Monday, August 07, 2006

The Devonian Mass Extinctions

I have been doing a lot of reading about the mass extinctions that have bedeviled or blessed our ecosystems through time. Part of the reason for this was the discussions that Carlos and I had. Part of this would be because once upon a time, I thought I would be a paleontologist, but alas, that didn't happen. The interest never faded so much as I put it on the back burner. Since those discussions I have been upping that priority. I un-niced it for those computer geeks out there. Finally, its necessary research for a novel setting I am cobbling together. That 'Great Vision' novel that I mentioned before needs a bit of research and this is part of it.

Everyone has heard of the KT Event when the dinosaurs, ave our feathered friends, got wiped. This is probably the singular most popular extinction to study and discuss. The results were pretty profound. The reasons are pretty obvious these days. The studies that have been done are pretty extensive. A few less have heard of the Permian Extinction where a lot more species died out, but it has been popularized a lot in the last decade so that more people have heard of it. There are a few other mass extinctions that people haven't heard so much about. The Ordovician was older and pretty important if largely kept to the sea.

There are two that are a bit more controversial. They may not have even happened: The Cambrian extinction so popularized by the late Stephen J Gould and the End Triassic may not have even been extinctions at all, or so its starting to appear, but rather are just statistical blips (so say recent sources) that are from sampling biases. That stance though is pretty controversial, especially with respect to the Triassic-Jurassic Boundary. The Cambrian Extinction seems to be largely consigned to history, but that may not remain so.

Of course, there's the one going on right now. This is supposed to be the worst extinction since the KT Event. It's ongoing and very political since we - humanity - are the primary, and possibly sole cause. It's a bit depressing, but ought to be considered in the Drake Equation that intelligent species might bring about their own demise ecologically. I have faith in humanity to make it through, but I mourn the passing of so many species.

However, there is another extinction event that hasn't been touched on as much. That is the Late Devonian Mass Extinction:

The Late Devonian Event is particuarly controversial at present. Indeed, one evolutionary biologist has bemoaned, "The Late Devonian extinction is one where not even the major facts are ageed on yet" (Van Valen 1984).

Or so sayeth George McGhee in his work on the subject, The Late Devonian Mass Extinction. Hallam and Wignall called it "one damn thing after another" in their book Mass Extinctions and their Aftermath. Unfortunately, it doesn't look like anything is really agreed by anyone about anything here with respect to this extinction except that a a helluvalota species died.

One of the best things to start with is the fact that there is a lot of disagreement about exactly when the timing of events happened in the Devonian. The Mesozoic, the Tertiary, and even the Permian (end Paleozoic) are very well documented in terms of their geochronology compared to the Devonian. Truthfully, the Permian work has been all done in the last decade, but the Devonian seems to be nothing short of a dog's breakfast right now. The different zones are simply not well documented and the work there is very young at best (IMO) based on reading McGhee and Hallam's work. Until this is done with a lot of cross checking and the timeline generally accepted, the debate will rage for a long, long time.

Another point that has a lot of contention here is the reconstruction of the continents during the Devonian. There seems to be a lot of argument over what the positions of the continents were. The arguments go back and forth it seems over geomagnetic data and the fossils found. They are often in disagreement. The fossils for certain aquatic biomes end up in the wrong places if they are put where they ought to be for the geomagnetic fossils and vice versa for the marine zoological fossils.

The Devonian Extinctions were actually three different events that fit within the Devonian. The first was the Taghanic. None of the sources that I read discuss this one almost at all. This was during the Givetian stage and lasted into the Frasnian. The next was the one that McGhee concentrated on and Hallam and Wignall discussed quite a bit. This one is termed either the Kellwasser Event or the F-F Boundary Event. This extinction happened on the boundary between the stages of the Frasnian and Famennian geological stages.

Next it might be best to explain what happened in terms of species and fossils. While McGhee has the book developed to the whole crisis, it is actually Hallam and Wignall's work that gives a more detailed discussion of what died and lived. The one failing of H&W's work is that it glazes over the land animals to a degree that is disappointing. This isn't just true of the Devonian, but also of the other extinction events that they cover (all of them). It's to be understood since they're marine paleontologists, but...still disappointing. They do have some excellent visual aids on pages 74 and 75 on the pain that was inflicted on diversity changes. The most profound changes that they note there are the fall from grace of the agnathans (jawless fish) and the placoderms (armored fish) during the Devonian events. This had huge implications for the fish of the world because our familar actinpterygians (ray finned fish) took over after the Placs were knocked on their collective ass-fins into extinction. At any rate, the total amount of families and species that died may actually be greater through the Devonian that at the KT Event. That too is a controversial stance, but one supported by Sepkoski's great database.

The question then is, "what killed them all?" The answer is not quite so clear.

McGhee is a strong supporter of the impact theory as to at least one, if not all three peaks of the Devonian Extinctions. The so-called F-F Crisis appears to have a string of craters that impacted in a relatively linear fashion going slightly from the southwest to the northeast in the reconstruction that he favours for the continental positions. He notes that one should not take that as anything other than artifact, but after watching the impacts of Shoemaker-Levy on Jupiter, I have to wonder. The problem is with the impact theory is that, frankly, there's little supporting evidence. There are no global indicators about these impacts. If they were as devastating as the KT impact then there ought to be evidence scattered everywhere. yet. it's not. Indeed, to have three major sets of impacts at tens of millions of years apart hsn't been explained either.

Hallam and Wignall favor that it was oceanic anoxia that did in the Devonian species. Once they seem to favour a global turnover of the ocean, but mostly they seem to lean very strongly towards a model where regression of the ocean shorelines takes place. Then anoxia develops in the oceans. Finally there's a transgression across the continents of the oceans agian that brings the anoxic waters up into former habitats wipes out the local species. In support of this there are tons of so-called black shales where organics were laid down, unoxydized nor munched on by bottom dwellers. On the other hand, this doesn't explain why it seems that the extinction effected the land and fresh water dwellings critters too. Anoxia of the oceans wouldn't swack them as much as it appears to have.

Hallam also wrote a more recent work that indicates that there might be some link to global cooling as well, but that link was rather tenuous.

Despite this being a very important chapter in the extinction book, so to speak, this one is largely unwritten. It had huge implications for life later in the world's history, but we really don't understand it. What ways this influenced our tetrapod ancestors and their fish and arthopod nemeses are quite important and also very poorly studied. Much more needs to be done before we can look at the Devonian Extinctions as settled as the work done for the KT Event or even the Permian Event(s).

That said, next up is the Permian Events. This one is something that almost cooked my goose, irl wrt to my job. I won't belabor that point, but the event has tempered my interest in the PT Event considerably. That said, the reading for that post is done. I actually need to go back and reread some of my notes before that post goes up. It will almost certainly happen before the end of the month though.

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