still with a relative abundance of Ediacaran holdovers that was developed in the wider period of tiering complexification and diversification of ecospaces, feeding habits and optimization, with the generalized appearance of deeper deposit-feeding patterns in the offshore during the Ordovician radiation.