Paired fins of jawless vertebrates and their homologies across the "agnathan"- gnathostome transition

The origin of jawed vertebrates (Gnathostomata) is one of the greatest events in vertebrate evolution, and one of the most poorly understood to this day. Among the many features of gnathostomes shared with possible precursors in jawless ("agnathan") vertebrates are paired fins. "Agnathan" paired fin-like structures occurred in many species of anaspids, thelodonts, and osteostracans. These early paired fins are not all the same: some taxa have pectoral precursors, and others have pelvic precursors. At least one thelodont probably had both, the only "agnathan" known to share this feature with gnathostomes. Some "agnathan" lineages likely lost either pectoral fins (furcacaudiforms) or pelvic fins (osteostracans, perhaps some thelodonts) that were present in their ancestors. Pectoral and pelvic fins or their precursors differed fundamentally in position and structure even be- fore the origin of jaws, and within most of the major groups of early jawed vertebrates.