The arrangement of the heart chambers and associated blood vessels in the Devonian osteostracan Norselaspis glacialis. A reinterpretation based on recent studies of the circulatory system in lampreys

Abstract

A brief account is given of the different arrangements of the heart and associated blood vessels of ammocoetes and adult lampreys, the significance of the oesophageal suspension in ammocoetes and the structure of the rigid cartilaginous pericardium which is developed during lamprey metamorphosis. These accounts have then been used, in conjunction with the size, shape and position of the apertures and recesses of the ossified pericardial capsule of the Devonian osteostracan Norselaspis glacialis, to reinterpret the way in which the heart chambers were arranged within this fossil agnathan and to elucidate the vessels responsible for returning blood to its heart. The dorsal aorta of osteostracans undergoes a similar right‐hand curve just behind the postbranchial wall to that exhibited by the dorsal aorta of larval lampreys. This indicates that the dorsal aorta of N. glacialis was likewise shifted to the right by vertical strands of connective tissue which suspend the oesophagus from the under surface of the notochord and between two ducts of Cuvier and the heart. The size and shape of apertures and recesses in the pericardial cavity of Norselaspis glacialis supports the view that, in this osteostracan, two ducts of Cuvier entered the pericardial cavity through a dorsal opening on the posterior part of the capsule and then joined with the sinus venosus. The marginal sinuses/veins in the anterior part of the body of N. glacialis, from which blood passed horizontally into the respective ducts of Cuvier, are now regarded as homologues of the anterior cardinal veins. While there is evidence that, as in ammocoetes, the right duct of Cuvier was larger than the left duct in N. glacialis, the relative size and position of its proposed atrium and ventricle was apparently more like that of adult lampreys. Despite differences in the mode of formation of the ossified pericardium of osteostracans and the cartilagenous pericardium of adult lampreys, these structures presumably had a similar function. This would involve generating negative pressure within the pericardial chamber during ventricular systole, thereby aiding atrial diastole and thus the influx of blood into the heart from the main venous vessels, such as the inferior jugular vein(s) and the ducts of Cuvier. The inner side of the pericardial capsule of osteostracans, like that of adult lampreys, appears to have possessed an extensive vascular network which could thus have likewise played an important role in providing fluid for lubricating the surface of the heart.