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 living Agnatha (lampreys and hagfish) are known as cyclostomes. Recent molecular data from rRNA, and from mtDNA, show that these living agnathans are monophyletic. There are about 100 species. Hagfish are vertebrates but do not have vertebrae. It is believed that they lost their vertebrae during their lifestyle adaptations.
- Agnatha: the jawless fish. Cambrian to present day.
The oldest fossil agnathans are found in Cambrian deposits.
Many Ordovician, Silurian, and Devonian agnathans were armoured with heavy bony-spiky plates. The first armoured agnathans were the Ostracoderms ("shell-skinned"). By the upper Silurian the agnathans had reached the high point of their evolution. They declined in the Devonian and never recovered.
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