Agnatha (Lampreys, Hagfishes, Ostracoderms)

Abstract

Agnathans are an ensemble of jawless (nongnathostome) vertebrates, some of which are more closely related to the gnathostomes than others. Living agnathans fall into two groups, hagfishes (Hyperotreta) and lampreys (Hyperoartia), united into a higher group, the cyclostomes (Cyclostomi).

Key Concepts

  • Phylogenomics now demonstrates that cyclostomes are a monophyletic group contrary to what morphological features suggested.
  • The rise of bone and dentine occurred after the divergence of the cyclostomes.
  • The first mineralised skeleton was in the form of small ‘odontodes’ made up by acellular bone and dentine in the dermis, which later fuse into larger plates.
  • All jawless vertebrates that produce bone and dentine are stem jawed vertebrate, or gnathostomes, and informally referred to as ‘ostracoderms’.
  • Nonmineralised soft tissues can sometimes be preserved in fossils under particular environmental conditions, and provide information about the early history of cyclostome and jawless fishes in general.

Keywords: hyperotreta; hyperoartia; cyclostomes; vertebrates; gnathostomes

Figure 1. Living agnathans. (a) Hagfishes; top, the Pacific hagfish Eptatretus in lateral view; bottom; the head skeleton of the Atlantic hagfish Myxine. (b) Lampreys; top, the brook lamprey Lampetra in lateral view; bottom, the head skeleton of the brook lamprey.
Figure 2. Attempted reconstructions of fossil agnathans. (a) Haikouichthys, Lower Cambrian of China; (b) the hagfish Myxinikela, Carboniferous of USA; (c) the lamprey Mayomyzon, Carboniferous of USA; (d) the anaspid Rhyncholepis, Silurian of Norway; (e) the thelodont Loganellia, Silurian of Scotland; (f) the arandaspid Sacabambaspis, Ordovician of Bolivia; (g) the heterostracan Tartuosteus, Middle Devonian of Estonia; (h) the heterostracan Poraspis, Lower Devonian of Spitsbergen; (i) the pituriaspid Pituriaspis, Middle Devonian of Australia; (j) the osteostracan Zenaspis, Lower Devonian of Scotland; (k) the galeaspid Polybranchiaspis, Lower Devonian of China; (l) the euconodont Clydagnathus, Carboniferous of Scotland. The sizes of the fish range from about 3–4 cm (a,l) to 1 m (g) in length.
Figure 3. Interrelationships and distribution of the vertebrates in time.
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References

Kuraku S , Hoshiyame D , Katoh K , Suga H and Miyata T (1999) Monophyly of lampreys and hagfishes supported by nuclear DNA‐coded genes. Journal of Molecular Evolution 149 (6): 729–735.

Further Reading

Aldridge RJ , Briggs DEG , Smith MP , Clarkson ENK and Clark NDL (1993) The anatomy of conodonts. Philosophical Transactions of the Royal Society of London series B – Biological Sciences 340: 405–421.

Bardack D and Zangerl R (1971) Lampreys in the fossil record. In: Hardisty MW and Potter IC (eds) The Biology of Lampreys, vol. 1, pp 67–84. Academic Press: London.

Bardack D (1991) First fossil hagfish (Myxinoidea): a record from the Pennsylvanian of Illinois. Science 254: 701–703.

Evans TM , Janvier P and Docker MF (2018) The evolution of lamprey (Petromyzontida) life history and the origin of metamorphosis. Reviews in Fish Biology and Fisheries 28 (4): 825–838. DOI: 10.1007/s11160‐018‐9536‐z.

Hardisty MW and Potter IC (eds) (1974–1982) The Biology of Lampreys. Academic Press: London.

Janvier P (1993) Patterns of diversity in the skull of jawless fishes. In: Hanken M and Hall BK (eds) The Skull, pp 131–188. University of Chicago Press: Chicago.

Janvier P (1996) Early Vertebrates. Oxford University Press: Oxford.

Janvier P (1996) The dawn of the vertebrates: characters versus common ascent in the rise of current vertebrate phylogenies. Palaeontology 39: 259–287.

Janvier P (2008) Early jawless vertebrates and cyclostome origins. Zoological Science 25: 1045–1056. DOI: 10.2108/zsj.25.1045.

Janvier P (2015) Facts and fancies about early fossil chordates and vertebrates. Nature 520: 483–489.

Janvier P and Sansom RS (2015) Fossil hagfishes, fossil cyclostomes, and the lost world of ‘ostracoderms’. In: Edwards S and Goss G (eds) Hagfish Biology, pp 73–93. Taylor & Francis: Boca Raton.

Jørgensen JM , Lomholt JP , Weber RE and Malte H (eds) (1998) Biology of Hagfishes. Chapman & Hall: London.

Løvtrup S (1977) The Phylogeny of Vertebrata. Wiley: New York.

Yalden DW (1985) Feeding mechanisms as evidence for cyclostome monophyly. Zoological Journal of the Linnean Society 84: 291–300.

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How to Cite close
Janvier, Philippe(Oct 2020) Agnatha (Lampreys, Hagfishes, Ostracoderms). In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029214]