Cephalaspidomorphs are a taxon of jawless fishes named for the cephalaspids, a group of osteostracans. Most of the members of this group are extinct; however, it interests modern biologists because it may include the lampreys. If so, the lampreys extend the known range of the group from the Silurian and Devonian periods to the present day.

In the 1920s, the biologists Kiaer and Stensiö first recognized the Cephalaspidomorphi as including the osteostracans, anaspids, and lampreys, because all three groups share a single dorsal "nostril", now known as a nasohypophysial opening.

Since then, opinions on the relations among jawless vertebrates have varied. Most workers have come to regard the agnatha as paraphyletic, having given rise to the jawed fishes. Because of shared features such as paired fins, the origins of the jawed vertebrates may lie within the Cephalaspidomorphi. Some biologists no longer use the name Cephalaspidomorphi because relations among osteostraci and anaspida are unclear, and the affinities of the lampreys are also contested. Others have restricted the cephalaspidomorphs to include only groups more clearly related to the Osteostraci, such as Galeaspida and Pituriaspida, that were largely unknown in the 1920s.

Lampreys

Many reference works still regard Cephalaspidomorphi as a Linnean class whose sole living representaties are the lampreys.

A lamprey (sometimes also called lamprey eel) is a jawless fish with a toothed, funnel-like sucking mouth.

Mouth of a river lamrey

Lampreys live mostly in coastal and fresh waters, although at least one species, Geotria australis, probably travels significant distances in the open ocean, as evidenced by the lack of reproductive isolation between Australian and New Zealand populations, and the capture of a specimen in the Southern Ocean between Australia and Antarctica. They are found in most temperate regions except Africa. Their larvae have a low tolerance for high water temperatures, which is probably why they are not found in the tropics.

Outwardly resembling eels, in that they have no scales, an adult lamprey can range anywhere from 13 to 100 centimetres (5 to 40 inches) long. Lampreys have no paired fins, large eyes, one nostril on the top of the head, and seven gills on each side. The unique morphological characteristics of lampreys, such as their cartilaginous skeleton, mean that they are the sister taxon (see cladistics) of all living jawed vertebrates (gnathostomes) and are not classified within the Vertebrata itself.[citation needed] This is disputed by some, who place lampreys within Vertebrata. Hagfish, which superficially resemble lampreys, are the sister taxon of the lampreys and gnathostomes (a clade termed the Craniata).

Studies reported in Nature suggest that lampreys have a unique type of immune system with parts that are unrelated to the antibodies found in mammals. They also have a very high tolerance to iron overload, and have biochemical defenses to detoxify this metal.

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Taxonomy

The taxonomy presented here is that given by Fisher, 1994. This work classifies lampreys as the sole living members of the class Cephalaspidomorphi. The lampreys entail the single order Petromyzontiformes and family Petromyzontidae.

Within this family, there are 40 recorded species in nine genera and three subfamilies:

• Subfamily Geotriinae
  • Genus Geotria
    • Pouched lamprey, Geotria australis (Gray,1851)
• Subfamily Mordaciinae
  • Genus Mordacia
    • Mordacia lapicida (Gray, 1851)
    • Mordacia mordax (Richardson, 1846)
    • Mordacia praecox (Potter, 1968)
• Subfamily Petromyzontinae
  • Genus Caspiomyzon
    • Caspiomyzon wagneri (Kessler, 1870)
  • Genus Eudontomyzon
    • Eudontomyzon danfordi (Regan, 1911)
    • Eudontomyzon hellenicus (Vladykov, Renaud, Kott and Economidis, 1982)
    • Eudontomyzon mariae (Berg, 1931)
    • Eudontomyzon morii (Berg, 1931)
    • Eudontomyzon stankokaramani (Karaman, 1974)
    • Eudontomyzon vladykovi (Oliva and Zanandrea, 1959)
  • Genus Ichthyomyzon
    • Ichthyomyzon bdellium (Jordan, 1885) - Ohio lamprey
    • Ichthyomyzon castaneus Girard, 1858 - chestnut lamprey
    • Ichthyomyzon fossor (Reighard and Cummins, 1916) - northern brook lamprey
    • Ichthyomyzon gagei (Hubbs and Trautman, 1937) - southern brook lamprey
    • Ichthyomyzon greeleyi (Hubbs and Trautman, 1937) - mountain brook lamprey
    • Ichthyomyzon unicuspis (Hubbs and Trautman, 1937) - silver lamprey
  • Genus Lampetra
    • Lampetra aepyptera (Abbott, 1860) - least brook lamprey
    • Lampetra alaskensis (Vladykov and Kott, 1978)
    • Lampetra appendix (DeKay, 1842) - American brook lamprey
    • Lampetra ayresii (Günther, 1870)
    • Lampetra fluviatilis (Linnaeus, 1758)
    • Lampetra hubbsi (Vladykov and Kott, 1976) - Kern brook lamprey
    • Lampetra lamottei (Lesueur, 1827)
    • Lampetra lanceolata (Kux and Steiner, 1972)
    • Lampetra lethophaga (Hubbs, 1971) - Pit-Klamath brook lamprey
    • Lampetra macrostoma (Beamish, 1982) - Vancouver lamprey
    • Lampetra minima (Bond and Kan, 1973) - Miller Lake lamprey
    • Lampetra planeri (Bloch, 1784)
    • Lampetra richardsoni (Vladykov and Follett, 1965) - western brook lamprey
    • Lampetra similis (Vladykov and Kott, 1979) - Klamath lamprey
    • Lampetra tridentata (Richardson, 1836) - Pacific lamprey
  • Genus Lethenteron
    • Lethenteron camtschaticum (Tilesius, 1811)
    • Lethenteron japonicum (Martens, 1868)
    • Lethenteron kessleri (Anikin, 1905)
    • Lethenteron matsubarai (Vladykov and Kott, 1978)
    • Lethenteron reissneri (Dybowski, 1869)
    • Lethenteron zanandreai (Vladykov, 1955)
  • Genus Petromyzon
    • Petromyzon marinus (Linnaeus, 1758) - sea lamprey
  • Genus Tetrapleurodon
    • Tetrapleurodon geminis (Alvarez, 1964)
    • Tetrapleurodon spadiceus (Bean, 1887)

Some taxonomists place lampreys and hagfish in the phylum Chordata under the super-class Agnathostomata (without jaws). The other super-class of the phylum is Gnathostomata (jaw-having) and includes the classes Chondrichthyes, Osteichthyes, Amphibia, Reptila, Aves, and Mammalia.