Thelodus is a coelolepid thelodontiform jawless fish from the Llandovery-Early Devonian of the United Kingdom, Canada, Estonia, Norway, Russia, Sweden, and Ukraine. It was named in 1839 by Louis Agassiz. It was one of the many jawless fishes to swim in the Silurian seas, as well as one of the more advanced ones.
Physiology[]
Even though Thelodus had many generic jawless fish traits, such as a mobile tail, a large, oval-shaped body, and eyes positioned near the sides of its head, it was slightly more advanced as well. It had a larger tail fin, a dorsal (back) and anal (near the base of the tail) fin, tiny pectoral (arm) fins, and a forward-facing mouth (rather than one that faced downwards). Its body would have been covered in scaly skin.
Diet[]
Thelodus was a planktivore, sucking up colonies of plankton drifting through the water. Its small, toothless mouth would have been used to filter plankton from the water.
Ecology[]
So far, all we know about Thelodus is that it would have had a build that rendered it extremely adapted to life in open water (in this case, a larger tail fin for faster swimming, a dorsal and anal fin, and small pectoral fins for steering), and that it had a forward-facing mouth that made it easier for it to filter plankton from the water; as well as this, due to the fact that it had a more slender body than other jawless fishes, it would have relied on swift swimming speed rather than thick armor to escape predators such as Pterygotus.