Placentation in dolphins from the Amazon River Basin: the Boto, Inia geoffrensis, and the Tucuxi, Sotalia fluviatilis

Little is known of placentation in whales [1]. Turner [2] showed that they have epitheliochorial placentation. For the baleen whales (suborder Mysticeti) our knowledge has advanced little further [3], since available specimens often have been in too poor condition for histology [4]. There is an excellent study of the early development of the fetal membranes in the humpback whale [5], but it does not extend to the establishment of the placenta. Rather more is known about the toothed whales (suborder Odontoceti), although based on descriptions of single specimens of bottlenose dolphin [6], harbour porpoise [7,8], killer whale [9], right whale dolphin [1] and Commerson's dolphin [1]. A brief account of placentation in the Ganges river dolphin was based on examination of four pregnant females [10].Several lines of evidence point to a close relationship between the mammalian orders Cetacea and Artiodactyla (even-toed hoofed mammals). The argument is supported by recent fossil evidence [11,12] as well as by molecular phylogenetics, which has Cetacea nested within Artiodactyla and closely related to hippopotami [13-18]. This new interpretation of the phylogenetic affinities of cetaceans makes it interesting to re-examine their placentation and ask what morphological transformations occurred in the lineage of these aquatic mammals.The Amazon River Basin is unusual in harbouring two sympatric species of cetacean, the small gray dolphin or tucuxi (Sotalia fluviatilis [19]) and the pink dolphin or boto (Inia geoffrensis [20]). Features shared by Inia and Sotalia include a reproductive cycle determined by the annual flooding cycles of the Amazon. Calving takes place when water levels are low or declining and fish more concentrated and susceptible to predation [21]. It is thought that Inia entered South America in the Miocene, at which time drainage of the Amazon Basin was towards the Pacific [20]. Subsequent uprising of the Andes led to clockwise reversal of the drainage of the