by Greg Mayer

Linnaeus thought hagfish were worms, not fish, but there has been considerable controversy about which fish they are closest to. Are hagfish the earliest diverging of all extant vertebrates, or are they closer to lampreys?  The latter hypothesis, which we might call the cyclostome hypothesis (because hagfish and lampreys have been grouped in the taxon Cyclostomata), was favored for many decades. But in the late 20th century, people began to argue that lampreys were closer to jawed fish (gnathostomes), making cyclostomes paraphyletic (i.e. ancestral rather than sister to gnathostomes), which would mean that hagfish were their own group, an early and primitive branch.

In my post, I said recent molecular data had moved us back to the cyclostome hypothesis. Philippe Janvier, one of the most prominent proponents of the paraphyly hypothesis, has come round back to the cyclostome hypothesis, and has an excellent, brief, discussion of the history of the issue and the evidence.  So, hagfish, it seems, are cyclostomes. Money quote:

The results of Heimberg et al. (11) are certainly certainly the most convincing contribution ever published in support of cyclostomes monophyly…, Although I was among the early supporters of vertebrate paraphyly (6, 7), I am impressed by the evidence provided by Heimberg et al. (11) and prepared to admit that cyclostomes are, in fact, monophyletic [i.e. holophyletic].

This is, by the way, an excellent example of how a good scientist accepts new evidence, and alters his views accordingly.

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Janvier, P. 2010. microRNAs revive old views about jawless vertebrate divergence and evolution. Proceedings of the National Academy of Sciences (USA) 107:19137-19138. pdf (may not be open access)