Paleobet and Cambrian Fossils

 
PZ has discovered palaeobet¹ so, naturally, I had to post my initials as well.


Some of you may not recognize "laggania." It's Laggania cambria, one of several species related to Anomalocaris. Collectively they are known as Anomalocarids.

Here's a fossil of Laggania cambria from the Burgess Shale (right). It just so happens that I was looking at this very fossil on Saturday during our visit the the Royal Ontario Museum. The Burgess Shale fossils are stuck in a corner of the museum where they can easily be missed by people entering the dinosaur rooms. That's a shame since these are unique fossils and very few museums have such a wonderful collection of Cambrian fossils.

Most of you are probably more familiar with Anomalocaris canadensis, a much more fierce-looking cousin of L. cambria (see below). A comparision of the two species can be found on The Anomalocaris Homepage.

Anomalocaris and Laggania were among the species made famous by Stephen Jay Gould in his excellent book Wonderful Life. Gould pointed out that these species so not fit neatly into any of the existing phyla, although they have some of the characteristics of arthropods and onychophora (velvet worms).

Lumpers will now include them in Arthropoda and splitters assign them to a separate, extinct, phylum called Dinocaridida. What's clear is that there are no modern species that can trace their ancestry directly to the anomalocarids. They represent a body plan that has not survived and this lends support to Gould's idea that there were more fundamentally different kinds of animals in the past that we see today. As he put it on page 208 ...

The Burgess Shale includes a range of disparity in anatomical design never again equaled, and not matched today by all the creatures in the world's oceans. The history of multicellular life has been dominated by decimation of a large initial stock, quickly generated by the Cambrian explosion. The story of the last 500 million years has featured restriction following by proliferation within a few stereotyped designs, not general expansion of range and increase in complexity as our favored iconography, the cone of increasing diversity, implies. Moreover, the new iconography of rapid establishment and later decimation dominates all scales, and seems to have the generality of a fractal pattern.

Scientists have been chipping away at Gould's thesis over the years since the publication of Wonderful Life in 1989. Several problematic species have been reliably assigned to existing phyla and others have been tentatively squeezed into the standard animal phyla. The goal is to discredit the idea that life was more diverse (disparate) during the Cambrian and the conclusion that the evolution of animals is characterized by the extinction of major lines.

I think Gould's main point is still valid and I don't understand why so many people find it troubling. It may have something to do with people's perception of evolution as progress.

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1. Fossil animals for each letter of the alphabet.

[Hat Tip: P (pteraspis) Z (zalambalestis) Myers]