MicroRNAs Resolve an Apparent Conflict Between Annelid Systematics and their Fossil Record  

Both the monophyly and interrelationships of the major annelid groups have remained intractable, despite intensive efforts by both morphologists and molecular phylogeneticists.  Morphological cladistic analyses indicate that Annelida is monophyletic and consists of two major monophyletic groups, the clitellates and polychaetes, whereas molecular phylogenetic analyses suggest that polychaetes are paraphyletic and that sipunculids are crown-group annelids.  Both of these hypotheses are in conflict with the annelid fossil record – the former because Cambrian taxa are similar to modern polychaetes in possessing biramous parapodia, suggesting that clitellates are derived from polychaetes; the latter because although fossil sipunculids are known from the Lower Cambrian, crown-group annelids do not appear until the uppermost Cambrian.  New data – the presence and absence of specific microRNAs – show that both hypotheses are incorrect: annelids are monophyletic with respect to sipunculids, and polychaetes are paraphyletic with respect to the clitellate Lumbricus, as suggested by the fossil record.  Further, our data resolve sipunculids as the sister group of the annelids, rooting the annelid tree, and hence revealing the polarity of morphological change within this diverse lineage of animals.