When Trilobites have a bad day

Mass extinctions are relatively common on planet Earth

© Bryan Zepp Jamieson

3/13/05

http://zeppscommentaries.com/S&E/trilobates.htm

The Guardian had a piece on the curious periodicity of extinction level events in earth’s history, stating that they occurred every 62 million years (British years, that is), and that we were overdue for the next one. I posted the story to my newsreaders under the helpful Sunday morning greeting, "Good morning. We’re all going to die. Have a nice day," and moved on to the next topic.

But something about the story nagged at me. It wasn’t that the mass die-offs occurred every 62 million years, or that the last one was 69 million years ago. It was that the number seemed too big. Or too small. I vaguely remembered that there had been the die off that killed the dinosaurs, making life much more convenient for us mammals, but the previous really big one was 250 million years ago, not 130 million. One hundred and twenty million years might not seem so long when you are preparing for a tax audit, but it seems a rather big bump on a frequency of 62 million years.

On the other hand, the ice ages caused die-offs, surely, and the last one of those was only 12,000 years ago. An eye-blink. Even in British years.

So I decided to find out where the truth lay.

Well, I don’t know if I found much in the way of truth. What I did find was that opinions on mass extinctions varied. There may have been five since the first one we know about, some 450 million years ago. Or there might have been 23. There’s five that scientists are absolutely positive about, and the rest are more conjectural, and depend, in large part, on how you define a mass extinction. One thing seems clear: we are in the middle of one right now, and it’s mostly man-made.

The good news is that life seems to bounce back and variegate with amazing rapidity once things stabilize after such events. When I was in school (shortly after the last ice age) we were taught that evolution was a slow, steady process, and were left with the impression that woolly mammoths took one hundred million years to grow an extra eyelash, and then went extinct. Apparently superfluous eyelashes was a bad move. A couple of million years later, dogs and women were domesticated, ending evolution forever. Naturally, none of us remembered any of these lessons because dinosaurs were so much cooler, even though the dinosaurs of my day were slow, sluggish, cold-blooded and died because they were stupid and politically conservative. Nobody knew about the Chicxulub crater, or had thought through the global ramifications of an asteroid strike at that point, and "too stupid to live" seemed as good a theory to explain the demise of the dinosaurs as anything.

I was correct in remembering that prior to the Cretaceous-Tertiary extinction (the asteroid that killed off the dinosaurs) the previous really big one was the Permian-Triassic extinction, 251 million years ago. It killed off 95% of all species. There are some oddities about that one that I’ll get to in a minute.

The number of mass extinctions cited depends on three factors; how big the extinction level was, how much evidence there is for it, and the percentage of species wiped out. Time frames are malleable; the Triassic extinction is described as occurring "199 to 214 million years ago" A catastrophe that takes 15 million years to unfold strikes me as a kind of a leisurely affair. On the other hand, it resulted in a sizeable ocean – the Atlantic – where there hadn’t been any ocean at all before, so maybe it’s just as well that it took its own sweet time.

The further back in time you go, the less evidence there is for extinction events, partly because fossils are destroyed by seismic events and meteorological wear and tear over the eons, and partly because back about 450 million years ago, the time of the first such event, all life was invertebrate and most had soft exoskeletons, and thus didn’t fossilize well.

The Chicxulub crater is the only thing we can point at and say, "There! That’s what caused that mass extinction!" The crater, some 150 kilometers across, is in the Gulf of Mexico and traverses the Yucatán peninsula, and is where the "dinosaur killer" asteroid struck. In all the other cases, we can’t even guess at the nature of the catastrophe that triggered the mass extinction, let alone its location. Guesses at such causes range from wild bouts of volcano eruptions to various debris from space, such as comets or asteroids, striking, to clouds of cosmic dust, and even an unnoticed companion star to the sun.

About the only thing they have in common is that there is a sharp and fairly severe change in the climate patterns. A large number of volcanic eruptions could create a fimbulwinter, or "nuclear winter," in which world-wide temperatures drop by twenty degrees or more. We know that something like that happened in the sixth century, and caused world-wide crop failures for two years running. Every advanced civilization collapsed, and in Europe, the following 500 years were known as "The Dark Ages."

But the thing about the mass extinctions is that they generally had little effect on plant life – there are a lot of plant species that were around 250 million years ago, but only one in a hundred animal species, and the effects were less severe for marine life. This is a good thing, since it means a reservoir of simpler and more adaptable life forms – insects and sea creatures – are there to replenish the biosphere when die-offs take out the more advanced creatures, such as dinosaurs.

The one exception to this is the Permian-Triassic. It seems to have hit plant and marine life almost as hard as it hit land-based animal life, and eliminated 95% of all species on earth. Nobody is quite sure what happened, but one scientist, a, Gregory J. Retallack from the University of Oregon, wrote a paper noting that the dominant life form of the Permian, a family of critters known as therapsids, all but vanished, but that one species survived into the Triassic, and Retallack noted that it had a much shorter nares and a barrel chest, suggesting that it evolved to survive in an atmosphere where the ambient amount of oxygen was around 12%, and from that surmised that some sort of wild greenhouse effect must have taken place. Retallack doesn’t try to make any firm conjectures from his data, but merely suggests it as a possibility. Other scientists have suggested that a wide-spread form of vulcanism known as mantle-plume eruptions might have pushed vast quantities of CO2 into the atmosphere, creating runaway global warming.

Of course, there is some good news out of this. We can’t begin to imagine what it was like on earth 251 million years ago that could cause such a fantastic die off. But in a remarkably short period of time – perhaps ten million years – life on earth was back to the full richness and diversity that it had been before the great Permian-Triassic die off.

We are slowly learning that evolution is a far quicker process than anyone imagined. For example, a new species of finch evolved on the Galapagos Island in the 1980s, one with a different sized beak. It arose in response to a drought which stressed the local food plants, causing them to produce smaller seeds than the regular finches could break with their existing beaks. The new species could break those seeds and access the nutrients inside.

Similarly, a scientist in Russia has, in less than fifty years, produced a domesticated fox that has such unique features as piebald fur markings, floppy ears, a tail that arches up over the spine, and which is much more tractable to humans than the untamable wild fox is. Vulpus domesticus.

So evolution is far faster, more adaptable, and less random than we had supposed. That’s the good news.

The bad news is that we are not only overdue for a mass extinction (even if the 62 million cycle story turned out to be bollocks), but we are, by the standards applied to the five biggest ones, already in the middle of one such.

And it’s pretty unlikely that this one will take 15 million years to become a problem for the top of the food chain.