Is this the end of Paradoxopoda?
Arthropods are the most successful phylum of animals that ever lived. The “true arthropods,” the Euarthropoda, which excludes the velvet worms (Onychophora) and the water bears (Tardigrada) (members of both of these phyla are sometimes referred to as arthropods), represent over 80% of living animal species and comprise over 1.1 million species. Arthropods have external skeletons, segmented bodies and jointed legs. It is generally agreed that Arthropoda is a monophyletic group; that is, all arthropods have a common ancestor. It has even been suggested from the fossil record that arthropods pre-date the so-called Cambrian explosion and arose in the in the Ediacaran Period (see, e.g., M.F. Glaessner, “New Fossils from the Base of the Cambrian in South Australia,” 81 Trans. Roy’l Soc’y of South Australia 185 (1958) (open access pdf file)).
The problem, however, is how to organize the classes within Arthropoda. Euarthropods consist of: horseshoe crabs, spiders, mites and scorpions (Chelicerata), insects (Insecta), millipedes, centipedes (Myriapoda), lobsters, shrimp, crabs (Crustacea), and marine sea spiders (Pycnogonida). As Amy Maxmen explains in a Nature News report of August 10, the problem has arisen pitting morphologists against molecular biologists. Morphologists, who tend to trust what they see, grouped myriapods, insects and crustaceans together based on the morphology of the head (the have similarly segmented heads, mouth parts and mandibles) and are therefore grouped into Mandibulata. This group is distinguished from the chelicerates (spiders, etc.), which developed pincers instead of mandibles. The mandibles of the mandibulates are derived from the legs of the third or fourth segment.
Some molecular biologists, however, have erected a phylogeny which puts myriapods (centipedes and milipedes) in the same clade as spiders, scorpions and horseshoe crabs in a clade called “Paradoxopoda” by some, but Myriochelata, by those presumably contending the proposal not to be ludicrous. The molecular biologists require the morphologists to disbelieve their “lying eyes” because constructing such a clade would mean that either the mandible arose the same way several times in Arthropoda or it disappeared in all Paradoxopods other than the myriapods without leaving a trace. As an intuitive matter, this seems to violate the principles of parsimony. So it is a great relief to all concerned that Omar Rota-Stabelli, of the University College London, and others, wrote “A congruent solution to arthropod phylogeny: phylogenomics, microRNAs and morphology support monophyletic Mandibulata.” This paper seems to be so welcome that the Royal Society has made its full text freely available online for those of us who waiting to see what the Discovery Institute would have to say about it in court papers. The authors used micro-RNA analysis to put a stake through the heart of Paradoxopoda:
“we address this issue by analysing two independent molecular datasets: a phylogenomic dataset of 198 protein-coding genes including new sequences for myriapods, and novel microRNA complements sampled from all major arthropod lineages. Our phylogenomic analyses strongly support Mandibulata, and show that Myriochelata is a tree-reconstruction artefact caused by saturation and long-branch attraction. The analysis of the microRNA dataset corroborates the Mandibulata , showing that the microRNAs miR-965 and miR-282 are present and expressed in species sampled, but not in the chelicerates. Mandibulata is further supported by the phylogenetic analysis of a comprehensive morphological dataset covering living and fossil arthropods, and including recently proposed, putative apomorphies of Myriochelata.”
This appears to be the end of Paradoxopoda. It might be of some consolation to the spiders, scorpions and horseshoe crabs that sea spiders (pycnogonids) might be part of their group (Chelicerata), under the same analysis, and that including them keeps the clade monophyletic (i.e., having a single ancestor). Unfortunately, however, Cycloneuralia becomes paraphyletic, but all we can do is let nematodes fend for themselves.
Not satisfied to simply make morphologists happy, the authors went on to heap additional scorn on previous molecular phylogenetic analysts. It seems that they failed to find the correct relationships owing to the dreaded “long branch attachment” effect. But we will draw the curtain over this shame to molecular biologists. Those wishing to see the true degradation that molecular phylogenetics can descend to when it runs amock will have to read the paper.