Fishes were the first vertebrates. Understanding the evolutionary history of fishes is therefore important not only for what it tells us about fish groups, but for what it tells us about evolution of the vertebrates and ultimately our own species. Innovations during fish evolution that were passed on to higher vertebrates include dermal and endochondral bone and their derivatives (vertebral centra, bony endoskeletons, brain cases, teeth), jaws, brains, appendages, and the internal organ systems that characterize all vertebrate groups today. During 500 million years of evolution, fishes colonized and dominated the seas and fresh waters and eventually emerged, at least for short periods, onto land. Major clades prospered and vanished, or were replaced by newer groups with presumably superior innovations.
Extant (“living”) fishes therefore represent the most recent manifestations of adaptations and lineages that have their roots in the early Paleozoic. The more than 27,000 species of extant fishes constitute only a fraction of the diversity of fishes that has existed historically, as should be evident from the long lists of extinct forms given here (which in turn represent a select fraction of the diversity of former taxa). Many of the extinct forms are exotic in their appearance, whereas others are remarkably similar to living forms, at least in external morphology. A major challenge to ichthyology involves unraveling the evolutionary pathways of both modern and past fish taxa in the process of determining relationships among groups. Which of the many fossil groups represent ancestral types? Which were independent lineaes that died out without representation in modern forms? What are the links between and among groups of the past and present? What do fossilized traits tell us about ancient environments? Where do similarities represent inheritance, convergence, or coincidence among extinct and living groups? And how have past adaptations influenced and perhaps constrained present morphologies and behaviors?
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