The earliest record of a sclerobiont assemblage from a methane-seep ecosystem (upper Silurian, Morocco)

Sclerobionts colonizing brachiopod shells derived from the oldest-known methane seep deposit (upper Silurian, Moroccan Meseta) are reported for the first time. Despite a large number of the brachiopod shells collected, all representing the atrypide species Septatrypa lantenoisi Termier, very few showed the presence of sclerobionts. The majority of the shells were taphonomically altered: worn, exfoliated and corroded. The sclerobionts are represented by cornulitid tentaculitoids and less common auloporid tabulate corals and epibiontic brachiopods. Although the observed assemblage certainly does not reflect the original taxonomic composition of the sclerobiont assemblage, it provides the first insight into the Palaeozoic encrusters inhabiting methane-seep ecosystems. As implied by the moderately 13C-depleted carbon isotope composition and simple, micrite-dominated paragenetic succession of the associated seep carbonates, the investigated community most likely represents a period of slow, diffuse fluid seepage, an interpretation consistent with the common perception of the seep-related brachiopods as non-chemosymbiotic, perhaps opportunistic seep-dwellers unable to colonize sites with more vigorous fluid flow. Likewise, the studied encrusters most likely were non-obligate inhabitants of the seep ecosystem, using the brachiopod shells as firm attachment sites and elevation above the seafloor. This colonization pattern would have probably improved their access to the prolific suspended organic particles and potentially limited their exposure to the highest environmental toxicity. As in the case of geologically much younger serpulid polychaetes, also the cornulitid tubeworms colonized the methane seeps relatively shortly after their first, Middle Ordovician appearance in the fossil record.