The butterflyfish is not closely related to any other species, but it belongs to a large group called the bonytongues, which contains a mishmash of primitive fish such as freshwater elephantfish and knifefishes. (newscientist.com)
The middle ear evolved as primitive fish (top) became more advanced (bottom): The spiracle enlarged as the proportions of other bones (yellow, blue) changed. (sciencemag.org)
Previous studies seeking to address the question have only looked at primitive fish with cartilaginous skeletons such as sharks, in which scientists have failed to find pain receptors. (sciencemag.org)