Philippe Janvier, Muséum National d'Histoire Naturelle, Paris, France
Published online: April 2001
DOI: 10.1038/npg.els.0001532
Agnathans are an ensemble of jawless (non-gnathostome) 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).
Keywords: hyperotreta; hyperoartia; cyclostomes; vertebrates; gnathostomes
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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.
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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.
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Figure 3. Interrelationships and distribution of the vertebrates in time.
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| 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 (1991) First fossil hagfish (Myxinoidea): a record from the Pennsylvanian of Illinois. Science 254: 701–703. | |
| book 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. London, Academic Press. | |
| book Hardisty MW and Potter IC (eds) (1974–1982) The Biology of Lampreys. London, Academic Press. | |
| book Janvier P (1993) "Patterns of diversity in the skull of jawless fishes". In: Hanken M and Hall BK (eds) The Skull, pp. 131–188. Chicago: University of Chicago Press. | |
| book Janvier P (1996) Early Vertebrates. Oxford: Oxford University Press. | |
| Janvier P (1996) The dawn of the vertebrates: characters versus common ascent in the rise of current vertebrate phylogenies. Palaeontology 39: 259–287. | |
| book Jørgensen JM, Lomholt JP, Weber RE and Malte H (eds) (1998) Biology of Hagfishes. London: Chapman & Hall. | |
| book Løvtrup S (1977) The Phylogeny of Vertebrata. New York: Wiley. | |
| Stock DW and Whitt GS (1992) Evidence from 18S ribosomal RNA that lampreys and hagfishes form a natural group. Science 257: 787–789 | |
| Yalden DW (1985) Feeding mechanisms as evidence for cyclostome monophyly. Zoological Journal of the Linnean Society 84: 291–300. | |
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