My last scholarly reference on vertebrate evolution, Michael Benson’s 1991 Vertebrate Paleontology, has this to say:
Essentially, the situation at that time was that individual orders were fairly clearly mapped out – the interrelationships of perissodactyls or of carnivores as amongst themselves were known – but that what orders were most closely associated with what other orders was still hotly debated. (For those confused, the placentals are the vast majority of living mammals, which bring forth their young in a relatively developed state after nurturing them with a fully developed placenta for much longer than do marsupials. Placentalia, AKA Eutheria, comprises a clade assigned Infraclass status in most traditionalist works, with a double dozen orders (Primates, Carnivora, Perissodactyla, Rodentia, Cetacea, etc.) assigned to it.
As of the time Benton wrote, there was a general agreement, supported by both skeletal/physiological and genetic/biochemical markers, that the Edentata/Xenarthra (sloths, anteaters, and armadillos) were an “outgroup” to the remainder. But which groups among the remainding 20-odd orders were most closely related to which other groups, forming intermediate-level clades of “Cohort” or “Superorder” status, was argued at great length by mammalogists, mammalian paleontologists, and other interested parties. Hyraxes, for example, were either most closely related to elephants and sirenians, or to perissodactyls. Whales and dolphins, either to the extinct mesonychids or to artiodactyls. Rabbits and pikas, either to rodents or to some of the primitive hoofed forms. For every given scientist with a clear theory, there were five equally skilled scientists prepared to refute his and advance their own in its place – and for the most part those five did not concur on a whole lot.
My question is if any consensus has been reached in the 15 years since Benson wrote, if so what it is, and if not, what progress has been made towards resolving some of the debated issues. Links and/or pointing me to intelligent discussion online would be appreciated; a post with a good less-than-totally-arcane summary of the situation today would be appreciated even more.