Reconstruction of the cranial vessels in the Early Cretaceous mammalVincelestes neuquenianus: implications for the evolution of the mammalian cranial vascular system

Abstract

Vincelestes neuquenianus from the Early Cretaceous of Argentina is the earliest therian mammal known from nearly complete crania. Described here is the structure of the petrosal bone and other cranial elements inferred to be associated with the vascular system. From comparisons with Recent amniotes, the major basicranial arteries and veins that we reconstruct for Vincelestes represent a composite of the vascular patterns in monotremes and some placentals (e.g., lipotyphlan insectivorans). Vincelestes had a well-developed stapedial system that was supplied principally via the arteria diploëtica magna, a transpromontorial internal carotid artery, and a well-developed lateral head vein that drained the prootic sinus via a prootic canal. As in monotremes and extinct “non-therian” mammals, Vincelestes had an enlarged anterior lamina of the petrosal (=lamina obturans), through the base of which (lateral flange) ran the ramus superior of the stapedial artery. Reconstruction of the major basicranial arteries and veins in Vincelestes facilitates identification of homologous channels in extinct “non-therian” mammals and non-mammalian cynodonts. We recognize a homologous system of vascular channels on the lateral braincase wall that we interpret as for the arteria diploëtica magna and ramus superior that occur in Recent mammals. The gradual reduction of this system and the transformation of the channels from wholly extracranial to wholly intracranial positions are traced. Problematic are the courses of the major basicranial veins, the lateral head vein and prootic sinus, in non-mammalian cynodonts and the earliest mammals. We disagree with previous models that reconstruct the lateral head vein dorsal to the lateral flange of the prootic and we place that vessel ventral to the middle-ear roof. Complicating venous reconstruction is the discovery of a “prootic canal” in the non-mammalian cynodonts Probainognathus and Massetognathus, because some taxa appear to have had two vessels connecting the prootic sinus with the lateral head vein: one ran dorsal to the lateral flange and the other was enclosed in the prootic (petrosal). The venous and arterial patterns of Recent mammals are compared to those interpreted to have been present in ancestral mammals. Monotremes retain a primitive venous pattern, but the arterial system is modified in different ways in the platypus on the one hand and the echidna on the other. In marsupials, both the arterial and venous patterns have been markedly altered, with the reduction of the stapedial system, arteria diploëtica magna, and lateral head vein. Primitive placentals retain all the major components of the arterial pattern of ancestral mammals, but their venous system is further modified.