extinctions. An asteroid impact was proposed as the prime cause of this faunal turnover. The impact that created the Siljan Ring either was just before the Kellwasser event or coincided with it. Most impact craters, such as the Kellwasser-aged Alamo, cannot generally be dated with sufficient precision to link them to the event; others dated precisely are not contemporaneous with the extinction. Although some evidence of meteoric impact have been observed in places, including iridium anomalies and microspherules, these were probably caused by other factors. Supernova A more recent explanation suggests that a nearby supernova explosion was the cause for the specific Hangenberg event, which marks the boundary between the Devonian and Carboniferous periods. This could offer a possible explanation for the dramatic drop in atmospheric ozone during the Hangenberg event that could have permitted massive ultraviolet damage to the genetic material of lifeforms, triggering a mass extinction. Recent research offers evidence of ultraviolet damage to pollen and spores over many thousands of years during this event as observed in the fossil record and that, in turn, points to a possible long-term destruction of the ozone layer. A supernova explosion is an alternative explanation to global temperature rise, that could account for the drop in atmospheric ozone. Because very high mass stars, required to produce a supernova, tend to form in dense star-forming regions of space and have short lifespans lasting only at most tens of millions of years, it is likely that if a supernova did occur, multiple others also did within a few million years of it. Thus, supernovae have also been speculated to have been responsible for the Kellwasser event, as well as the entire sequence of environmental crises covering several millions of years towards the end of the Devonian period. Detecting either of the long-lived, extra-terrestrial radioisotopes 146Sm or 244Pu in one or more end-Devonian extinction strata would confirm a supernova origin. However, there is currently no direct evidence for this hypothesis. Other hypotheses Other mechanisms put forward to explain the extinctions include tectonic-driven climate change, sea-level change, and oceanic overturning. These have all been discounted because they are unable to explain the duration, selectivity, and periodicity of the extinctions. See also Evolutionary history of plants Notes References Sources Racki, Grzegorz, "Toward understanding Late Devonian global events: few answers, many questions" in Jeff Over, Jared Morrow, P. Wignall (eds.), Understanding Late Devonian and Permian-Triassic Biotic and