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Author Topic:   Chordates in the Cambrian?
caffeine
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Message 1 of 2 (867256)
11-22-2019 5:21 PM


This may be a long post for a very short thread, but at least it keeps me off the streets.

Many creationists are very fond of the concept of the Cambrian Explosion. By ignoring details and paying no attention to what a phylum is, it's possible to take cavalier assertions that 'all the major phyla appeared at once' as some sort of support for special creation.

I don't want to talk about how bad this reasoning is, though, so much as to question how good the evidence is for the base assertion that 'all the major phyla appeared at once'. I'm going to talk about chordates, because I've been thinking a lot about early vertebrate evolution recently, and the thought struck me today that it's not even clear if there is such a thing as a Cambrian chordate.

Chordates are, of course, our phylum, and so they are of considerable interest. Possibly the most famous of Cambrian chordates is Pikaia gracilens from the Canadian Burgess Shale:

Pikaia was, of course, initially interpreted as a polychaete. Polychaetes are nothing to do with chordates - if Pikaia was a polychaete, then it belongs in the same phylum as the earthworms in your garden, notin ours. Here's a living polychaete:

The reason for this radically different interpretation is a bunch of weird appendages and tentacles visible in the 'head' of Pikaia fossils that do not make sense for a chordate; the absence of any evidence of gills; and the fact that it's body segments are not the same shape as chordate ones. Despite it's popular status as iconic poster-rock for early chordates, Pikaia is still considered a protostome (and so very distantly-related) by some workers.

We have much more than the Burgess Shale to rely on for chordate origins, though - thanks to the discovery of the fantastically preserved Chengjiang fauna in China. Here, we can find vetulicolians:

Vetulicolians are, to use the technical terminology, weird as fuck. Though sometimes considered chordates, they were originally interpreted as arthropods - like insects and trilobites, because they look in some ways a lot like old arthopods with segmented posterior and a bivalved carapace. I say 'originally', but that should not give the impression that this is a view of the past - some still hold it today. Vetulicolians are often considered to be closely related to Banffia, and animal from the Burgess Shale. Here it is:

I didn't mention Banffia together with Pikaia because, well, it looks nothing like a chordate. It has no sign of gill openings (erm.. like Pikaia and has appears to have hindgut diverticula - which imply it's a protostome (ie. not a chordate). Nevertheless, it is also very similar to vetulicolians.

A cladistic analysis done in 2007 produced an interesting result. Including only two vetulicolians led to them being classified as sister group to tunicates (a type of chordate). Including other vetulicolians, however, completely changed the analysis. Now they showed as the sister to kinorynchs (a type of arthropod). To understand how little sense that makes, see below some living tunicates and a kynorynch:

That article, by the way, is worth a read if you're interested in the ideas I'm trying to cover in brief here, since it speculates about a dozen different wildly different hypotheses of what the hell vetulicolians actually are.

Not every fossil 'chordate' is as weird as this. Yunnanozoans are widely accepted as chordates. Here's Haikouella, a good chordate:

Except, is it? An exceptionally well-preserved yunnanozoan was discovered about 20 years ago, and the researchers who described it noted that it seemed to lack the diagnostic features of chordates. They allied it with vetuicolians, which as we've seen tells us little.

What is the point of all this rambling? I'm not really trying to claim there are no chordates in the Cambrian (for less controversial examples see, for example Metaspriggina and Myllokunmingia). My point here was more to show how meaningless the appearance of chordates in the Cambrian is for creationists. The animals we're discussing are not similar to anything alive today. Some are indeed representatives of living phyla. But the fact that scientists cannot agree on which phyla, if any, they represent is not the pattern we should expect from the creation of immutable species. This is the pattern of evolution from a common ancestor, where the first chordates, echinoderms, annelids and arthropods are still similar enough to each other to mistake for one another.


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Message 2 of 2 (867348)
11-23-2019 5:39 PM


Thread Copied to Biological Evolution Forum
Thread copied to the Chordates in the Cambrian? thread in the Biological Evolution forum, this copy of the thread has been closed.

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