As early as the late Silurian period, when placoderms had just appeared, the most primitive branch of bony fish—the spiny fish—quietly stepped onto the evolutionary stage.
The Late Silurian and Early Devonian acanthodians can be considered representative of early acanthodians. They were only a few tens of centimeters long, their bodies gradually tapering from front to back, curving upwards at the end to form a heterocercal tail. They had two large, triangular dorsal fins, each composed of a dermal membrane, with a powerful bony spine supporting the leading edge of the fin. On the lower part of the body, symmetrical to the dorsal fins, was an anal fin of equal size and similar shape. Anterior to the anal fin was a pelvic fin. Behind the skull was a pair of pectoral fins. Between the pectoral and pelvic fins, along both sides of the abdomen, were five pairs of smaller fins. These smaller, "extra" fins each had a spine supporting the leading edge, a characteristic feature of acanthodians.
Acanthodian fish lived in rivers, lakes, and swamps during the Middle and Late Paleozoic Era. They reached the peak of their evolution in the Early Devonian Period, after which they declined and disappeared from the historical stage by the end of the Paleozoic Era.
Razorfish
