Rhizodonts were fish that grew to as much as 7 meters long, around 22 feet, the largest freshwater fish known. They got their start during the Devonian, but really proliferated during the Mississippian, only to go extinct by the end of the Pennsylvanian.
The rhizodonts were a fish group whose limbs – fins – showed close affinities to the tetrapods, the primitive amphibians that were invading the land during the Mississippian. The fins, especially the front fins, were strong, with strong supporting tissues, and the rays of the fins were very much like fingers.
They had highly maneuverable jaws, and they were the greatest predators in the Mississippian lakes and rivers where they could have attacked things like lungfish and early amphibians. Some species had large tusks, and the name of the group, rhizodont, means “root-tooth” because their long teeth, some as long as 15 centimeters, or 6 inches, had distinctive root systems, another trait that connects them to the tetrapods and later animals.
Study of rhizodonts is an active area of paleontology today because of their relatively close relationship to the tetrapods, the ancestors of amphibians, reptiles, birds, and mammals. The fossil record of tetrapods, which began during the Devonian, still contains many gaps even millions of years later in the Mississippian.
—Richard I. Gibson
Image from Palaeos.
Reference: Gaining Ground: The Origin and Evolution of Tetrapods, by Jennifer A. Clack, Indiana University Press, 2012, p. 75-78.