Molecular phylogenetic studies of extant organisms have shown that those branches of the Bacteria and Archaea that lie closest to the "last common ancestor" of all life are occupied by hyperthermophiles. At the same time, the search for former life on Mars has focussed on thermal spring deposits. For these reasons there is interest in the palaeobiology of ancient thermal spring deposits on Earth. Many such deposits are known but very few have been studied by palaeobiologists. The Devonian sinters of the Drummond Basin, Australia, rank with the Rhynie cherts of Scotland as the oldest well established examples of fossil subaerial hot springs. The Drummond Basin sinters are closely comparable with modern examples in Yellowstone National Park, Wyoming, and elsewhere. It is possible to recognise a range of palaeoenvironments from high temperature vents through former hot-water channelways and terraces to ambient temperature marsh deposits. Cyanobacterial stromatolites and microfossils are abundant in those palaeoenvitronments characterised by temperatures well above ambient, and the marsh and other ambient temperature palaeoenvironments contain abundant herbaceous lycopsids and sphenopsids encrusted by microbial overgrowths.