Palaeoscolecid worms from the Lower Ordovician Fezouata Lagerstätte, Morocco: Palaeoecological and palaeogeographical implications

Abstract

The Lower Ordovician Fezouata Lagerstätte from Morocco (central Anti-Atlas, Zagora area) has yielded abundant and diverse soft-bodied fossils. Most described taxa were epibenthic or pelagic, so that little is known about the endobenthic components of the fauna. Here we report the discovery of a locally abundant element of the biota, a palaeoscolecid worm, which may have played an important ecological role in the community. Palaeoscolecids had a long, annulated body with an eversible tooth-bearing pharynx. They probably represent stem priapulids, but unlike them, they were protected by biomineralized (phosphatic) microscopic plates covering most of the body. These sclerites are commonly found isolated within SSF assemblages, thus considerably extending the fossil record of the group that ranges from the Cambrian Series 2 (Stage 3) to the upper Silurian. Like priapulids, palaeoscolecids might have been important bioturbators, contributing to the substrate colonization in the lower Palaeozoic. The discovery of abundant material (38 specimens) in the Fezouata Shale (Lower Ordovician, Morocco) allows us to reconsider their possible lifestyle, habitat and feeding habits. All the specimens belong to 'Palaeoscolex? tenensis', a species previously only known from a fragment of cuticle from the Floian of Bohemia (Czech Republic). Some exhibit remains of the gut and aboral spines, features that are highly informative with regard to the ecology of these extinct organisms, but rarely preserved. The phosphatic nature of the plates is demonstrated by compositional analyses and interpreted with regard to the diagenetic context of the Fezouata Shale as primary. We also hypothesize that (at least) some palaeoscolecids were makers of 'Tomaculum'-type ichnofossils, which are elongate clusters of faecal pellets, and show that they ingested a notable amount of sediment during the normal course of feeding. The presence of 'Palaeoscolex? tenensis' in the Lower Ordovician of Morocco further documents the great faunal similarities between Perunica and northern Gondwana at that time.