Those of you that have been reading this blog for a while know that I have a fascination with mass extinctions. What causes such traumatic changes to the biodiversity, their resultant evolutionary consequences, and the what-if buried there-in happens to be a subject that absolutely fascinates me. It probably steps from the combination of interests of dinosaurs that grew when I was a very small boy, alternate history and what-if's that I picked up from SHWI, and the interest in modeling some of these events. I am sure there's a touch of that mad and half-baked world builder that all SFnal nuts have as well. In learning about these extinction events and writing them up, I have come across something that has started to annoy me a great deal. I've been putting in jabs at it from time to time, Now its time to take off the gloves and beat the subject senseless.
The YAGUMET subject in question is actually a rather human problem: we all want to remembered for doing something great, leaving our mark, or however you want to describe the problem. This extends to mass extinction theories as well. The scientists involved are perfectly human and they make human mistakes (stooo-pid hooo-mons!). One of these mistakes has risen up as I read and research to the point of a major annoyance. It feels like every single theory for a mass extinction is turned into a panacea for describing all the mass extinctions of times past. I call these offenses "Yet Another Grand Unified Mass Extinction Theory" or YAGUMET. What it boils down to is the scientists in question want to be known as the guys/gals that figured it all out for everything. Wah-lah! Wave the hands and everything falls into place. Except, it doesn't really work out that way. We have precisely two extinction events that have been solidly backed with data that we can say that we think we have them 'solved.' The problem is that both of these major extinctions have very different causes.
Explained Mass Extinctions
The first of the two 'explained' extinctions is the most famous. That would be the KT Extinction or the rock drop that saw the dinosaurs off the dance floor. The evidence is pretty solid to the extreme, Gerta Keller et al notwithstanding (see Stop Dreaming...), that the Chixiculub Crater is the grave marker for the end of the Mesozoic Era and the beginning of the rise of the mammals. There are some details to be worked out, but it appears that there was a nasty cold and darkness snap combined with the direct impact effects that would see the dinosaurs off. Anything that relied on the photosynthetic cycle and had a higher than a certain amount of caloric intake requirements was wiped out for sure with others being less lucky. This whole scenario was brought forward by Alvarez et al and has held sway since for the KT Event as the research grew to support the hypothesis and it made its transition to a solid theory during the 1980s and early 1990s. It's chronicled elsewhere and in quite a bit more detail than I can ever do here. I'll do a post in the future to cover the KT Extinction, but that's for another time. However, I will say that they even believe they know where the asteroid came from that took out the dinos, exactly.
On the other hand, the Permian Extinction (or Permian Triassic Extinction) was even more profound than the one that was first successfully investigated. More types of critters died there than at the KT Boundary. While the KT Extinction saw a massive pulse of life dying, it also saw life recover PDQ. While the actors on the stage were not at all the same as before, there was not a twenty million year recovery phase like at the PT Boundary nor were there multiple pulses of extinctions like the PT Event either. The pattern of dying was rather different than at the KT Event too. The PT Event had the deep water critters getting wiped out first. Then the extinctions crept up from the depths to the shallows and coastlines and from there to the land whereas the KT extinction had the ecologies that were photodependent (like the phytoplankton based ecologies in the shallow marine waters or terrestrial ecologies) go dead all quickly while the deep water ecology was not nearly as brutalized. That would strong suggest that the KT and PT Events had rather different causes. As research has shown in the last five to ten years, this has turned out to be true: the PT Event was caused by the Siberian Traps - flood basalt volcanoes - triggering a nasty, prolonged feedback cycle that munched life in a big way. Read more here.
The problem is that the people investigating the extinctions have gelled into two groups: the vulcanists and the impactists. The vulcanists have proposed that volcanoes and their effects are the root cause for all mass extinctions. The impactists are predictably arguing that bollides - meteors of large size in this case - from space are the root cause of all the mass extinctions. The impactists have gone as far as to claim that every 26 to 30 million years there is another round of impacts that causes another pulse extinctions as per the Periodicity Theory espoused by Raup et al (re Nemesis). It does not appear to be the case, but we'll visit that again another time with another post. In both cases, both sides have argued theirs is the One True Way and that the other is incorrect at best. The most either will concede is that maybe one of the extinctions was caused by the other side's mechanism, but all the rest are their's (Courtillot, most famously, on the vulcanist's side conceding the KT, frex, but maintaining that the rest are volcano victims). Either way, they both maintain that their theory is the Grand Unifying Mass Extinction Theory. For a time, both sides would argue and sort through data to try to make their case, but until recently neither could make a solid case for their extinction mechanism to have caused another of the Big Five mass extinctions. That appears to be the changing.
