The epicardium is the outer layer of the vertebrate heart. Both the embryonic epicardium and its derived mesenchyme are critical to heart development, contributing to the coronary vasculature and modulating the proliferation of the ventricular myocardium. The embryonic epicardium arises from an extracardiac, originally paired progenitor tissue called the proepicardium, a proliferation of coelomic cells found at the limit between the liver and the sinus venosus. Proepicardial cells attach to and spread over the cardiac surface giving rise to the epicardium. Invertebrate hearts always lack of epicardium, and no hypothesis has been proposed about the origin of this tissue and its proepicardial progenitor in vertebrates. We herein describe the epicardial development in a representative of the most basal living lineage of vertebrates, the agnathan Petromyzon marinus (lamprey). The epicardium in lampreys develops by migration of coelomic cells clustered in a paired structure at the roof of the coelomic cavity, between the pronephros and the gut. Later on, these outgrowths differentiate into the pronephric external glomerulus (PEG), a structure composed of capillary networks, mesangial cells, and podocytes. This observation is consistent with the conclusion that the primordia of the most anterior pair of PEG in agnathans have been retained and transformed into the proepicardium in gnathostomes. Glomerular progenitor cells are highly vasculogenic and probably allowed for the vascularization of a cardiac tube primarily devoid of coronary vessels. This new hypothesis accounts for the striking epicardial expression of Wt1 and Pod1, two transcription factors essential for development of the excretory system.