Enrolled agnostids from Cambrian of Spain provide new insights about the mode of life in these forms

Enrolled agnostids have been known since the beginning of the nineteenth century but assemblages with high number of enrolled specimens are rare. There are different hypotheses about the life habits of this arthropod group and why they enrolled. These include: a planktic or epiplanktic habit, with the rolled-up posture resulting from clapping cephalon and pygidium together, ectoparasitic habit or a sessile lifestyle, either attached to seaweeds or on the sea floor. Herein we describe two new assemblages from the middle Cambrian of Purujosa (Iberian Chains, North Spain) where agnostids are minor components of the fossil assemblages but occasionally appear enrolled. The taphonomic and sedimentological data suggest that these agnostids were suddenly buried and rolled up as a response to adverse palaeoenvironmental conditions. Their presence with typical benthic components supports a benthic mode of life for at least some species of agnostids.