THE ANTITROPICAL FACTOR IN CETACEAN SPECIATION

It is well known that several cetacean genera, and possibly even some species, are discontinuously distributed on either side of the equator. Such distributions, often described as "bipolar," have been widely discussed for other marine animals-notably by Berg (1933) and by Ekman (1953)--but no one has attempted a systematic examination of the phenomenon in regard to the whales and dolphins. Because most so-called "bipolar" marine distributions are actually "bitemperate" or even "bisubtropical," Hubbs (1952) suggested the term "antitropical" as more appropriate and all-inclusive. An attempt is made here to review the nature of antitropical distributions in the Cetacea, to account for them, and to suggest something of their evolutionary significance. Present evidence, both specific and circumstantial, suggests that the earlier cetaceans lived in a warm-water environment. Thus the earliest known forms are zeuglodonts from the Middle Eocene of Egypt, Nigeria, and Texas and the distribution of existing species still suggests an aversion to cold: only three genera (Balaena, Delphinapterus, and Monodon) breed in polar waters, none breeds in Antarctic polar waters, and the majority of species frequenting cold water appear to move equatorward to breed. By the end of the Eocene, zeuglodonts had spread north to British Columbia, England, and Germany and probably south to Australia and New Zealand; at any rate they were certainly in southern waters by the Oligocene. Toward the end of the Eocene also, the first archaic toothed whales (Odontoceti) appeared, but are known only from North America. Although a few zeuglodonts persisted in the early Miocene, both of these early stocks apparently declined markedly during the Oligocene without being immediately replaced by newer forms. Kellogg (1928) remarked on "the scarcity of fossil cetaceans in Oligocene formations" and pointed out that this "seems all the more surprising in view of the wide variety of types that make their appearance in formations of lower Miocene age. " In addition to the now extinct squalodonts, the modern groups of dolphins, sperm whales, beaked whales, and river dolphins all appeared about this time, together with the first of the whalebone whales-the cetotheres. The marked turnover of forms at about the Oligocene-Miocene boundary appears to be correlated with the cooling of world climates during the Tertiary, which seems generally to have been reflected in oceanic temperatures. The writer has recently suggested (Davies, 1958) that an apparently sudden enlargement of range and speeding of evolutionary rates in the Pinnipedia was associated with this expansion of the coldwater environment, and it seems likely that the mid-Tertiary revolution in whale stocks arose for a similar reason. The older, warm-water types failed to adapt themselves to the growing new cold-water environments and the newer types, which appeared in force in the Miocene, evolved and spread rapidly in response to these new opportunities. There are some indications in the distribution of post-Oligocene as compared with pre-Miocene fossils and in the almost complete fossil taxonomic discontinuity between northern and southern hemispheres that the equatorial warm-water zone was an important barrier to dispersal throughout the later Tertiary. However the distribution and relationship of existing Cetacea show that, while the equatorial zone is still a barrier, there have been relatively recent opportunities for increased dispersal