Cnidarians of the Cabrières Lagerstätte (Lower Ordovician, France): diversity, taphonomy, palaeoecology and palaeobiogeography

Like most other cnidarian-bearing biotas of Early Ordovician age, the cnidarian component of the late Floian Cabrières Biota (southern Montagne Noire, France) consists exclusively of the hard part remains of polypoid medusozoans, namely Sphenothallus sp. and Archaeoconularia insignis. Located on or adjacent to some specimens are small rhynchonelliform brachiopods (orthids indet.) and/or low-spired tergomyan molluscs (Carcassonnella), all of which lived in close proximity to each other on the muddy seafloor. Whereas Sphenothallus probably was epibenthic, Archaeoconularia may have lived with its apical portion embedded in mud; both were toppled prior to final burial, which was also preceded by exposure and fragmentation of recumbent remains on the sea floor. The medusozoans were micro-predators, with Sphenothallus standing up to about 60 mm above the seafloor and the conulariids standing even higher than Sphenothallus and the largest sponges, which together with long-stemmed eocrinoids (Lingulocystis) may have competed with the medusozoans for microscopic plankton. Neither Sphenothallus, nor the conulariids terminate in an apical wall (schott), and no specimen exhibits definite epibionts or evidence of predation or parasitism by bacteria or fungi. Finally, an updated table of documented occurrences of Lower Ordovician Sphenothallus and conulariids is presented, and actual or probable occurrences of conulariids attached to hard biological substrates are summarized.