| Notochords are the axial skeleton of invertebrate chordates, of agnathan fish, and of amphibian tadpoles. |
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| The term agnathan unites several groups that do not necessarily have a common ancestry. |
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| One hypothesis is that jaws developed from the first gill arches in as yet undiscovered or unidentified agnathan fish. |
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| One landmark event in vertebrate evolution was the evolution of jaws from agnathan ancestors, which was followed by the first appearance of the stomach. |
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| The diverse agnathan and fish remains include the psammosteids Pycnosteus tuberculatus, Pycnosteus sp. |
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| Agnathan vertebrates in the Lower Silurian of Gotland, Sweden. |
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