198 PALAEOZOIC/Devonian of Gondwana (present day South America), ice-caps that extended to sea-level Global sea level had begun to fall earlier in the Famennian, and continued to fall throughout the Famennian, indicating the probable formation of high-altitude mountain glaciers considerably before the end of the Famennian The end-Devonian extinctions began slightly earlier in the marine realm than the terrestrial, but the entire period of extinctions appears to have spanned only 100 000 years Although the observed extinctions of marine animals were not as great as those in the Late Devonian mass extinction (Figure 4), the effect of the end-Devonian extinction on land plants and on phytoplankton in the sea was much more severe In particular, the towering Archaeopteris trees that had been so widespread and characteristic of Late Devonian forests rapidly declined in abundance during the Late Famennian climatic cooling, barely surviving into the Early Carboniferous before dying out The end-Devonian ice caps were clearly the result of global cooling, which itself was most likely triggered by the dramatic decrease in atmospheric carbon dioxide throughout the latter half of the Devonian (Figure 2) By the end-Devonian, carbon dioxide concentrations in the atmosphere were only three to four times higher than those at present (Figure 2) The Late Devonian Biodiversity Crisis The principal cause of the Late Devonian extinction also appears to have been global cooling, but the pattern of cooling has a very different signature than that seen at the end of the Devonian, and the cause of the cooling remains controversial Palaeobotanical evidence shows that the Frasnian world was quite warm, with extensive shallow seas covering much of the continental landmasses In these warm seas the great Devonian reefs (see Fossil Invertebrates: Corals and Other Cnidaria), and other high-diversity marine ecosystems, proliferated In contrast to Frasnian climates, the Early Famennian world appears to have been cold and arid The great forests shrank in their areal extent, and were confined to low-latitude equatorial regions The uplands and high-latitude regions of the Earth supported only very sparse vegetation in conditions that were not only very cold but were also very dry, without perennial snow In the oceans, huge reef tracts died out all over the world during the latest Frasnian The few reef organisms that managed to survive into the Famennian were confined to what few warm waters still existed on the Earth, in restricted regions along the equator It is still not clear what triggered the collapse of the Frasnian hot climates, and the rapid fall of global temperature into the cold Famennian world The rapidity of the Late Frasnian cooling is particularly noteworthy, and quite different from the gradual pattern of cooling seen at the end of the Devonian that culminated in continental glaciation Geochemical evidence indicates that tropical sea-surface temperatures fell by as much as 5 to 7 C in two abrupt pulses, separated by about 300 000 years, in the Late Frasnian to Early Famennian interval Three competing hypotheses are at present being debated concerning the ultimate cause of the Late Devonian biodiversity crisis The ‘Devonian plant hypothesis’ proposes that both the Late Devonian and end-Devonian biodiversity crises were triggered by the evolution and spread of extensive land plant cover (Figure 1), and the effects of that new terrestrial plant biomass on the atmosphere (Figure 2) and on the oceans (particularly the massive increase in nutrient input to marine ecosystems) However, the Late Devonian biodiversity crisis occurred some 10 million years before the end of the Devonian (Figure 4) All attempts to find geologic evidence of glaciation in the Late Frasnian to Early Famennian interval to date have failed Global sea-level was at an all-time high for the Devonian, indicating there was no major buildup of glacial ice in this time interval (although apparently rapid oscillations in sea-level during the Late Frasnian and Early Famennian may indicate the formation of small, high-altitude alpine-type glaciers) Atmospheric carbon dioxide concentrations were six times higher than at the present day (Figure 2), and approximately twice as high as those present during the formation of the end-Devonian glacial ice-caps Many argue that these facts, particularly the rapidity of the temperature drops during the Late Devonian biodiversity crisis, indicate a substantially different triggering mechanism than that seen for the end-Devonian extinctions The ‘volcanic winter hypothesis’ proposes that an interval of catastrophic flood-basalt volcanism occurred during the Late Frasnian One of the climatic effects of such extensive volcanism is hypothesized to have been rapid global cooling, due to the vast amount of debris, ash, and gasses injected into the Earth’s atmosphere A volcanic-produced global dust and gas cloud would have blocked light from the sun, from reaching the Earth’s surface, triggering planet-wide lethally cold temperatures even at the equator The catastrophic volcanism scenario is based upon the recognition that there have been other flood-basalt episodes in Earth history, during which enormous amounts of lava, gasses, and volcanic dust have been produced in volcanic eruptions of almost unimaginable magnitude These flood-basalt fissure eruptions are produced by gigantic plumes of molten rock that originate deep in the Earth’s mantle, which then