Paleontology

1979 Edition

Agnatha

  • Robert H. Denison
Reference work entry
DOI: https://doi.org/10.1007/3-540-31078-9_3

Agnatha, a class of primitive aquatic vertebrates, are distinguished by the fact that none of their gill arches has been modified to form jaws. Living ones, the eel-like cyclostomes, including hagfishes ( Myxine ) and lampreys ( Petromyzon ), lack a mineralized skeleton and so are very rare as fossils; the only one that has been discovered is Mayomyzon , a lamprey from the Pennsylvanian of Illinois. Other fossil Agnatha (Stensiö, 1964; Romer, 1966; Obruchev, 1967; Moy-Thomas and Miles, 1971; Halstead and Turner, 1973), sometimes called ostracoderms, occur in Ordovician, Silurian, and Devonian rocks in North America, Europe, Asia, and Australia. Agnatha are classified in two subclasses: (1) Monorhina with a single, median nostril, including cyclostomes and the fossil Osteostraci and Anaspida; and (2) Diplorhina with paired nostrils, known only from fossil Heterostraci and Thelodonti.

Heterostraci

Order Heterostraci (Middle Ordovician to Upper Devonian) includes the earliest and some of...
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References

  1. Halstead, L. B. and Turner, S., 1973. Silurian and Devonian ostracoderms, in A. Hallam, ed., Atlas of Palaeobiogeography. Amsterdam: Elsevier, 67–79.Google Scholar
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  3. Obruchev, D. B., 1967. Fundamentals of Paleontology, vol. 11, Agnatha, Pisces (transl. from Russian). Jerusalem: Israel Program for Scientific Translations, 36–167.Google Scholar
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Cross-references

Copyright information

© Dowden, Hutchinson & Ross, Inc. 1979

Authors and Affiliations

  • Robert H. Denison

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