The Underexplained Mass Extinctions
Recently, I blogged about the Late Triassic Mass Extinction. The science is so fresh and new that the mortar between the bricks is extremely wet. For the moment, it does appear that the vulcanists have a very strong lead in this case. They have a lot of data that signatures of the mass extinction seem to correspond to the LTE being caused by volcanoes run amok, but on a smaller scale than the Permian Extinction. Specifically, it appears that the fissioning of the supercontinent to create the Atlantic was the culprit. Furthermore, the science is very very solid and the evidence for impact has faded very badly as the geochronology of the Late Triassic has been sorted out. However, this work hasn't had the time for the responses that the other two extinctions have had, so I cannot in good faith say that the case is closed. Yet it does appear that a convincingly vulcanism initiated mass extinction at the T/J Boundary would bolster the vulcanists' position, right? Well, uh, there are two other extinctions that are part of the Big Five that ought to be considered. One does have a cause that appears to be agreed on and the other is just a mess. Neither fits either of the GAMETS!
The one that is the mess is the Late Devonian Mass Extinction. Wignall and Hallam called it 'one damned thing after another.' There were three pulses. At the middle and nastiest pulse was the so-called Frasnian/Famennian Boundary. Interestingly, there happens to be a crater smack in the middle of this. At the same time, there's evidence of anoxia in the oceans. That would be a symptom of warm temperatures which are in turn a symptom of rampant vulcanism. There was also a flood basalt eruption around that tiume period according to Courtillot. The multi-pulse extinctions, as superficially appears here, seems to hint that this might be tied to those eruptions. However, there's evidence for glaciation which might be related to the evolution of trees. Hallam and Wignall point of that a combination of regression and then transgression of the oceans...which may or may not be related to the glaciation. Or vulcanism. Ward puts forward the idea that its all hypoxia's fault and its even responsible for Romer's Gap. Right now, there's no real clear theory that can explain the situation as we understand it during the LDME. More research needs to be done. A lot more. To make matters worse, there is one really serious problem with studying the LDME: the geochronology of the whole Period is royally frakked up. Van Valen, in a somewhat dated quote, stated "The Late Devonian extinction is one where not even the major facts are agreed on yet." Until they are, the timing of various events are going to be extremely difficult to pull together for a sound theory of what happened. That said, what evidence that is available is not terribly clear.
So at this point we have a clear meteor strike, one clear vulcansim, one probable vulcanism, and one mass extinction that's up in the air. That's not terribly conclusive for any one theory and would cast doubt, at least in my mind about any single theory having a preeminent position to explain all mass extinctions. If what has been highlighted so far should raise the doubts, what is going to written about next ought to cast serious doubt about this position for any of the previous theories being all encompassing.
The Rebel (with a cause) Extinction
There is one more mass extinction in the so-called Big Five. It is the Ordovician Mass Extinction. I have to admit that to me this is the least interesting extinction. It was a time when the ecologies of the world were restricted almost exclusively to the marine or other aquatic environments and, personally, I find that the terrestrial environment is far, far more interesting. That said, there is a fascinating and completely unexpected twist to the extinction causes: the consensus among the workers in the field that are concentrating on the Ordovician Extinction is that it was not a meteor nor volcanic eruption. It was caused by loss of habitat due to glaciation. Both the impactists and the vulcanists have tried to claim that this was caused by their preferred method, but the evidence isn't there. The method of mass death by vulcanism is through global warming, anoxia, and hydrogen sulfide poisoning with a good sterilizing blast of UV radiation. To make matters worse, the geochemical evidence isn't in favor of a volcano run amok either. Impacts kill with cold snaps, even months long ones with accompanying - global - darkness. However, based on the KT Extinction, it doesn't trigger an ice age whatsoever. That would mean there would have had to have been a string of timely impacts that would alter the climate on a prolonged period...which is highly unlikely. Unfortunately the for impact folks, there's no evidence of such at the Ordovician's end. What does this mean? It means that all the work done on the Ordovician Mass Extinction to date contradicts the idea that one theory with exceptions can explain all mass extinctions: this one was caused by a rapid change in climate from a warm period to a cold period and back again: this was due to the loss of the shallow water habitats and when they were regained, the anoxic/hypoxic waters flooding them.
