Micropalaeontology of a Moroccan Ordovician deposit yielding soft-bodied organisms showing Ediacara-like preservation

A newly discovered locality of an hitherto unknown age, but now estimated to the Late Ordovician, in south-eastern Morocco has yielded numerous specimens of problematic soft-bodied metazoans preserved in coarse sandstones. The excellent preservation of the recovered specimens permits an improved understanding of both the palaeobiology of these Ordovician metazoans and the taphonomic processes associated with them. This style of soft-tissue preservation is closely comparable to that of the late Neoproterozoic problematic Ediacaran organisms, and is virtually absent in the Phanerozoic. Palynological analysis (chitinozoans and acritarchs) of shaly intercalations in an ophiuroid-bearing sandstone slab facilitates accurate identification of the site as belonging to the Upper Ktaoua Formation and thus a relative dating of the associated fossils. The chitinozoan assemblage is relatively well preserved, moderately diverse and closely comparable to the previously described chitinozoan fauna from the Upper Ktaoua Formation (Rawtheyan, late middle Ashgill age). Acritarchs are abundant but mostly ill-preserved; nonetheless they clearly indicate an undifferentiated Ashgill age, consistent with the chitinozoan chronostratigraphic result. These datings prove that the newly discovered Moroccan metazoan assemblage contains one of the oldest known paropsonemids, and possibly some of the youngest protolyelloids and aglaspidids. The new finds also show that Ediacara-like coarse-elastic preservation did not completely vanish with the onset of the Phanerozoic. (C) Etditions scientifiques et medicales Elsevier SAS