Fast post-mortem encrustation of razor shells: examples from the Irish Sea and palaeontological implications

Abstract

Instructive taphonomic principles are demonstrated by the skeletons of dead invertebrates. Bivalves have resistant skeletons and are common fossils. The thin-valved razor shells, Ensis spp., have a good fossil record despite being fragile. This may be due, in part, to rapid post-mortem encrustation of valves by mineralized invertebrates. Two Recent specimens of Ensis siliqua (Linnaeus), encrusted post-mortem, are described from the Irish Sea coast of Southport, Merseyside. An articulated shell with an intact ligament is encrusted posteriorly on all surfaces by the balanid Balanus crenatus Brugiére. Barnacles inside the shell are smaller than those externally, yet may represent the same spatfall; those inside were constrained by growing in an enclosed space. To a palaeontologist, a mollusc valve encrusted inside and out by cementing organisms would be interpreted as having had a long residence time on the seafloor. This specimen demonstrates the disjunction between loss of soft tissues (days?) and loss of the ligament (weeks, perhaps months), between which encrusters may settle inside the shell, early in its post-mortem history. Similar patterns of encrustation by balanids are now known two species of Ensis and the cockle Cerastoderma edule (Linnaeus).A fragment of a single valve is encrusted only on the internal surface by serpulids and bryozoans. This is balanulith-like, but is only encrusted on the inner surface and not by balanids. Ensis valves can be reinforced by a range of calcareous encrusting organisms; an Ensis fragment encrusted both inside and out by serpulids would be worthy of being named a serpulith.