EVOLUTION: Of Whale Knuckles and Placental Trees -- de Muizon 309 (5734): 559 -- Science:
Cetaceans are probably the most extraordinary mammals. They are highly adapted to life in water and strictly dependent on their aquatic environment. As a consequence, their anatomy and physiology have been strongly modified and do not resemble those of other mammals. Because cetaceans are so drastically transformed from their terrestrial ancestors, their affinities with other mammals have long been debated. Although researchers agreed that cetaceans had their origin in some group of land mammals that lived during the early Tertiary, there was no consensus on the identity of the group that subsequently evolved into whales and dolphins. Whales have been seen as closely related to seals, creodonts (hyaenodonts), ungulates, and mesonychid condylarths (large carnivorous to omnivorous archaic ungulates). In the absence of a better candidate, the mesonychids were on the verge of becoming accepted when molecular biologists claimed that cetaceans were most closely related to artiodactyls (even-toed ungulates)--specifically to hippos. A few years later, paleontologists discovered postcranial remains of early Eocene cetaceans that demonstrated the presence of a double-pulleyed astragalus (like the sheep ankle bones that the Romans used to play at knucklebones), a characteristic of all artiodactyls and exclusively found in that order. This discovery, among the most important paleontological finds of the past hundred years, led to an immediate consensus on cetacean ancestry. It also demonstrated a remarkable complementarity of two different approaches to the study of the evolution and phylogeny of mammals.
This from a review of
The Rise of Placental Mammals : Origins and Relationships of the Major Extant Clades by Kenneth D. Rose, J. David Archibald
Sounds like a good book.