during the Devonian. Duration and timing Extinction rates appear to have been higher than the background rate, for an extended interval covering the last 20–25 million years of the Devonian. During this time, about eight to ten distinct events can be seen, of which two, the Kellwasser and the Hangenberg events, stand out as particularly severe. The Kellwasser event was preceded by a longer period of prolonged biodiversity loss. The Kellwasser event, named for its type locality, the Kellwassertal in Lower Saxony, Germany, is the term given to the extinction pulse that occurred near the Frasnian–Famennian boundary (372.2 ± 1.6 Ma). Most references to the "Late Devonian extinction" are in fact referring to the Kellwasser, which was the first event to be detected based on marine invertebrate record and was the most severe of the extinction crises of the Late Devonian. There may in fact have been two closely spaced events here, as shown by the presence of two distinct anoxic shale layers. Potential causes Since the Kellwasser-related extinctions occurred over such a long time, it is difficult to assign a single cause, and indeed to separate cause from effect. From the end of the Middle Devonian (382.7 ± 1.6 Ma), into the Late Devonian (382.7 ± 1.6-358.9 ± 0.4 Ma), several environmental changes can be detected from the sedimentary record, which directly affected organisms and caused extinction. What caused these changes is somewhat more open to debate. Possible triggers for the Kellwasser event are as follows: Weathering and anoxia During the Late Silurian and Devonian, land plants, assisted by fungi, underwent a hugely significant phase of evolution known as the Silurian-Devonian Terrestrial Revolution. Their maximum height went from 30 cm at the start of the Devonian, to archaeopterids, at the end of the period. This increase in height was made possible by the evolution of advanced vascular systems, which permitted the growth of complex branching and rooting systems, facilitating their ability to colonise drier areas previously off limits to them. In conjunction with this, the evolution of seeds permitted reproduction and dispersal in areas which were not waterlogged, allowing plants to colonise previously inhospitable inland and upland areas. The two factors combined to greatly magnify the role of plants on the global scale. In particular, Archaeopteris forests expanded rapidly during the closing stages of the Devonian. These tall trees required deep rooting systems to acquire water and nutrients, and