Abstract
One of the main limitations to understanding the evolutionary history of hydrothermal vent and cold seep communities is the identification of tube fossils from ancient deposits. Tube-dwelling annelids are some of the most conspicuous inhabitants of modern vent and seep ecosystems, and ancient vent and seep tubular fossils are usually considered to have been made by annelids. However, the taxonomic affinities of many tube fossils from vents and seeps are contentious, or have remained largely undetermined due to difficulties in identification. In this study, we make a detailed chemical (Fourier-transform infrared spectroscopy and pyrolysis gas-chromatography mass-spectrometry) and morphological assessment of modern annelid tubes from six families, and fossil tubes (seven tube types from the Cenozoic, 12 Mesozoic and four Palaeozoic) from hydrothermal vent and cold seep environments. Characters identified from these investigations were used to explore for the first time the systematics of ancient vent and seep tubes within a cladistic framework. Results reveal details of the compositions and ultrastructures of modern tubes, and also suggest that two types of tubes from ancient vent localities were made by the annelid family Siboglinidae, which often dominates modern vents and seeps. Our results also highlight that several vent and seep tube fossils formerly thought to have been made by annelids cannot be assigned an annelid affiliation with any certainty. The findings overall improve the level of quality control with regard to interpretations of fossil tubes, and, most importantly, suggest that siboglinids likely occupied Mesozoic vents and seeps, greatly increasing the minimum age of the clade relative to earlier molecular estimates.
Highlights
Hydrothermal vents and cold seeps are remarkable sites in the deep sea, characterized by the ejection of chemically reduced fluids from the seafloor that fuel abundant life through the process of chemosynthesis (Van Dover 2000)
To advance the understanding of the evolutionary history of deep-sea hydrothermal vents and cold seep communities, this study aims to improve the taxonomy of the abundant but problematic fossil annelid tubes from Phanerozoic vents and seeps
Assessment of the organic constituents of alvinellid, chaetopterid and siboglinid tubes confirms the different compositions of the tubes built by these families, thereby suggesting that tube organics can be taxonomically informative
Summary
Hydrothermal vents and cold seeps are remarkable sites in the deep sea, characterized by the ejection of chemically reduced fluids from the seafloor that fuel abundant life through the process of chemosynthesis (Van Dover 2000). Not long after the discovery of vent and seep ecosystems in the late 1970s and early 1980s (Lonsdale 1977; Corliss et al 1979; Paull et al 1984), multiple fossil analogues of these communities were described. Many tubular fossils are referred to as ‘worm tubes’, rather than being assigned to specific modern or fossil lineages, or have received controversial assignments (Campbell 2006; Vrijenhoek 2013). This has hindered our ability to understand the evolutionary history of chemosynthetic communities
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