Are hydrothermal vent animals living fossils?

Since their discovery in 1977, hydrothermal vent communities have provided many surprises about life in the deep sea and in extreme environments. It has been suggested that vent communities contain many living fossils and that deep-sea chemosynthetic environments, such as vents and hydrocarbon seeps, are buffered from extinction events that affect the photic zone. This hypothesis is based on the dependence of these deep-sea communities on a geochemical energy source and the considerable levels of taxonomic novelty that they contain. Here, we review recent evidence from the fossil record of hydrothermal vents and hydrocarbon seeps, together with molecular phylogenies of several dominant hydrothermal vent and seep taxa. In spite of significant discrepancies between the fossil record and molecular divergence estimates for several important taxa, we show that most modern vent animal groups arose relatively recently and that the taxonomic composition of vent communities has changed considerably through time.