I investigated cranial and dental morphology in 37 species of fossil and living peccaries, Family Tayassuidae, from North and South America. PAUP analysis of 94 apomorphous character states among 33 species represented by adequate samples produced a set of trees (C.I. = 0.74) which unequivocally support the monophyly of several clades within the Tayassuidae. The subfamily Tayassuinae comprises the three living species of peccary and 23 Miocene and younger species. The Miocene taxa Hesperhys, "Thinohyus" siouxensis, Floridachoerus, "Cynorca" sociale, and two unnamed species form a clade which is the sister group of the Tayassuinae. The Oligocene species Perchoerus probus is the sister group of the clade comprising Hesperhys and the Tayassuinae; another Oligocene species, Thinohyus lentus, is the sister group of all tayassuids surveyed. Analysis of a set of predominantly tayassuine taxa yielded a set of trees (C.I. = 0.84) which resolve relationships among several tayassuine clades. Tayassu is the sister group of a clade comprising Prosthennops (restricted to P. serus), Mylohyus, and Platygonus. Catagonus and Dicotyles are sister groups, and together form a clade which is the sister group of the clade which unites Tayassu and Platygonus. Progressively distant sister groups of that clade include three unnamed Miocene species, Macrogenis, "Prosthennops" xiphidonticus, "Prosthennops" niobrarensis, "Cynorca" occidentale, and Dyseohyus. Miocene and younger tayassuids possess within their nasal cavities an elaborate complex of bony structures that is unique among mammals. The floor of the nasal cavity is developed into a pneumatic, labyrinthine structure. The vomer is also pneumatic, and is developed into a pair of large, bilaterally symmetrical chambers. Many members of the clade of late Miocene and younger tayassuines that includes Macrogenis and its sister taxa have wing-like zygomatic processes; possession of these processes is primitive for that clade. Primitively both sexes (diagnosed by discretely bimodal canine diameters) possessed them, but they are sexually dimorphic in some species of Platygonus. Zygomatic processes were reduced in size in as many as five clades; their reduction was accompanied by reduction in the degree of canine dimorphism. A minimum of four dispersal events is required to account for the diversity of South American Tayassuidae.

Microwear analysis has been severely underutilized as a dietary technique due to numerous constraints involved in employing traditional scanning electron microscopy. A new methodology is described that greatly simplifies the assessment of microwear scar features for the discernment of the trophic adaptations of living and fossil taxa. A standard stereomicroscope and a fiber-optic light source have replaced specimen preparation tools such as venting and plating apparatus and the high-tech scanning electron microscope. Several new microwear variables supplement traditional quantification of pits and scratches. Significant niche partitioning in extant browsers, grazers, and mixed feeders is apparent and habitat differences within each broad trophic group are discernible by scoring relative pit sizes, scratch textures, and gouges in addition to quantifying scratches and pits. Variations in scratch number ranges, scratch textures, and relative pit sizes are the most useful variables for partitioning living ungulates into more refined trophic categories. Pit numbers and scratch textures are most effective for distinguishing fine versus coarse browsing, as well as leaf browsing versus fruit browsing. Scratch textural differences and scratch numbers distinguish fresh grass grazing versus coarse or mixed grass grazing; C3 versus C4 grazing, coarse bark and stem feeding, and seasonal or regional mixed feeding versus a meal-by-meal alternation between browse and grass. Heavy gouging is used to distinguish significant grit encroachment upon food items. Seasonal or regional mixed feeders have microwear that is more similar to browsers, whereas meal-by-meal mixed feeders have wear more similar to grazers. Three trophic phases are identified within extant ungulates by partitioning taxa into three potential raw scratch ranges: traditional browsing and grazing phases, comprised by surprisingly few species, and a browsing-to-grazing transitional phase where the majority of taxa are found, including all of the mixed feeders. The new microwear methodology is used to test two hypotheses regarding large shifts in dietary strategies in fossil ungulates from the North American Great Plains region: the supposed browsing-grazing transition in the fossil record of North American equids and the supposed shift to coarser browse in the fossil ruminant artiodactyl family known as the Dromomerycidae.