Craniate

Cyclostomes and craniates

In 1758, Linnaeus classified hagfishes as Vermes, not vertebrates. In 1806, André Marie Constant Duméril grouped hagfishes and lampreys in the taxon Cyclostomi, characterized by horny teeth borne on a tongue-like apparatus, a large notochord as adults, and pouch-shaped gills (Marsupibranchii). The cyclostomes were seen as either degenerate cartilaginous fishes or primitive vertebrates. In 1889, Edward Drinker Cope coined the name Agnatha ("jawless") for a group that included the cyclostomes and fossil groups in which jaws could not be observed. Vertebrates were subsequently divided into two major sister-groups: the Agnatha and the Gnathostomata (jawed vertebrates). In 1927, Erik Stensiö suggested that the two groups of living agnathans (i.e. the cyclostomes) arose independently from fossil agnathans. In 1977, Søren Løvtrup argued that lampreys are more closely related to gnathostomes, based on characters such as radial muscles in the fins, true lymphocytes, neuromasts in the inner ear, and a cerebellum. This implied that Vertebrata and Craniata were distinct taxa. The validity of the taxon "Craniata" was examined in 2002 by Delarbre et al. using mtDNA sequencing, concluding that Myxini is more closely related to Hyperoartia than to Gnathostomata - i.e., that modern jawless fishes form a clade called Cyclostomata. This implies that Vertebrata should return to its old content (Gnathostomata + Cyclostomata) and the name Craniata is a junior synonym of Vertebrata. In 2010, the debate concluded when the French paleontologist Philippe Janvier stated that he accepted that both vertebrates and cyclostomes were monophyletic, and that "the intuitions of 19th century zoologists were correct in assuming that [cyclostomes] (notably, hagfishes) are strongly degenerate and have lost many characters over time."