|
|
 |
|
|
Actinopterygian node B: Distinct acrodin crown on all teeth, differentiated postcleithrum from anterior body scales.
The polypterids - or bichirs - are the most primitive actinopterygians still extant today. They are currently restricted to swampy streams and lakes of tropical Africa. These are heavily armoured predators covered in thick rhomboid scales, capable of breathing air through a pair of lungs connected ventrally to the front end of the gut, bulging around it and expanding dorsally. The dorsal fin runs down the whole length of the body, separated into segments, each possessing a spine on its rostral margin.
The bichirs are believed to have remained virtually unchanged since their first appearance in the Devonian, having never been outcompeted by more advanced actinopterygians. This may be due in part to the fact the polypterids have kept functional lungs, while later forms evolved a swimbladder solely functional as a buoyancy-regulating organ, this would permit bichirs to remain better adapted to the oxygen-poor waters they are found in.