Temporal range: Cambrian – Recent
The Agnatha as a whole may be paraphyletic. This means it is a convenient holdall term, which does not follow the rules of cladistics. For example, most extinct agnathans belong to the stem group (ancestral group) of gnathostomes. But according to the rules, one sister group should not contain ancestors of another sister group,
The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today: the lampreys and the hagfish, with about 100 species in total. Hagfish are not members of the subphylum Vertebrata, as hagfish do not have vertebrae; they are rather classified in the more inclusive group Craniata. In addition to the absence of jaws, modern agnathans are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. There is a light sensitive pineal eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not have an identifiable stomach or any appendages. Fertilization and development are both external. There is no parental care in the Agnatha class. The Agnatha are exothermic, with a cartilaginous skeleton, and the heart contains 2 chambers.
Agnatha have existed since the Cambrian, and continue to live in modern times. In addition to the absence of jaws, the Agnatha are characterised by absence of paired testicles, the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. There is a light sensitive pineal eye. There is no identificable stomach. The Agnatha are cold-blooded, with a cartilaginous skeleton, and the heart contains two chambers.
[change] Fossil agnathans
The oldest fossil agnathans are found in Cambrian deposits.
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans were the Ostracoderms ("shell-skinned"). Not to be confused with the Osteichthyes ("bony fish"), who were ancestors of the bony fish, and hence of tetrapods (including human beings). By the upper Silurian the agnathans had reached the high point of their evolution. agnathans declined in the Devonian and never recovered.
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