Discussion of Phylogenetic Relationships
Node 1: tentacles
Node 2: large ventrolateral slime glands, esophago-cutaneous duct on the left side, elongate body shape.
Myxinidae: pair of common external branchial openings.
There is no detailed phylogeny of hagfishes. However, Myxine and Neomyxine can reasonably be regarded as forming a clade, the Myxinidae, because of their unique pair of external branchial openings. Whether the Eptratretidae (Eptatretus and Paramyxine) are a clade or not is still undecided, yet tooth morphology suggests monophyly.
The only fossil hagfishes known to date are Late Carboniferous in age (about 330 million years). They are represented by Myxinikela, from the Pennsylvanian of Illinois, which shares with extant hagfishes the tentacles and long nasopharyngeal duct, but differs from them by its stout body shape and larger caudal fin. Another fossil, Gilpichthys, from the same locality and age, displays nearly the same morphology, except for the tentacles, and is questionably referred to hagfishes.
Hagfishes have long been included in the clade Cyclostomi, together with lampreys, because of their peculiar "tongue" apparatus, horny teeth and pouch-shaped gills. The cyclostomes were regarded as the sister-group of the Gnathostomata (jawed vertebrates). There is now overwhelming evidence that the cyclostomes are paraphyletic, and that hagfishes are the sister-group of lampreys and the gnathostomes; that is, the Vertebrata in the current sense. Some zoologists, however still consider the cyclostome characteristics are synapomorphies and reject cyclostome paraphyly.
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