To Live and Die in the Phanerozoic
If we were to take what we know about the Big Five Mass Extinctions and take the latest research as solid enough for us to call what they've theorized with strong backing evidence, it appears that there is no single theory that can be used to cover all the events with an exception here or there at this point. There would be only two extinctions right now with a common cause: the Permian and the Late Triassic seem to be united by vulcanism. The KT was bollide induced. The Ordovician seems to have glacier/climate changes as its root cause. Finally, the Devonian is up in the air. From what it looks like there it could be multiple reasons: Erwin's Midnight Express raises it's ugly head again. However the research is underway there and hopefully in ten years we'll have some better information.
There are still a host of lesser mass extinctions that need explaining. It's quite likely that those extinctions will have been caused by one of the three explanations that have been seen here: meteor, glacier, & lava if you will, something of a mass casualty paper-rock-scissors "game." Those extinctions need to be looked at in detail. Carefully. Meticulously. Then, when the data has been compiled, and the profiles compared to the baselines, then their causes can be figured out. It is not until then that we will really be able to say what killed what when and how much. Most likely nothing will come out as the dominant killer. There are simply too many ways to wipe out life on a grand scale. Wishing and throwing temper tantrums that your way is the right way for everything is a bad case of Teh Stoopid.
As I have said before, mass extinctions are not events that you can simply graft your favorite theory onto for their cause. Even if the timing is approximately right your an event you think might have caused the extinction you still have to go back and check all the other data besides the timing (see Stop Dreaming again). Mass extinctions are perfect storms. They have to be the right perfect storms though. I said it before and now let me do a bit more of a generalization: these "baseline" extinctions - the Ordovician, Cretaceous, and Permian - all three affected different organisms in different ways. Trying to pound them into the wrong shaped hole is a waste of time. Instead of becoming zealous advocates and fighting resource wasting sometimes personality driven wars, it would be far better if scientists would move over to the idea that there are diagnostic characteristics for different ways of killing off organisms in large batches. Use those, forgive me, checklists to rule out or confirm a theory. The "checklist" happen to be more than merely whether or not there was a crater or a large scale basalt eruption that is geochronologically convenient.
One of the really nice aspects about moving to the checklist instead of espousing Yet Another Grand Unified Mass Extinction Theory is that there's always the possibility that the extinction event will be because of something new, something that doesn't fit a lava-meteor-glacier profile and you will have been able to make a case for something unique very, very quickly because you are approaching it from an unbiased position instead of a "I'm out to prove theory X!" one. To me, at least, a lot more credibility is to be found there.
So, please, please, please, PLEASE, can we render the YAGUMET extinct?
(that almost needs a blink tag)
There's no ONE TRUE WAY to kill to cause a mass extinction.
1. There are guerrillas in both cases waging what looks like a lost cause. However, science isn't the march of armies and the logistics are not the ones that the militaria types have to worry about, it still remotely possible the guerrillas in the proverbial paleontology hills will come out on top. It's probably less than a 2% chance, but it's still there.
2. To be fair there are other contenders for YAGUMETs. One of those is that the sun's orbit around the galactic plane goes up above and below the disk. Claims have been made that when this happens, life takes it in the shorts. It could be that there is a lot more radiation and that life gets killed off by this mechanism. The problem is that at least to date, there's no support for this in the isotope ratios to date. Another candidate is climate change. There are more.
3. There's another cause, too, but we'll get to that after I do the KT Extinction post. It's more controversial than most. With good reason.