First discovery of Early PalaeozoicBathysiphon(Foraminifera) – test structure and habitat of a ‘living fossil’

Abstract

AbstractThe giant, agglutinated foraminiferanBathysiphonSars, previously Triassic–Recent, occurs in much older sedimentary rock (Early Ordovician, late early Tremadocian) of Avalonia. The genus extends back toc. 485 Ma based on its discovery in platform mudstone of the Chesley Drive Group in Cape Breton Island, Nova Scotia. Elongate (up to 60 mm), epibenthicBathysiphontubes occur in wave-rippled, green-grey mudstone with a low diversity, probably dysoxic fauna. The mudstone is coeval with and lithologically similar to the Shineton Formation in Shropshire and the Welsh Borderlands. Scanning microscopy of theBathysiphonwalls shows imbricated mica grains that parallel the long axis of the tests. The lumen has a mélange of packed sediment grains, some of which are spherical structures of siliciclastic mud studded with tetrahedral pyrite crystals. A felt-like, agglutinated test, a lumen packed with spherical structures (probable stercomata) and the domal ends of some specimens are consistent with modernBathysiphon. This report is the first time that cytoplasmic activity and stercomata formation have been used to refer fossil protists to a modern group.Bathysiphondiffers from the Cambrian foraminiferanPlatysolenitesPander, which has an open lumen without stercomata, but support a comparable, sediment deposit-feeding niche.Bathysiphonis truly a ‘living fossil’, with a mode of test construction, cytoplasmic activity that formed stercomata and a niche unchanged for almost 500 million years. Foraminiferans have not been found prior to the Cambrian Period, and the Early Cambrian appearance of agglutinated foraminiferans is part of the radiation of Phanerozoic communities.