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
Halstead, L. B. and Turner, S., 1973. Silurian and Devonian ostracoderms, in A. Hallam, ed., Atlas of Palaeobiogeography. Amsterdam: Elsevier, 67–79.
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Denison, R.H. (1979). Agnatha . In: Paleontology. Encyclopedia of Earth Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-31078-9_3
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