Communities and palaeo-ecophysiology of fossil hot spring ecosystems through time

Anatomical evidence plays an important role in elucidating the relationships of plant fossils and in the ways in which plants grew and functioned -physiology. Silicification of plant tissues results in the most faithful preservation of cellular detail and occurs in two principal ways, within volcaniclastic deposits by precipitation of silica dissolved from ashes or as silica supersaturated waters flow from hot springs. The latter is particularly important because eruptions cause flooding of vegetation in the vicinity of vents and thus not only engulfs growing plants but also animals and microbes, even whole ecosystems in situ. Such occurrences are rare in the fossil record, but provide unique snapshots of past life. Perhaps the best known hot spring deposit is the Lower Devonian Rhynie Chert of Aberdeenshire, Scotland. However, studies of present-day vegetation growing in the vicinity of hot springs e.g. Yellowstone, USA and Iceland, demonstrate that the plants (and associated ecosystems) that are most likely to be flooded are usually hydrophytes or tolerant of flooding and are capable of withstanding normally high and potentially toxic levels of salt, heavy metals and pH extremes. Indeed similar plant and animal associations are found around brackish water associated with coastal marshes or ephemeral evaporation dominated inland water bodies (e.g. salt lakes). This suggests that fossiliferous hot spring deposits such as the Rhynie Chert do not reflect the most common vegetation but are highly specialised. However testing of such an hypothesis at Rhynie is highly unsatisfactory because we have no fossils from contemporaneous rocks in coastal or lacustrine settings, the Rhynie Chert plants are dominated by soft tissues unlikely to be preserved unless permineralised and, apart from a lycophyte, they have no living relatives, the evolution of the remaining lineages of vascular plants having occurred in the intervening 400 million years. Exploration by gold mining companies has identified numerous, more-recent (Tertiary & Mesozoic) plus one earlier Silurian, hot spring deposits with potentially fossiliferous silica sinters and associated wetland environments. By far the most extensive are confirmed richly-fossiliferous Jurassic (c. 200Ma) deposits within the Deseado Massif, Patagonia. Preliminary results indicate that unlike the Rhynie Chert, some of the Patagonian fossils can be related to living forms e.g. the monkey puzzle conifer family, and further there are richly-fossiliferous rocks recording the vegetation peripheral to the hot springs, bordering lakes and rivers, plus in stressed environments such as coastal fringes. A major component of the proposed project will be to reconstruct the Jurassic hot spring ecosystem including plants, bacteria, fungal decomposers, algae and animals. Building on this, following plant identification with assistance of Argentine colleagues, we will compare diversity (species list) from the various types of rock and estimate the degree to which the hot spring ecosystems are typical of either 'normal' dry-land/wetland, or salinity stressed wetland ecosystems. Following detailed anatomical description we will detect any modifications at the cellular level which are indicative of adaptation to water stress/physiological drought, or are connected with withstanding heavy metal toxicity. Similar but probably less rigorous analyses, due to time constraints, will be applied to Carboniferous, Cretaceous and Miocene hot springs, to attempt to demonstrate convergence in anatomical and physiological responses in disparate plant lineages. Particularly exciting is the prospect of the discovery of 3-dimensionally preserved angiosperms at the Chinese locality, Dongfanghong, part of an extensive gold field situated within the same Lower Cretaceous province and close to localities that have yielded the earliest semiaquatic angiosperms plus birds and feathered dinosaurs.