Yellowstone hot spring environments and the palaeo-ecophysiology of Rhynie chert plants: towards a synthesis

Abstract

Background: The Rhynie chert, Aberdeenshire, encapsulates the ecophysiological and anatomical information on all components, including plants, animals and micro-organisms, of an early terrestrial ecosystem as it existed some 400 million years ago (Early Devonian), as preserved by siliceous waters emanating from a hot spring system. Aims: This paper concentrates on the higher plants (tracheophytes) and brings together information on the habitats of the plants and the environmental pressures that they endured to answer issues relating to their endemism and their ecophysiology. Methods & Results: The synthesis includes detailed information on the palaeo-environments recorded in the chert, plus anatomical and autecological data from the plants themselves, and makes comparisons with the abiotic and biotic data obtained from an extant analogue, the alkali–chloride geothermal systems at Yellowstone National Park. Particular attention is paid to the physiological basis and evolution of osmotic and chemical tolerance of halophytes, the dominant colonisers of Yellowstone's wetlands and, to a lesser extent, metallophytes. Conclusions: The Rhynie plants colonised wetlands at the low temperature fringes of a hot spring system and were versatile, but physiologically highly specialised, capable of withstanding osmotic and chemical stresses in a dynamic environment, but were probably out-competed by mesophytic vegetation elsewhere.