Acanthodians (class Acanthodii) are jawed fish that lived from the early Silurian to the end of the Permian. Despite being called spiny sharks, they actually predate the first sharks by 50 million years, and it is now theorized that they gave rise to the first cartilagenous fish. In 2017, a study of the morphology of Doliodus reveals that it shows a mosaic of shark and acanthodian features, making it a transitional fossil and therefore improving the likeliness of this theory being true.
At the end of the episode, acanthodians can be seen swimming in groups and also feeding on a dead nautiloid. They are also hunted by Pterygotus, the descendant of the Ordovician eurypterids.
The acanthodians survived the Devonian extinction, along with trilobites (Hell on Earth), but they and the trilobites died out during the Permian extinction, the worst extinction on Earth.