Biology Encyclopedia	Agnatha Author, 3000 “no jaws”
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Common Names

Agnathans, Jawless Fish

Latin Synonyms

Linnaean Rank

Grade, or Superclass if considered monophyletic.

Ancestral Phylogeny and Taxonomy

Explanation of phylogenetic trees

Cladistic classification of Agnathans is very difficult and there are many different schemes.  Analysis of the two extant groups, lampreys and hagfish is difficult enough as it is not certain if the two form a monophyletic clade excluding jawed vertebrates, Gnathostomata, or if lampreys are closer relatives to gnathostomates than hagfish as many morphologists believe.  When considering fossil forms of Agnathans, typically called Ostracoderms that extend back into the Cambrian period, classification becomes even more difficult.  Recent molecular data does suggest that extant lampreys and hagfish do form a monophyletic clade, but even if this is certain it is still very unclear which jawless fish fossils are closer relatives of gnathostomates and which are closer to ancestral forms of lampreys and hagfish.  For now it seems best to consider lampreys, hagfish, and all fossil jawless fish as a grade, Agnatha (non-italicized), until further information becomes available. 

Lineage

Terrabiota, Cytota, Neomura, Eukaryota, Unikonta, Opisthokonta, Animalia, Metazoa, Eumetazoa, Bilateria, Deuterostomia, Cyrtotreta, Chordata, Notochordata, Vertebrata

Descendant Phylogeny and Taxonomy

Several differing taxonomies have been devised for Agnathans.

The following paragraph is quoted from Fishes of the World, 4th ed., by Joseph Nelson (ISBN: 0-471-25031-7):

Are agnathans (jawless fishes) monophyletic? This is a bothersome question that concerns one of several major conflicts in fish phylogeny between the results of molecular biology and morphological studies. The term "Agnatha" is no longer used as a taxon name, as it was in the 1994 edition; it applied in a conventional sense to all taxa from Myxini to Pituriaspida, recognized now as a paraphyletic group. However, the term "agnathan" is still a useful one that can be used to describe these jawless fishes. Two groups of jawless fishes that are also considered here to be paraphyletic groups are the cyclostomes and the ostracoderms. The term "cyclostome" is used for the living jawless fishes (hagfishes and lampreys); this group is considered by most paleontologists and others using morphological evidence to be a paraphyletic group, and is so recognized here. However, there is molecular evidence from Mallatt and Sullivan (1998), Mallatt et al. (2001), Kuraku et al. (1999), Delarbre et al. (2002), and Takezaki et al. (2003) that supports the monophyly of cyclostomes (an older idea, termed the "cyclostome hypothesis," and rejecting the hypothesis that lampreys are more closely related to gnathostomes than to hagfishes, termed the "vertebrate hypothesis"). This is a serious conflict with the phylogenetic ideas accepted here and one that must be resolved, ideally by obtaining independent results that are in agreement from both molecular biology and morphological studies, before we can be confident in the basic phylogeny of early craniates. Meyer and Zardoya (2003) suggested the desirability of having larger data sets with greater taxon sampling to better support either the cyclostome hypothesis or the vertebrate (lamprey-gnathostome) hypothesis. It is interesting, though, that lampreys are now placed in the next sequenced group up from hagfishes following Donoghue et al. (2000), rather than in the class Cephalaspidomorphi, and sister to the Anaspidiformes. This suggests that they could have diverged within a relatively short time span. The term "ostracoderm" is used for the fossil armored jawless fishes; this is agreed to be a paraphyletic group but phylogenetically closer to the jawed vertebrates than to either hagfishes or lampreys. Forey (1995) reviewed past theories of relationships of agnathans and gnathostomes and of character evolution.

Morphology

The living lampreys and hagfish superficially resemble eels.  They do not have scales, although many extinct forms had hard bony armor plating.  All extant forms are ectothermic.

Distribution

Ecology

Ethology

Evolution

Development

Ethnobiology

Population

Notes and Comments

References

Literature

Carroll, R.H. 1988.  Vertebrate paleontology and evolution.  W.H. Freeman & Co. New York.

Nelson, Joseph.  2006.  Fishes of the world, 4th ed.  John Wiley & Sons, Inc.  ISBN: 0-471-25031-7