Cyclostomata

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This article is about the class of jawless fish. For the ancient order of stenolaemate bryozoans, see Cyclostomatida.
Cyclostomes
Temporal range: 430–0Ma
Silurian - Recent
Sea lamprey from Sweden
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Craniata or Vertebrata
Superclass: Cyclostomata
Duméril, 1806
classes

Cyclostomata is a group of chordates that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have round mouths that lack jaws but have retractable horny teeth. The name Cyclostomata means "round mouths".[1][2] Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.

Possible relationships

This taxon is often included in the paraphyletic Class Agnatha, which also includes several groups of extinct armored fishes called ostracoderms. Most fossil agnathans, such as galeaspids, thelodonts, and osteostracans, are more closely related to vertebrates with jaws (called gnathostomes) than to cyclostomes.[3][4] Cyclostomes seem to have split off before the evolution of dentine and bone, which are present in many fossil agnathans, including conodonts.[5]

Biologists disagree about whether cyclostomes are a clade. The "vertebrate hypothesis" holds that lampreys are more closely related to gnathostomes than they are to the hagfish. The "cyclostome hypothesis", on the other hand, holds that lampreys and hagfishes are more closely related, making cyclostomata monophyletic.[6][7]

Most studies based on anatomy have supported the vertebrate hypothesis,[8] while most molecular phylogenies have supported the cyclostome hypothesis.[1][6][9][10]

There are exceptions in both cases, however. Similarities in the cartilage and muscles of the tongue apparatus also provide evidence of sister-group relationship between lampreys and hagfishes.[11] And at least one molecular phylogeny has supported the vertebrate hypothesis.[12] The embryonic development of hagfishes was once held to be drastically different from that of lampreys and gnathostomes, but recent evidence suggests that it is more similar than previously thought, which may remove an obstacle to the cyclostome hypothesis.[13] There is at present no consensus on the correct topology.

References

  1. ↑ 1.0 1.1 Kuraku, Shigehiro, Ota, Kinya G., & Kuratani, Shigeru, S. Blair (2009b). "Jawless fishes (Cyclostomata)". In S.B. Hedges & S. Kumar. Timetree of Life. Oxford University Press. pp. 317–319. ISBN 978-0-19-953503-3. 
  2. ↑ Duméril, A.M. Constant (1806). Zoologie analytique, ou me´thode naturelle de classification des animaux, Rendue plus facile a l'Aide de Tableaux Synoptiques. Paris: Allais. 
  3. ↑ Zhao Wen-Jin, Zhu Min (2007). "Diversification and faunal shift of Siluro-Devonian vertebrates of China". Geological Journal 42 ((3–4)): 351–369. doi:10.1002/gj.1072. 
  4. ↑ Sansom, Robert S. (2009). "Phylogeny, classification, & character polarity of the Osteostraci (Vertebrata)". Journal of Systematic Palaeontology 7: 95–115. doi:10.1017/S1477201908002551. 
  5. ↑ Baker, Clare V.H. (2008). "The evolution and elaboration of vertebrate neural crest cells". Current Opinion in Genetics & Development 18 (6): 536–543. doi:10.1016/j.gde.2008.11.006. PMID 19121930. 
  6. ↑ 6.0 6.1 Delabre, 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. doi:10.1006/mpev.2001.1045. PMID 11820840. 
  7. ↑ Stock, David; Whitt GS (7 August 1992). "Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group". Science 257 (5071). doi:10.1126/science.1496398. PMID 1496398. Retrieved 22 November 2011. 
  8. ↑ Janvier, Philippe (2003). Early Vertebrates. Oxford University Press. pp. 1–408. ISBN 978-0-19-852646-9. 
  9. ↑ Kuraku, Shigehiro, Meyer, Axel, & Kuratani, Shigeru (2009a). "Timing of Genome Duplications Relative to the Origin of the Vertebrates: Did Cyclostomes Diverge before, or after?". Molecular Biololgy & Evolution 26 (1): 47–59. doi:10.1093/molbev/msn222. PMID 18842688. 
  10. ↑ Heimberg, Alysha M.; Cowper-Sallari, Richard; Sémon, Marie; Donoghue, Philip C. J.; Peterson, Kevin J. (2010). "microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate". PNAS 107 (45): 19379–19383. doi:10.1073/pnas.1010350107. PMID 20959416. 
  11. ↑ Yalden, D.M. (1985). "Feeding mechanisms as evidence for cyclostome monophyly". Zoological Journal of the Linnean Society 84 (3): 291–300. doi:10.1111/j.1096-3642.1985.tb01802.x. 
  12. ↑ Gürsoy, Halil-Cem; Koper, Dorota, & Benecke, Bernd-Joachim (2000). "The Vertebrate 7S K RNA Separates Hagfish (Myxine glutinosa) and Lamprey (Lampetra fluviatilis)". Journal of Molecular Evolution 50 (5): 456–464. doi:10.1007/s002390010048. PMID 10824089. 
  13. ↑ Kuratani, Shigeru, & Ota, Kinya G. (2008). "Hagfish (Cyclostomata, Vertebrata): searching for the ancestral developmental plan of vertebrates". BioEssays 30 (2): 167–172. doi:10.1002/bies.20701. PMID 18197595. 
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