Nature's whack at the Dinosaurs' grandparents

kw: book reviews, nonfiction, geology, paleontology, palaeontology, disasters, extinction

My favorite science authors write about difficult concepts in ways that those with less training can understand, or at least appreciate. Douglas H. Erwin has written a great detective story, with the culprit still not fingered, but whose outline is beginning to emerge from the shadows. Extinction: How Live on Earth Nearly Ended 250 Million Years Ago is about an event even more significant for life on Earth than the Dinosaur-killing asteroid that whopped into the proto-Caribbean some 65 million years ago.

What is the Permian? It isn't just a high school in central Texas, nor only a series of ridges the school is named for. Those ridges are named for a geological period four times as distant in time as the demise of the dinosaurs. In Texas, the Permian reefs are a fantastic source of oil, and oil is the heart of Texas's economy. But the Permian is famous for more than that.

To a geologist, the Permian era lasted from 298 million years ago to just before 250 million years ago (abbreviated "ma" hereafter). It is one of ten major divisions of "deep time", stretching from about 2 ma to about 600 ma. It is very nearly in the middle. One of the later ones, the Cretaceous, ended with the asteriod impact that took out all the larger dinosaurs, leaving us with the little ones, that became the birds. In each of these eras, rocks worldwide contain specific kinds of fossils, and record a suite of ecological conditions that changed rather abruptly from one to the next. At least five of these abrupt changes included a mass extinction, a short interval (a million years or so? or perhaps only a few years...or less?) during which many species vanished and after which new species arose. The great extinction at the end of the Permian era is the most severe.

We know the following: about 90% of marine species, and 75% of land species became extinct. This compares to about 25% marine and 20% land species that died with the dinosaurs in the Cretaceous extinction event. The kinds of rocks being laid down everywhere changed, from massive limestone reefs to thinly layered mudstones and clays. The thin layers are particularly clean because there were no bottom-feeding critters to stir them up or burrow through them...for a few million years. Even for those species that weren't eliminated, only perhaps a percent or two of the individual animals or plants then living survived the catastrophe.

What catastrophe? Another asteroid? Lotsa volcanoes? What?? Dr. Erwin makes it clear he cannot give us a final answer, but he is able to eliminate some possibilities, and outline the most likely few that are able to explain the clues that we have.

The fact is, we have lots of clues. Problem is, they are not sufficient to define exactly what happened or in what order. Finely precise age dating of rocks has yielded an upper limit for how long the disaster took: 160,000 years. Other indications hint that it was probably much, much shorter, but the evidence for that is still being eagerly and energetically gathered. Further, the destruction of land species and marine species occurred at the same time. Thus, we can rule out suspects that operate slowly, such as mega-volcanic eruptions occurring over millions of years. We can also rule out most scenarios in which land species died out because the marine species died; we retain only those in which the marine die-off was so abrupt and so pervasive that the land die-off followed within a very short time.

However, we cannot completely rule out mega-volcanoes, if we can determine that the huge volumes of basalt that were erupted in Siberia at that time were emplaced quickly enough. It is kind of hard: The volume of a small continent was laid atop central Asia. Could that have happened in a matter of a few thousand years? Imagine hundreds of Mt Pinatubo, or thousands of Mt St Helens, going off every year for thousands of years. Actually, the style of eruption was more liquid, like tens of thousands of Kilauea eruptions, every year. It beggars belief.

The author presents all the suspects in Chapter 2, presents and explains all the evidence in several following chapters, then discusses the current understanding in the last two chapters. It is fascinating. He tends to favor what he calls Murder on the Orient Express, in which all the suspects participated in a murder. Scientists are not comfortable with scenarios that include too many players. It smacks of "throw in enough special cases until you have everything covered." But it is unlikely that such a significant event in life's history on Earth could have a single, simplistic cause.

I once favored another asteroid hit, bigger than the dino-killer. There may indeed have been one, though perhaps not any bigger. Dr. Erwin presents Jack Sepkoski's proposition, that some kind of comet or asteroid has clipped Earth about every 26 million years... but he doesn't go into it. The mega-volcanic episodes are much rarer, only every 200-300 million years, perhaps. Should an asteroid have barged in just as the Siberian eruption was well under weigh, it could have provided sufficient extra mayhem to make the effects of the eruptions much deeper and more widespread. If big space rocks are landing on a regular schedule, one is bound to arrive when something else significant is happening. So I also now favor the multi-cause scenario.

The mystery isn't yet solved. There's lots of exciting work to be done. What fun!