Figure 1. One view of vertebrate phylogeny. Incorporated is the notion that vertebrates evolved in estuaries (mouths of rivers) and then radiated into other habitats (red dots). Contemporary jawless fish (hagfish and lampreys) are living representatives of the once abundant class Agnatha which reached its peak diversity during the Silurian. Most fossil Agnatha were Ostracoderms, of which many were heavily armored, possibly as defense against predaceous freshwater eurypterids (sea scorpions) with which they coexisted. The living species, cyclostomes, lack bony skeletons, in which regard they are most likely degenerate.
The Agnatha gave rise to the first jawed fishes called Placoderms (not shown), which are now extinct. This group included ferocious parrot-beaked Arthrodires that sported armored heads and jointed necks (hence the name). Some placoderms also had paired fins, organs that would later be transformed into legs. The group was abundant during the Devonian, but disappeared before the end of the Paleozoic.
Modern cartilaginous and bony fish are presumed to have evolved from placoderm ancestors. Bony fish divide into ray-fin (actinopterygii) and lobe-fin (sarcopterygii) varieties. The former include the teleosts, which comprise the vast majority of contemporary freshwater and marine fishes. Among the latter, were rhipidistians, such as Osteolepis, which is close to the stock that evolved into tetrapods.
The evolutionary relationships in this figure are derived from molecular evidence and, in some cases, conflict with the fossil record. For example, there is compelling evidence supporting the idea that terrestrial vertebrates are more closely allied to lobe-fin fishes than to lungfish. The postulated estuarine origins of the major groups is also debatable.