Temporal range: Cambrian – Recent
The Agnatha as a whole may be paraphyletic. This means it is a convenient holdall term, which does not follow the rules of cladistics. For example, most extinct agnathans belong to the stem group (ancestral group) of gnathostomes. But according to the rules, one sister group should not contain ancestors of another sister group,
The living Agnatha (lampreys and hagfish) are known as cyclostomes. Recent molecular data from rRNA, and from mtDNA, show that these living agnathans are monophyletic. There are about 100 species. Hagfish are vertebrates but do not have vertebrae. It is believed that they lost their vertebrae during their lifestyle adaptations.
Classification[change | change source]
- Agnatha: the jawless fish. Cambrian to present day.
Characteristics[change | change source]
Anatomy[change | change source]
Physiology[change | change source]
Fossil agnathans[change | change source]
The oldest fossil agnathans are found in Cambrian deposits.
Many Ordovician, Silurian, and Devonian agnathans were armoured with heavy bony-spiky plates. The first armoured agnathans were the Ostracoderms ("shell-skinned"). By the upper Silurian the agnathans had reached the high point of their evolution. They declined in the Devonian and never recovered.
References[change | change source]
|Wikispecies has information on: Agnatha.|
- Purnell, M.A; Briggs, Derek and Crowther P.R. (eds) 2001. Palaeobiology II. Oxford: Blackwell, p. 401. ISBN 0-632-05149-3
- Zhao Wen-Jin & Zhu Min 2007. Diversification and faunal shift of Siluro-Devonian vertebrates of China. Geological Journal 42, 351–369. 
- Sansom, Robert S. 2009. Phylogeny, classification & character polarity of the Osteostraci (Vertebrata). Journal of Systematic Palaeontology 7: 95–115. 
- Mallatt J. & Sullivan J. 1998. 28S and 18S ribosomal DNA sequences support the monophyly of lampreys and hagfishes. Molecular Biology and Evolution 15 (12): 1706–1718. 
- DeLarbre, Christiane et al 2002. Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: the comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. Molecular phylogenetics and evolution. 22, 2, 184–192. 
- Janvier, Philippe 2010. MicroRNAs revive old views about jawless vertebrate divergence and evolution. Proceedings of the National Academy of Sciences (USA) 107:19137-19138. 
- BBC News Science & Technology 1999
- Not to be confused with the Osteichthyes ("bony fish"), who were ancestors of the bony fish.