Relationships of Cetacea to Terrestrial Ungulates and the Evolution of Cranial Vasculature in Cete

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Abstract

Concomitant with the evolution of numerous aquatic adaptations, extant cetaceans have developed a cranial vascular system that is very different from those of terrestrial mammals. Terrestrial mammals rely on the internal carotid, external carotid, and vertebral arteries for blood supply to the brain. In extant cetaceans, however, these cranial vessels are either lost or extremely reduced early in ontogeny (such as the internal carotid and its branch the stapedial artery), or ramify along part of their course into anastomotic networks of small arteries (such as the vertebral artery and branches of the external carotid). These networks of arteries, and their morphologically similar complexes of veins, are termed retia mirabile, “wonderful nets.” In contrast to the typical ungulate cranial circulation in which a few large arteries make up the main supply channels to the brain, the cetacean cranial circulation is characterized by a series of the retia mirabile. The cranial vascular patterns of cetaceans (Boenninghaus, 1904; Slijper, 1936; Walmsley, 1938; Vogl and Fisher, 1981a) are so different from those of terrestrial eutherian mammals (Sisson, 1921; Miller et al., 1964; Bugge, 1974; Gray, 1974; Hunt, 1974; Presley, 1979; MacPhee, 1981; Wible, 1987) that they have attracted the attention of morphologists since the first half of the nineteenth century (Breschet, 1836; Stannius, 1841).