The phylogeny of arthropods has always been messy. One reason is that studies trying to discern their evolutionary relationships often use too few taxa (this is, after all, the most species-rich of all animal groups), and, especially, too few genes. Conclusions have been based, for example, on only 18S and 28S rRNA and mtDNA (the latter is, of course, effectively one gene). And this has led to conflicting conclusions, some of which contravene morphologically-based systematics.
For example, morphology seems to define a group called the “mandibulata”: all those arthropods that have mandibles. But some molecular work has lumped the myriapods (centipedes and millipedes), which have mandibles, together with the chelicerates, which don’t have mandibles but a nonhomologous biting structure called chelicerae. (Chelicerates include spiders, horseshoe crabs, pycnogonids, and the like; see Fig. 1 below).
Now, a new paper in Nature by Regier et al. has come up with a near-definitive…
View original post 389 more words