Cetacea (dolphins, porpoises, and whales) is a clade of aquatic species that includes the most massive, deepest diving, and largest brained mammals. Understanding the temporal pattern of diversification in the group as well as the evolution of cetacean anatomy and behavior requires a robust and well-resolved phylogenetic hypothesis. Although a large body of molecular data has accumulated over the past 20 years, DNA sequences of cetaceans have not been directly integrated with the rich, cetacean fossil record to reconcile discrepancies among molecular and morphological characters.

The Cenozoic diversification of placental mammals is the archetypal adaptive radiation. Yet, discrepancies between molecular divergence estimates and the fossil record fuel ongoing debate around the timing, tempo, and drivers of this radiation. Analysis of a three-dimensional skull dataset for living and extinct placental mammals demonstrates that evolutionary rates peak early and attenuate quickly. This long-term decline in tempo is punctuated by bursts of innovation that decreased in amplitude over the past 66 million years. Social, precocial, aquatic, and herbivorous species evolve fastest, especially whales, elephants, sirenians, and extinct ungulates. Slow rates in rodents and bats indicate dissociation of taxonomic and morphological diversification. Frustratingly, highly similar ancestral shape estimates for placental mammal superorders suggest that their earliest representatives may continue to elude unequivocal identification.

—We describe the odontocete, Ashleycetus planicapitis, gen. et sp. nov., based on a partial skull that was collected from the upper Rupelian (lower Oligocene) Ashley Formation near Charleston, South Carolina, and place it in the Ashleycetidae, fam. nov. Overall, the cranial morphology of this new species is plesiomorphic; it has an elongate, tabular intertemporal region, external nares well anterior to the antorbital notches, and anteromedially oriented lateral margins of the supraorbital processes. Xenorophus sloanii and Xenorophidae are rediagnosed, and the latter is also redefined as an apomorphy-based clade. This new definition should lead to greater stability because it excludes Archaeodelphis patrius, whose phylogenetic relationships have proven to be problematic. The holotype skull of the archaic odontocete Mirocetus riabinini, from probable upper Rupelian sediments of the lower Maikop beds of Azerbaijan, is redescribed, newly figured, and placed in the Mirocetidae, fam. nov. Previously regarded as an archaeocete, Mirocetus is here shown to be an odontocete. We added Ashleycetus, Albertocetus, and Mirocetus to a recently published supermatrix of morphological and molecular data. Although Ashleycetus, Archaeodelphis, and Mirocetus consistently occupy basal branch(es) within Odontoceti, their exact positions are sensitive to the value of a constant used in implied weighting for cladistics analysis. Three of the characters we find to be odontocete synapomorphies are closely associated with soft tissue structures implicated in the production and transmission of high-frequency sounds during echolocation. Detailed dissections of extant odontocetes are required to determine if these features can be considered evidence of echolocation in extinct taxa. http://zoobank.org/urn:lsid:zoobank.org:pub:21FEC587-D1C1-4968-814B-D66B00323FF7 SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Placental mammals constitute the most intensively studied large clade of organisms. Deciphering their relationships has important implications not only to understand evolutionary processes (Bininda-Emonds et al., 2007) but also to inform many aspects of biomedical research (Springer and Murphy, 2007). Springer et al. (2007) have recently discussed limitations of morphological analysis of placental phylogeny compared with reconstructions based on molecular data. They presented a pessimistic view of the potential of morphological characters for use in high-level placental mammal phylogeny reconstruction, as the morphological data set they scrutinized (from Asher et al., 2003) does not recover the four major clades of mammals (Afrotheria, Xenarthra, Laurasiatheria, and Euarchontoglires) that these authors regard as overwhelmingly supported.