See this graph (right hand side of Wikipedia page) of extinction events over time. Five of them have been given names and are discussed as the “Big Five” extinction events.
The first thing that really leaps out at me on this graph is that there are two really spiky extinction events that ought to be added to the five to make a “Big Seven” – so why aren’t they? Both of them are in the Cambrian period. Both of them are close to 40% of all marine life dead as a doornail calibre events, making them more lethal than any other events on the chart except the big Permian event.
The second thing I notice is that the Devonian extinction event doesn’t particularly stand out against its relative backdrop of dying-off of marine species. If i were eyeballing the chart (which, umm, I am), I’d be inclined to speak of a “Big Six” and leave the Devonian off and add the two Cambrian events.
Anyway, back to the Cambrian extinctions. Why don’t they get attention and a nomenclature and so forth? Is this chart somehow misleading?
On that graph it looks like the average level of background extinction has steadily decreased over time too. I imagine this is most likely an artifact of the fossil record, but it might explain why the Cambrian spikes are counted for less than the Devonian one, despite being higher. I don’t actually know, though. Very interesting question.
There’s a couple of reasons. First, the events are poorly documented since there isn’t that much of a fossil record so early in the Cambrian. Also, absolute diversity was lower so not that many species actually went extinct. Note that the graph you link to is base on the percentage of extinctions rather than absolute number. See here for a graph that looks at absolute numbers of genera. The number of genera, and hence the number of extinctions, was small compared to later extinction event. The Cambrian events do have names: the Botomian and the Dresbachian.
The Devonian eventwas actually rather prolonged and took place over perhaps 20 million years or more, so it doesn’t stand out as much. It may have been made up of several distinct events.
Thanks for the links and the lessons! I figured it was partially because these earlier extinctions were way the fuck back when the fossil record was thin, but it’s also nice to know that they do indeed have names, just not official recognition as one of the “Big Five”.
