Tuesday, April 28, 2009

With Neither a Bang, Nor a Whimper

(I really try hard not to blog about paleontology. But sometimes it just creeps out unexpectedly. This post was prompted by this article in Time and some accompanying wittering on the VP listserve. If you actually know anything about paleontology, then it's probably not for you)

One of the more frequent questions that I get asked when people find out I work in a paleontology collections is "what do you think killed all the dinosaurs, eh?" I suspect that this is because the questioner is stumped for something to say and this is the only thing paleontological that they can think of. So, after I've explained to them that there are still around 10,000 species of feathery theropods hopping around the planet, we get down to the vexatious question of what killed off the "other" dinosaurs. This is when you find out whether or not the listener has been exposed to "asteroid porn."

What is asteroid porn? Well, it is a description of the effects of an impact of some form of celestial body on the Earth, usually at the end of Cretaceous (although there are some rather good examples of sci-fi astro-porn that imagine what it would be like if we got hit by an asteroid now - see the 1998 Hollywood double-whammy that was Armageddon and Deep Impact). Asteroid porn is usually found in popular accounts, being poorly suited to the rather dry discussions of bolides, tektites, and breccia lenses that accompany more scholarly publications.

Here is a brief summary of a typical piece of asteroid porn. Dinosaurs are peacefully grazing (or browsing, or doing whatever) on a warm sunny day (or at sunset, or some other time of peacefullness) when they see a big fireball fall out of the sky. It hits the Earth so hard that lava comes out, like a big bursting geological zit. The lava shoots up hundreds of miles into the air and comes down, setting fire to, like, the whole planet. All the forests are on fire, and all the dinosaurs are on fire as well. Then there's this big blast wave, and it's so big it goes round the world, like, 5 times at the speed of Concorde, and when it hits the burning dinosaurs they all get blown into burning pieces. Again and again, 5 times. Then there's this huge tsunami, that's like 10 miles high, and it also goes around the world 5 times and floods the entire planet, which puts out all the bits of burning dinosaur, but now there's no water left in the sea because it's all on the land, so all the fish and ammonites and other things that live in the sea die. And all the water from the sea goes up in the air and mixes with the soot from all the burning trees and dinosaurs and makes acid rain, and the acid rain falls back to earth and dissolves all the burnt chunks of dinosaurs and trees and also the dead fish as well, and even the fish that survived are dead because the oceans are now all vinegary. And there's still a lot of soot in the air, and it cuts out all the light from the sun for a thousand years, so that any plants that had not been burnt, blown up, drowned, or dissolved by acid can't photosynthesize. So they all die, and everything that eats plants dies, and everything else dies. And gets frozen, because there's no sunlight and it's now very cold.

OK, I exaggerated a bit.

But the fact is that once you've read a few of these accounts then you start to wonder how anything survived the K/T boundary impact. And of course, lots of things did. Aside from a least some dinosaurs (the aforementioned birdies) they included crocodiles, turtles, lizards, snakes, mammals, amphibians, fish, flowering and non-flowering plants, and enormous numbers of invertebrates. So unless you believe that these things all managed to live underground for a long, long time (and reader, there are actually people who believe that) then any impact that took place at the end of the Cretaceous can't have been on a planet-killing scale.
The other thorny problem, of course, is that a large number of prehistoric pin-up species disappear from the fossil record long before the end of the Cretaceous, such as almost all of the pterosaurs and the big marine reptiles. Non-avian dinosaur diversity and abundance seems to decline over the latter part of the Cretaceous. And while there are big die-offs in marine invertebrates and plankton at the K/T boundary, a number of these also show decines in diversity and abundance that start well before the boundary.

So, what was the cause? The answer, as is so often the case, is that no-one really knows. There were a lot of things going on at the end of the Cretaceous - widespread falls in sea levels (known as marine regression) that would have affected the most species-rich parts of the oceans; global cooling associated with long-term volcanic activity; and yes, maybe an impact as well.

So I tell people this, and usually their eyes begin to glaze over a bit when I start talking about the Deccan Traps and sea level changes. Because, after all, nothing is quite as much fun as asteroid porn.

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