been criticized for its slow and weak effect on ozone concentrations, as well as its suspect rejection of volcanic influences. Alternatively, cosmic rays from a nearby supernova would be capable of a similar degree of ozone depletion. The impact of a nearby supernova can be supported or refuted by testing for trace amounts of Plutonium-244 in fossils, but these tests have yet to be published. Ozone depletion could just as easily be explained by an increase in greenhouse gas concentrations resulting from an intense period of arc volcanism. The spore malformations may not even be related to UV radiation in the first place, and could simply be a result of volcanism-related environmental pressures such as acid rain. Volcanism Evidence of coronene and mercury spikes occurring in the Tien Shan Mountains of southern Uzbekistan near the Devonian-Carboniferous boundary has led some researchers to hypothesise a volcanic cause for the Hangenberg event. The activities of the Kola and Timan-Pechora magmatic provinces have been proposed as other hypothetical causes for the Hangenberg event. Impact event An asteroid impact has been suggested as a possible cause of the Hangenberg event. However, most impact craters, such as the Hangenberg-aged Woodleigh Crater, cannot be dated precisely enough to determine any causal relationship between the impact and the extinction event. See also Late Devonian extinction Kacak Event References Devonian Germany Late Devonian extinctions Devonian events Shale formations Famennian extinctions