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Hagfish, which resemble lampreys, have traditionally been considered the sister taxon of the true vertebrates (lampreys and gnathostomes) [36] but DNA evidence suggests that they are in fact the sister taxon of lampreys. [37] The heart of the lamprey is anterior to the intestines.
For a fuller treatment of this taxonomy, see the vertebrate article. The position of hagfish in the phylum Chordata is not settled. Phylogenetic research in 1998 and 1999 supported the idea that the hagfish and the lampreys form a natural group, the Cyclostomata, that is a sister group of the Gnathostomata. [3] [4]
Since then, it has been reclassified by many workers as having many different affinities, such as an "unspecialized anaspid", [6] or as a sister taxon to the lampreys, [2] its difficulty in classification due to difficulties in reconstructing the anatomy; [2] it does not possess any usual chordate synapomorphies.
Cyclostomi, often referred to as Cyclostomata / s ɪ k l oʊ ˈ s t ɒ m ə t ə /, is a group of vertebrates that comprises the living jawless fishes: the lampreys and hagfishes.Both groups have jawless mouths with horny epidermal structures that function as teeth called ceratodontes, and branchial arches that are internally positioned instead of external as in the related jawed fishes. [1]
Dumeril (1806) [11] 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 (Marspibranchii). Cyclostomes were regarded as either degenerate cartilaginous fishes or primitive vertebrates.
Newer research suggests this pattern of evolutionary relationship is incorrect. Extensive molecular phylogenetic analysis has shown convincingly that the Cephalochordata is the most basal subphylum of the chordates, with tunicates being the sister group of the vertebrates.
Molecular data, however, indicates they are vertebrates closely related to lampreys. [49] [50] An older view is that they are a sister group of vertebrates in the common taxon of Craniata. [51] In 2019, Tetsuto Miyashita and colleagues reconciled the two types of analysis, supporting the Cyclostomata hypothesis using only morphological data. [52]
Most biologists regard this taxon as extinct, but the name is sometimes used in the classification of lampreys, because lampreys were once thought to be related to cephalaspids. If lampreys are included, they would extend the known range of the group from the Silurian and Devonian periods to the present day .