300861 Lecture Notes - Lecture 20: Devonian, Acanthodii, Placodermi
17 May 2019
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Document Summary
The next class of fishes to appear was the acanthodii, containing the earliest known jawed vertebrates, which arose in the late silurian, more than 416 million years ago. The acanthodians declined after the devonian but lasted into the. Early permian, a little less than 280 million years ago. The first complete specimens appear in lower devonian freshwater deposits, but later in the devonian and permian some members appear to have been marine. Most were small fishes, not more than 75 cm (approximately 30 inches) in length. We know nothing of the ancestors of the acanthodians. They must have arisen from some jawless vertebrate, probably in fresh water. They appear to have been active swimmers with almost no head armour but with large eyes, indicating that they depended heavily on vision. The rows of spines and spinelike fins between the pectoral and pelvic fins give some credence to the idea that paired fins arose from fin folds along the body sides.
