Tube-building annelids from hydrothermal vents and cold seeps: Tube morphology, fossilisation, and evolutionary history

Problems with the identification of tubular fossils from ancient hydrothermal vent and cold seep deposits have hindered understanding of the evolutionary history of vent and seep communities. This thesis aims to (1) improve knowledge of lesser-studied tubicolous annelids occupying vents and seeps, (2) study the diversity of tubes at vents and seeps, (3) investigate the fossilisation of tubes within modern vents and seeps, (4) better interpret the fossil record, and (5) provide insights into the palaeoecology these environments. Results presented here on investigations of Sclerolinum contortum, a species belonging to a little- studied genus of the major vent/seep dwelling annelid family Siboglinidae, demonstrate that it exhibits tube morphological plasticity, wide habitat preferences and a global distribution spanning the Arctic to the Southern Ocean. These results also suggest that this species has dispersed throughout this range using chemosynthetic habitats as stepping stones. A detailed investigation of the full mineralisation process of Alvinella (Alvinellidae) tubes at modern hydrothermal vents shows that these tubes are fossilised by pyrite and silica that template organic tube layers, and that microorganisms living on tube surfaces are also exceptionally well-preserved alongside the tubes. No known ancient vent tube fossils resemble mineralised Alvinella tubes. A major morphological and compositional comparison of both modern and fossil tubes from vents and seeps revealed that two fossil tube types from the Mesozoic were likely made by vestimentiferans (Siboglinidae), suggesting that this major vent and seep annelid lineage has a longer history within vents and seeps than proposed by molecular clock age estimates. This analysis also demonstrates the need for greater caution in assigning affinities to fossil vent and seep tubes. Finally, this thesis reports the remarkable preservation of filamentous microorganisms on the walls of Silurian vent tube fossils, giving the first insights into ecological associations between microbes and metazoans within the oldest known hydrothermal vent community.