Spider Evolution: A Theory in Crisis

Sea spiders look so similar to land spiders, everyone would have thought they were related.  They differ, however, in several significant ways, said Graham Budd and Maximilian Telford in Nature:¹  ’Their bodies are so slender that the digestive systems and gonads are squeezed into their limbs; they possess a forward-pointing proboscis with a terminal mouth; and the males brood the eggs.”  Now, additional observations “are bound to provoke controversy in an already acrimonious field,” the field of spider evolution.  Organs called chelifores near the proboscis of sea spiders are not related to the chelicerae of land spiders, reported Maxmen, Browne et al. in the same issue,² because they originate from different parts of the head.  “The association of chelifores and chelicerae with different parts of the brain implies that the two types of limb are not equivalent, but are derived from different segments,” Budd and Telford said.  These observations will “shake up the field of arthropod evolution.”
    A check under the hood shows there is more trouble in the engine of arthropod evolution.

This result cuts across previous results based on adult structure, and to see the wider implications we need some historical background.  The composition of the arthropod head is one of the bitterest and longest-running problems in animal evolution.  Unresolved after more than a century of debate, this sorry tale is (in)famously known as the “endless dispute”. (Emphasis added in all quotes.)

The only way to salvage the evolutionary model is to assume that sea spiders “are extraordinary living fossils, retaining an organization of their head that all other living arthropods lost hundreds of millions of years ago,” Budd and Telford suggested.  The caption of a phylogenetic chart explains how both possible interpretations are distasteful:

a, If pycnogonids branched off before the appearance of insects, crustaceans, myriapods and arachnids, we can interpret their protocerebral chelifores (red) as equivalent to the supposedly anterior great appendage of fossil groups such as Anomalocaris.  The labrum (green) would have evolved in the common ancestor indicated with a star.  b, But if pycnogonids are related to arachnids, then either their protocerebral chelifores could be an atavistic re-evolution of the great appendage, or the labrum must have evolved independently in arachnids and the other three taxa.  Both of these latter hypotheses are contentious, and could raise doubts about the conclusions of Maxmen and colleagues.

The former interpretation, taken by Maxmen et al., is that the chelifores are examples of convergent evolution.  “Pycnogonid chelifores and chelicerate chelicerae are convergent structures,” they decided, “innervated from different segmental neuromeres.”  Budd and Telford don’t seem ready to swallow that line.  They ended their analysis with more bitter words:

The conclusions of Maxmen et al. overturn entrenched ideas about the body plan of the sea spiders and, furthermore, lend support to some controversial theories of arthropod evolution.  Unlike their terrestrial analogues, sea spiders lack a poisonous bite, but this paper is bound to inject venom into what is already one of the most controversial of all zoological topics.

¹Graham Budd and Maximilian Telford, “Evolution: Along came a sea spider,” Nature 437, 1099-1102 (20 October 2005) | doi: 10.1038/4371099a.
²Maxmen et al., “Neuroanatomy of sea spiders implies an appendicular origin of the protocerebral segment,” Nature 437, 1144-1148 (20 October 2005) | doi: 10.1038/nature03984.

You probably didn’t even know that the Darwinists had this problem.  Behind the scenes, they have been injecting each other with venom and battling each other for over a century about where arthropods fit in the evolutionary tree, all the while telling the rest of us evolution is a fact.  Should we feel sorry for them?  Do you feel sorry for someone who builds a sand castle on a fault line?