Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls

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Abstract
Modern members of the mammalian order Cetacea (whales, dolphins and porpoises) are obligate aquatic swimmers that are highly distinctive in morphology, lacking hair and hind limbs, and having flippers, flukes, and a streamlined body. Eocene fossils document much of cetaceans' land-to-water transition, but, until now, the most primitive representative for which a skeleton was known was clearly amphibious and lived in coastal environments. Here we report on the skeletons of two early Eocene pakicetid cetaceans, the fox-sized Ichthyolestes pinfoldi, and the wolf-sized Pakicetus attocki. Their skeletons also elucidate the relationships of cetaceans to other mammals. Morphological cladistic analyses have shown cetaceans to be most closely related to one or more mesonychians, a group of extinct, archaic ungulates, but molecular analyses have indicated that they are the sister group to hippopotamids. Our cladistic analysis indicates that cetaceans are more closely related to artiodactyls than to any mesonychian. Cetaceans are not the sister group to (any) mesonychians, nor to hippopotamids. Our analysis stops short of identifying any particular artiodactyl family as the cetacean sister group and supports monophyly of artiodactyls. In contrast to the debate about the cetacean sister group, the relationships among Eocene cetaceans and the content of Cetacea itself are not controversial1-5. All phylogenetic studies indicate that pakicetids are more closely related to living cetaceans than to artiodactyls and mesonychians, and that pakicetids share the cetacean synapomorphies of the ear2,3,6. Pakicetids are followed by ambulocetids in the cladogram, and modern cetaceans (toothed and baleen whales) are closely related to late Eocene basilosaurids and dorudontids1,3-5. The most archaic cetacean for which the skeleton is known is the amphibious Ambulocetus natans7,8. It was a powerful, walrus-sized animal that lived in coastal environments and resembled a crocodile, with the exception of long hind limbs that were used in swimming9. Although Ambulocetus is unlike modern cetaceans, it also differs strongly from its land mammal relatives, be they artiodactyls or mesonychians. Adaptations for life in water in Ambulocetus and later whales complicate determination of their closest relatives among the land mammals. Data from fossil whales that are more basal on the cetacean phylogenetic tree and have fewer aquatic adaptations could presumably yield new phylogenetic insights10,11. Pakicetids are in this position and can be used to test mesonychian and hippopotamid hypotheses. There are three genera of pakicetid cetaceans: Pakicetus, Nalacetus and Ichthyolestes2. Pakicetus is the largest, followed by Nalacetus (approximately 5% smaller in linear dimensions), and Ichthyolestes (approximately 29% smaller). Until now, only teeth, jaws and one braincase have been described for pakicetids2,3,12. We excavated four partial skulls, two of which retain the orbital region, several snout fragments, and approximately 150 isolated postcranial bones of pakicetids from multiple individuals. These were found at a single site in the early Eocene Kuldana Formation of Pakistan. We use these fossils to show (1) that these archaic cetaceans were land mammals; and (2) that cetaceans are more closely related to artiodactyls than to mesonychians.