One problem of applying the scientific method to past events is there is seldom enough information to reach a proper conclusion. An obvious example is the mass extinction that we know occurred at the end of the Devonian period, and in particular, something called the Hangenberg event, which is linked to the extrinction of 44% of high-level vertebrate clades and 97% of vertebrate species. Only smaller species survived, namely sharks smaller than a meter in length and general fish less than ten centimeters in length. This is the time when most ammonites and trilobites, which had been successful for such a long time, failed to survive. One family of trilobites survived, only to be extinguished in the Permian extinction, another of those that wiped out 90% of all species.
So why did this happen? First, it is most likely the ecosystems had been stressed. The Hangenberg event occurred about 358 My ago, but before that, at about 382 My BP most jawless fish disappeared, while from 372 – 359 My BP there were a series of extinctions or climate changes known as the Kellwasser event (although it was almost certainly a number of events.) So for about 30 million years leading up to the Hangenberg event, there had been severe difficulties for life. At this stage, leaving aside insects and plants that had left the oceans, most life were in marine or freshwater environments and it was this life that appears to have suffered the most. That conclusion, however, may more reflect a relative paucity of land-based fossils. Climate change was almost certainly involved because over this period there was a series of sea level rises while the water became more anoxic. The causes of this are less than clear and there have been a numper of suggestions.
One possibility is an asteroid collision, and while impact craters can be found they cannot be dated sufficiently closely to be associated with any specific event. A more likely effect questions why anoxic? The climate should have no direct effect on this, although the reverse is possible. The question is really was it the seas only that became anoxic? One possibility is that on land the late Devonian saw a dramatic change in plant life. In the early Devonian, plants had made it to land, but they were small leafy plants like liverworts and mosses. In the late Devonian they developed stems that could move water and nutrients, and suddenly huge plants emerged. One argument is that this caused a flood of nutrients through the weathering of rocks caused by the extensive root systems to flow down into the sea, which caused algal blooms, which led to anoxic conditions. Meanwhile, the huge forests of the Devonian may have reduced carbon dioxide levels, which would lead to glaciation, and the sea level fall in the very late Devonian. However, it does not explain sea level rise earlier. That may have arisen from extensive volcanism that occurred around 372 My ago, which would enhance greehouse warming. You can take your pick from these explanations because even the experts in the field are unsure.
Accordingly, a new theory has just emerged, namely Earth was bombarded by cosmic rays from a nearby supernova (Fields, et al., arXiv:2007.01887v1, 3rd July, 2020). This has the advantage that we can see why it is global. The specific event would be a core-collapse supernova. If this occurred within 33 light years from Earth, it would probably extinguish all life on Earth, but one about twice as far away, 66 light years, would exterminate much life, but not all. The mechanism is in part ozone depletion, but there is the possibility of enhanced nitrogen fixation in the atmosphere, which might lead to algal blooms. One of the good things about such a proposition is it is testable. Such an event would bombard Earth with isotopes that would otherwise be difficult to obtain, and one would be plutonium 244. There is no naturally occurring plutonium on Earth, so if some atoms were found in the fossils or in accompanying rock, that would support the supernova event.
So, is that what happened? My personal view is that is unlikely, and the reason I say that is that most of the damage would be done to life on land, and as I gather, the insects expanded into the Carboniferous period. The seas would be relatively protected because the incoming flux would be protected by the water. The nitrate fixation might cause an algal bloom and while a lot of energy would be required to saturate the world’s oceans, maybe there was sufficient. The finding of plutonium in the associated deposits would be definitive, however. The typical deposits were black shales overlaid by sandstone, and are easy to locate, so if there is plutonium in them, there is the answer. If there is not, does that mean the proposition is wrong? That is more difficult to answer, but the more samples that are examined from widespread sources, the more trouble for the proposition.
My preferred explanation is the ecological one, namely the development of tree ferns, etc. The Devonian extinction was slow, taking 24 million years, and while most marine extinctions occurred during what is called the Hangenberg event, the word event may be misleading. That specific period took 100,000 – 300,000 years, which is plenty of time for an ecological disaster to kill off that which cannot adapt. To put it into perspective, Homo Sapiens has been around for only 30,000 years, and effective for only about 10,000 years. Look at the ecological change. Now, think what will happen if we let climate change get out of control. We are already causing serious extinction of many species, but the loss of habitat if the seas rise will dwarf what we have done so far because our booming population has to eat. We should learn from the late Devonian.