Rigor Vitae: Life Unyielding

Sunday, July 30, 2006

EXPLOITING THE CAMBRIAN EXPLOSION

Everyone's talking about the Cambrian Explosion, it seems. While evolutionary biologists discuss the actual rate of adaptive radiation, a handful of creationists have hailed it as proof of...something...I forget exactly what. The Cambrian Explosion was, of course, that event from half a billion years ago, when suddenly—well, relatively suddenly—over the course of about 8 million years, a great diversity of metazoan life appeared on Earth, with representatives of most of the phyla that have ever turned up on the planet. For most of the preceding 3 ½ billion years, life on Earth had been unicellular. The cause of this explosion is poorly understood. Is it an illusion suggested by gaps in our knowledge of the fossil record? Was it caused by climatic or geological changes, or the evolution of Hox genes or sexual reproduction? Or was it simply a response to opportunities that could only be exploited by multicellular life? How about a combination of factors? Whatever the case, it gives me a chance to dust off this old illustration I did for a geology calendar that my friend John Christensen and I never finished designing back in the '80s: each week was to represent 10 million years—the whole of metazoan life on Earth compressed into one year. This was to be the illustration for January, and it depicts some of those cool Cambrian denizens cavorting in a shallow sea before a stromatolite reef. So without further ado, let's introduce them:

From left to right, the arthropod Marella scuttles across the sea floor, as the priapulid Ottoia emerges from its burrow to attack the mollusc Hyolithes. A pair of onychophorans (Aysheaia) graze on sponges as three Odaraia swim by in the company of a little Sarotrocercus. Behind the sponge is the phyllocarid Pseudoarctolepis. Nearby, the peculiar mollusc-like Wiwaxia and the trilobite Tricrepicephalus creep along. Closer in is the arthropod Habelia, and closer still, another Marella. A Canadaspis feeds upon a dead trilobite. The peculiar, worm-like Amiskwia swims near the surface, while in the background, the Great White Shark of the Cambrian, the two-foot-long Anomalocaris carries a fresh-caught trilobite in its arms. In the center sits the arthropod Branchiocaris, and the flatworm-like chordate Pikaia crawls alongside another Hyolithes. To the right of Branchiocaris is the little Burgessia. In the foreground, another Wiwaxia creeps along as a Marella gleans the carapace of a Naraoia. Behind Naraoia is the trilobite Asaphiscus. Further back are three forms of sessile animal life: a pair of articulate brachiopods, several Dinomischus, and the early echinoderm Gogia. In the foreground, an acrobatic Opabinia struggles with its prey, the polychaete Canadia. Last of all, a pair of tiny Lejopyge trilobites ride air bubbles on the surface, while the little arthropod Plenocaris perches in the foreground.
______________________
illustration: BURGESS SHALE FAUNA (1989) acrylic 15" x 20"

14 Comments:

Blogger Steve Bodio said...

HOX genes sound good.

That's the first depiction of Anomalocaris I have ever seen that doesn't look like a plastic model.

1:00 PM  
Blogger cpbvk said...

Thanks! It is hard to avoid making them look like a plastic model. My secret was to have a captive in a fishtank to work from (don't tell the Fish & Game).

9:25 PM  
Anonymous Beau said...

That is really impressive, great work. I particularly like all the ecological interaction going on.

As a Postgrad studying the Ediacara fauna I would love to see your interpretation of an Ediacarian community in a similar fashion to this.

9:48 PM  
Blogger cpbvk said...

Thanks, Beau. Boy, that would involve expanding the scope of our non-existent calendar! I know virtually nothing about Ediacaran fauna. Can you recommend any good literature?

9:57 PM  
Blogger monado said...

That Anomalocaris looks positively zippy and somewhat apprehensive! No one else gives them personality.

The "explosion" may have had something to do with the development of hard parts so that more organisms were preserved: an "induration," as S E E Quine called it. Or eyes, or front ends, or teeth? Now I'm just guessing.

10:54 PM  
Blogger NelC said...

What, no Hallucigenia?

4:19 AM  
Blogger cpbvk said...

Monado: Thanks. Zippy is exactly what I was shooting for. You make a good pint about "hard parts." Thanks for the link to your post.
Nelc: Nope. When I painted it in 1989, There was so much controversy about the nature of Hallucigenia. I suspected it was just part of an organism at the time, and rather than speculate, I wussed out completely and omitted it--bad decision that I'm not proud of.

7:44 AM  
Anonymous Anonymous said...

Don't worry about missing Hallucigenia. You'd have painted it the wrong way up anyway.....

Great painting!

8:32 AM  
Blogger cpbvk said...

Thanks! Actually, I'd have probably painted a "dead" one lying on its side, kind of like I did with the Pseudoarctolepis. Painting fossil reconstructions always involves lots of speculation, and the high risk of being proven inaccurate by future discoveries. You just have to live with that.

8:56 AM  
Anonymous Roger B. said...

In my geology class we tried to persuade the teacher thaty trilobites could fly... exactly, why escapes me now (it was a very long time ago).

By the way Circus of the Spineless XI is now online at http://pinguicula.typepad.com/blog/

9:18 AM  
Blogger cpbvk said...

Now that would make a great painting! I'll get that link up to COTS pronto.

11:07 AM  
Anonymous Anonymous said...

"The cause of this explosion is poorly understood."

Which is probably why creationists use it. In my experience, they'll take any weakness in evolutionary theory as "proof" that it's a failure. Even if the weakness is simply "we don't know enough yet".

And speaking of hard parts, I've read that they also had an effect on the "explosion" by starting a kind of arms race, creating evolutionary pressures to continually evolve both better armor and better weaponry in the struggle to survive.

6:24 AM  
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