Archaeopteris trees at high southern latitudes in the late Devonian

Abstract

During the Devonian Period plants first reached forest stature, impacting chemical weathering of rocks, fluvial systems, atmospheric composition and possibly aquatic eutrophication. Hypothetically these factors contributed to increasing climatic instability culminating in the End Devonian Mass Extinction Event. Understanding the timing of the spread of forests is however a prerequisite to correlation with its proposed consequences. Though evidence for forests at low palaeolatitudes demonstrates their emergence by the mid Devonian, sparse high-palaeolatitude records almost entirely comprise herbaceous lycopods. By the Famennian forest ecosystems are widely evidenced at low palaeolatitudes, however high latitude palaeofloras are almost exclusively represented by a single locality, the Waterloo Farm lagerstätte from South Africa (approximate palaeolatitude, 70°S). Understanding climatic and ecological conditions at this locality is doubly important as it also hosts diverse vertebrate taxa, including the only high latitude Devonian tetrapods. Archaeopteris, the quintessential Late Devonian woody tree, has previously been identified at this locality on the basis of leafy branch system fragments, though some uncertainty has remained as to whether these represent tree sized organisms. Here we present co-occurring large axes, including a trunk base, attributable to Archaeopteris trees inferred to be in excess of 20 m height, the first demonstration of forest stature at high latitudes in the Devonian. This possibly reflects high latitude climatic amelioration, resultant from warm ocean currents circulating southwards in response to progressive closure of the Iapetus Sea. As such, changing continental configurations may have indirectly facilitated the spread of forest ecosystems and helped to drive climatic instability and ultimately extinctions towards the end of the Devonian.