Insights into soft-part preservation from the Early Ordovician Fezouata Biota

International audienceThe Fezouata Biota in Morocco is the only Lower Ordovician Lagerstätte yielding a biologically diverse assemblage in a fully marine environment, whilst also containing organisms typical of Cambrian Burgess Shale-type (BST) ecosystems. Fossils from the Fezouata Shale share the same mode of preservation as Cambrian BST biotas defined by carbonaceous compressions and accessory authigenic mineralization. Most organisms of the Fezouata Biota were already dead and decaying on the seafloor when they were buried insitu by occasional storm-induced deposits in an environment just below the storm-weather wave base. Pre-burial decay in the Fezouata Shale was responsible for the non-preservation of completely cellular organisms such as jellyfish. These conditions contrast with the processes described for soft-tissue preservation in the Burgess Shale (Canada) and the Chengjiang Biota (China). In these two Cambrian Lagerstätten, animals were transported alive or shortly after death by obrution events to an environment that was favorable for preservation. Despite preservational biases, the autochthonous assemblages of the Fezouata Shale offer a unique opportunity to decipher the structure of in-situ communities and ecological dynamics in Early Palaeozoic seas, when compared to the allochthonous communities of most Cambrian BST biotas