The embryonic development of a coronary circulation in birds and mammals occurs in concert with myocardial compaction, likely in response to myocardial hypoxia and the work of a cardiac chamber. Whether similar driving forces featured prominently during the evolution of the coronary circulation among chordates remains a mystery, in part because the arrangements for supplying oxygen to fish hearts are far more diverse than a simple coronary supply to compact myocardium. Indeed, it may be that the majority of fish species do not have a coronary circulation! As evident from extant cyclostome fishes, the archetype chambered, myogenic heart probably had no coronary circulation; a spongy ventricular myocardium receives its oxygen supply from systemic venous blood. Instead, the coronary circulation likely made its first appearance with the appearance of a jaw and a vertebral column advent in the elasmobranch fishes. While all elasmobranchs have compact myocardium in variable amounts in their ventricle, all boney fishes (teleosts) do not have a coronary circulation, suggesting an evolutionary loss of this cardiac oxygen supply route, a feature that persists in extant amphibians. Molecular phylogenies have shown where losses have occurred. Thus, many fish do not need a coronary circulation, even if the heart’s oxygen supply is precariously dependent on oxygen-depleted venous blood. Thus, beyond the highly developed coronary circulations of endothermic sharks, salmonids and tunas, cardiac evolution showed movement away from coronary dependence, beginning perhaps with the cyprinid lineage and persisting at least through to the amphibians. . The degree of ventricular compaction in boney fishes, in part, is tied to athleticism, as shown by salmon and tunas, suggesting a close tie between the degree of compaction and cardiac workload. Nevertheless, salmon can survival and swim after coronary artery ligation, which suggests only a facultative dependence under certain situations. In fishes, the fact remains that a coronary circulation is always associated with compact myocardium and this is true for cardiac muscle of the ventricle as well as the cardiac muscle of the outflow tract, the conus arteriosus. Where the greatest mystery lies is with the forerunners of the tetrapod lineage, the lungfishes and coelocanths, and with the extant species of basal teleosts, many of which breathe air as well as water. Often these fishes lack compact myocardium in their ventricle, but possess a coronary circulation, which raises the question of coronary vascularity in the spongy myocardium. While the evolution of air breathing in fishes certainly increased the security of the cardiac oxygen supply by bringing better-oxygenated blood to spongy myocardium, it clearly did not supplant the need for a coronary circulation. Curiously, the early evolution of cardiac compaction and a coronary circulation also introduced the possibility of coronary arteriosclerosis since it is very prevalent in migratory salmon.