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The key idea is that bony fishes have a skeleton made primarily of bone tissue. This ossified skeleton distinguishes them from other fish groups with different skeletal materials. Chondrichthyes are cartilaginous fishes, so their endoskeleton is cartilage rather than bone. Agnatha are jawless, representing an earlier vertebrate condition with no jaws. Placodermi were ancient armored fishes known for bony plates but are extinct. Because of the bone-based skeleton, the group that fits as bony fishes is Osteichthyes, the true bone fishes.