Xenacanthus
This kind of freshwater shark lived from the later Devonian period until the end of the Triassic. Scientists estimate that the sharks disappeared from the fossil record more than 202 million years ago. The Xenacanthus is often referred to as the “eel shark” because it has an elongated dorsal fin that extends along the spine.
These sharks were small, only three feet in length. The dorsal fin was ribbon-like, and it ran the entire length of the body to the tail, where an anal fin joined it. The closest living relatives are the stingray and other sea-dwelling species.