Taxonomy and Systematics
Taxonomists place lampreys and hagfish in the subphylum Vertebrata of the phylum Chordata, which also includes the invertebrate subphyla Tunicata (sea-squirts) and the fish-like Cephalochordata (lancelets or Amphioxus). Recent molecular and morphological phylogenetic studies place lampreys and hagfish in the superclass Agnatha or Agnathostomata (both meaning without jaws). The other vertebrate superclass is Gnathostomata (jawed mouths) and includes the classes Chondrichthyes (sharks), Osteichthyes (bony fishes), Amphibia, Reptilia, Aves (birds), and Mammalia.
Some researchers have classified lampreys as the sole surviving representatives of the Linnean class Cephalaspidomorphi. Cephalaspidomorpha is sometimes given as a subclass of the Cephalaspidomorphi. Fossil evidence now suggests lampreys and cephalaspids acquired their shared characters by convergent evolution. As such, many newer works, such as the fourth edition of Fishes of the World, classify lampreys in a separate group called Hyperoartia or Petromyzontida, but whether this is actually a clade is disputed. Namely, it has been proposed that the non-lamprey "Hyperoartia" are in fact closer to the jawed vertebrates.
The debate about their systematics notwithstanding, lampreys constitute a single order Petromyzontiformes. Sometimes still seen is the alternative spelling "Petromyzoniformes", based on the argument that the type genus is Petromyzon and not "Petromyzonta" or similar. Throughout most of the 20th century, both names were used pretty much indiscriminately, even by the same author in subsequent publications. In the mid-1970s, the ICZN was called upon to fix one name or the other, and after much debate had to resolve the issue by voting. Thus, in 1980 the spelling with a "t" won out, and in 1981 it became official that all higher-level taxa based on Petromyzon have to start with "Petromyzont-".
The following taxonomy is based upon the treatment by FishBase as of April 2012. Within the order, there are 10 living genera in three families. Two of the latter are monotypic at genus level today, and in one of them a single living species is recognized (though it may be a cryptic species complex):
- Family Geotriidae – Pouched Lamprey
- Genus Geotria
- Family Mordaciidae – southern topeyed lampreys
- Genus Mordacia
- Family Petromyzontidae – northern lampreys
- Genus Caspiomyzon
- Genus Entosphenus
- Genus Eudontomyzon
- Genus Ichthyomyzon
- Genus Lampetra
- Genus Lethenteron
- Genus Petromyzon
- Genus Tetrapleurodon
Read more about this topic: Lamprey