Zoology 10th Edition by Stephen Miller,John Harley

Exercise 6

The subphylum Craniata includes two subphyla, Hyperotreti and Vertebrata. Hyperotreti is composed of hagfishes and Vertebrata includes all other fishes (Petromyzontida, Chondrichthyes, Actinopterygii, and Sarcopterygii) and the tetrapods. The craniate lineage dates back more than 500 million years. The oldest fossils showing evidence of bone date back about 510 million years.
Why are all bony fishes no longer grouped together in the single class "Osteichthyes"?