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ABSTRACT-
A comparative analysis of the postcranial skeletal morphology of three species of pakicetid cetaceans provides new insights into their locomotor behavior. In gross morphology, they are similar to their smaller early artiodactyl relatives, lacking obvious signs of their transition to an aquatic niche. Features linking them are related to a cursorial adaptation centered in the reduction of joint mobility to the parasagittal plane, evident in both the elbow and the ankle. In addition to cursorial features of the limbs, the earliest whales and early artiodactyls both possess a long, stable lumbus and robust tails. The three pakicetid genera can be distinguished by size, proportion, and details of articular morphology. However, all pakicetid postcrania bear microstructural specializations commensurate with aquatic locomotion, and incompatible with cursoriality. The most striking modification is the presence of systemic increased bone density, likely used as skeletal ballast. When both postcranial morphology and microstructure are considered, it can be concluded that pakicetid cetaceans were highly adapted for an aquatic niche. As in several extant semiaquatic taxa, the pakicetid tail undoubtedly contributed to its locomotor repertoire, whether in propulsion or stabilization.
INTRODUCTION
THE PAKICETIDAE are the earliest known members of the mammalian order Cetacea (Gingerich et al., 1983: Thewissen and Hussain, 1998; Thewissen et al., 2001b), and can therefore document the initial morphological shift in their land to water transition. Given their basal position and the primitive nature of their aquatic specialization (Thewissen et al., 2001b), pakicetid fossils can also provide key insights in the relationships of whales to other extant and extinct mammalian groups (e.g., Geisler and Uhen, 2003). This is critical given the continuing disagreement between molecular and morphologically based phylogenetic analyses involving order Cetaeca, particularly the data conflict apparent in combined data sets (Nikaido et al., 1999; O'Leary and Geisler. 1999; O'Leary, 1999. 2001; Naylor and Adams, 2001; Thewissen et al., 2001b; Geisler, 2001 ; Geisler and Sanders, 2003; Geisler and Uhen, 2003).
More than 350 pakicetid postcranial elements from a single fossil locality have been recovered and prepared to date. H-GSP (Howard-Geological Survey of Pakistan) Locality 62 of the Lower Kuldana of the Ganda Kas area, northern Pakistan, is attributed to the early Eocene of Pakistan (Thewissen et al., 2001a). The locality represents a shallow ephemeral stream...