Chapter 1 reports on several fossil pinnipedimorph specimens from the Pacific Northwest. Two new taxa (“Enaliarctos” bertae and Proneotherium? borealis) are described from the late Oligocene of Washington and early Miocene of Oregon. The former is known from a small partial skull and mandible from the Pysht Formation of Clallam County, Washington. This specimen, likely the smallest mature stem-pinniped known, has unusually large postcanine alveoli. P. borealis , known from a nearly complete cranium from the Astoria Formation of Lincoln County, Oregon, shares many features with Pinnarctidion and Proneotherium, but differs in the shape of the zygomatic arch, the size of the molar alveoli, and the shape of the auditory bulla. The newly described taxa possess mosaic features which simultaneously indicate the diversity of form and function of stem pinnipeds but also complicate our understanding of the details of their evolutionary history. This chapter also reports new specimens referable to previously known taxa, “Enaliarctos” barnesi and Pinnarctidion iverseni. The new “E.” barnesi specimen preserves the basicranium–missing in the holotype–which thus helps refine how this taxon is related to other pinnipedimorphs. The new P. iverseni specimen preserves a nearly complete cranium, mandible, scapula, humerus, femur, tibia, and other elements yet to be prepared. This specimen adds important anatomical information missing from the holotype , such as details of the facial sutures, lateral basicranium, and postcranial skeleton. Lastly, we report on several marked differences between the type specimens of “Enaliarctos” mitchelli and the referred specimen, which indicate that the latter should be recognized as a separate taxon, “Enaliarctos” sullivani.

Chapter 2 takes a comprehensive look at the evolutionary relationships of pan-pinnipeds and their arctoid relatives. A broad sampling of taxa were used to determine whether enigmatic taxa such as Kolponomos, Amphicticeps, amphicynodontids, Potamotherium, and Puijila are allied with pinnipeds, or if they fit elsewhere on the arctoid tree. A new composite matrix of characters was assembled to parse the differences between canids, ursids, musteloids, pinnipeds, and their extinct relatives. Our results support Potamotherium and Puijila as basal pan-pinnipeds and Kolponomos as an ursid. Although some trees place Amphicticeps within Pan-pinnipedia, most suggest it diverged, along with other amphicynodontids, before the arctoid common ancestor. Our results concord with molecular evidence placing pan-pinnipeds closer to mustelids than to ursids. Amphicyonids have mixed results, either diverging from the canid branch or between canids and arctoids. Among pan-pinnipeds, we find that “Enaliarctos” is a paraphyletic grade, Prototaria and Proneotherium diverge before crown pinnipeds, phocids are the earliest-diverging pinniped family, and desmatophocids may be paired with, or nested within, odobenids. A series of twelve trial analyses were run to assess how ordered characters, weighted characters, implied weights, and molecular backbone constraints affect the branch support values of resulting maximum parsimony trees. The only statistically significant effect came from implied weights, which display a strongly negative relationship between Bremer support and bootstrap support values.

Chapter 3 examines the dental disparity and varied diets of arctoid carnivorans (the group including bears, weasels, and seals). Their feliform and canid relatives tend to retain the ancestral carnassial dentition for shearing meat, whereas many arctoids – such as ursids, procyonids, and pinnipeds – possess modified post-canines to feed on plants, invertebrates, or fish. Details of these morphological and corresponding dietary shifts are poorly understood, but can be inferred from the fossil record. The upper fourth premolar (P4), which is preserved often and comparable across disparate groups, serves as a reliable proxy for diet in otherwise enigmatic fossil taxa. The P4s of a variety of extinct and extant arctoids were analyzed using 2D landmark morphometrics. Canids and many early arctoids retain carnassial-like premolars and cluster together as hypercarnivores. Some enigmatic early arctoids, such as Kolponomos and Eoarctos, occupy morphospace that overlaps with extant hypocarnivores, suggesting they may have eaten more plant material than previously believed. Among pan-pinnipeds, “Enaliarctos” species fit intermediately between invertebrate and fish consumers, whereas Pinnarctidion and other later-diverging pinnipedimorphs have more overlap with modern piscivorous pinnipeds.