Chondrostei
Why did Chondrosteans react with such total reorganization? Obviously selection pressure must have been very strong. However, in one way they were almost unique. Rarely amongst animals, they underwent polyploidy, several times. Instead of two sets of chromosomes (diploid), the Chondrosteans have 8 or 16 sets of chromosomes. Such a dramatic genetic change may have given the potential for a rapid, drastic reorganization of the body plan.