High Quality Content by WIKIPEDIA articles! Rhizodonts (Order Rhizodontida) are an extinct group of predatory lobe-finned fishes. They are known from many areas of the world from the Givetian through to the Pennsylvanian - the earliest known species is about 377 Ma, the latest around 310 Ma. Rhizodonts lived in tropical rivers and freshwater lakes and were the dominant predators of their age. They reached huge sizes - the largest known species, Rhizodus hibberti from Europe and North America, was an estimated 7 metres in length, making it the largest freshwater fish known.