In the five chapters of this dissertation, I investigate the paleobiology of SouthAmerican native ungulates (SANUs) and caviomorph rodents. Although these studies involve different taxonomic groups and employ disparate methods, most rely on previously undescribed fossils to interrogate broader evolutionary questions. In the first chapter, I describe early Miocene litopterns (a group of SANUs) from Pampa Castillo, Aysén Region, Chile, and perform a phylogenetic analysis of one of its subgroups, Proterotheriidae. Litoptern taxa from Pampa Castillo include the macraucheniid Theosodon and proterotheriids Thoatherium and Picturotherium, corroborating the fauna’s assignment to the Santacrucian South American land mammal age (SALMA). My phylogenetic analysis, which indicates that “Anisolambdidae” forms a non-monophyletic cluster within Proterotheriidae, is the foundation of a new stem-based definition for Proterotheriidae. This chapter was published in the Journal of Systematic Palaeontology in 2020. My second chapter describes two new proterotheriids from the middle Miocene of Quebrada Honda, Tarija Department, Bolivia, and analyzes litoptern diversity and body size evolution in a phylogenetic context. These taxa, Olisanophus riorosarioensis gen. et sp. nov. and Olisanophus akilachuta gen. et sp. nov., greatly clarify proterotheriid evolution in mid-latitude South America. The diversity analysis indicates that these groups were more diverse than previously appreciated, particularly during the Paleogene. Macraucheniids increased in body size throughout the Cenozoic, whereas proterotheriids followed a similar trend during the Paleogene, but did not change in size during the Neogene. This chapter was published in Ameghiniana in 2020. Cavioid, chinchilloid, and erethizontoid caviomorph rodents from Pampa Castillo are described in the third chapter. Cavioids are represented by Luantus minor, Eocardia cf. excavata, and Neoreomys australis, the last of which is the most abundantly represented mammal in the Pampa Castillo fauna. Chinchilloids are represented by four species of Perimys (P. erutus, P. onustus, P. intermedius, and a yet unnamed species previously described from the Pinturas Formation), Prolagostomus pusillus, Scleromys quadrangulatus, and a probable new species of Scleromys. This assemblage represents a mixture of two well-known, contemporaneous fossil faunas previously recognized from the lower + middle sequences of the Pinturas Formation and the Santa Cruz Formation. The phylogenetic affinities and relative abundance of these taxa suggest the paleoenvironment of Pampa Castillo was similarly intermediate between these two better-known faunas. In the fourth chapter, I describe the octodontoid caviomorphs from Pampa Castillo and conduct a faunal similarity analysis of Pampa Castillo and ten other early to middle Miocene (Colhuehuapian–Friasian/Colloncuran SALMAs) Patagonian rodent faunas. Eight octodontoids occur at Pampa Castillo, three of them likely representing new species of Caviocricetus, Dudumus, and Prostichomys. The other five are referred to previously described taxa: Acarechimys minutus, Acarechimys constans, Acarechimys cf. minutissimus, Acaremys cf. murinus, and Spaniomys cf. riparius. Faunal similarity analyses yielded inconsistent results depending on whether genus- or species-level data were used. The combined results of these analyses and recently published geochronological data suggest that the ‘Pinturan’ biochronologic interval is not valid; faunas referred to it should instead be assigned to the Santacrucian SALMA. I analyze the proximal ankle bones of litoptern and notoungulate (another SANU group) from the early Miocene Santa Cruz Formation, Argentina with the aim of assessing their utility in systematic studies and whether linear measurements or twodimensional (2D) landmarks are more effective for assessing morphological differences. The ultimate goal of this research is to allow robust inferences about the locomotory habits of the sampled taxa. These results suggest that isolated tarsals may be identified to family and genus-level, but this conclusion needs further testing. Quantitative tests show that tarsals of different SANU taxa may be slightly more reliably distinguished by 2D landmarks than by linear measurements. However, 2D landmarks are clearly superior from the perspective of ease of measurement, replicability, and applicability to taxonomically diverse samples. Although the lack of modern taxa in the sample limited the paleobiological inferences that could be drawn, body mass, phylogeny, and possibly locomotor behavior, clearly influence tarsal morphology. Throughout this dissertation, I have used the description of new taxa and specimens as the foundation for phylogenetic, paleoenvironmental, biochronological, and ecomorphological analyses. These studies have advanced our knowledge of mammal evolution in South America, particularly in the early and middle Miocene.

Two new Oligocene “notohippids” from the Abanico Formation, a geographically widespread volcanic and volcaniclastic lithostratigraphic unit in the Andean Main Range of central Chile, are described and subjected to phylogenetic analysis. “Notohippidae” is a paraphyletic assemblage of medium-sized South American native ungulates from the Casamayoran (middle Eocene) to Santacrucian (early Miocene) SALMAs (South American Land Mammal Age). This group belongs to Notoungulata, the most diverse clade of native South American ungulates.

The two new taxa are represented by SGOPV 3750, consisting of a cranium and partial upper dentition recovered near Upeo, Chile (~200 km south of Santiago), which represents a new genus, and SGOPV 3221, a nearly complete upper dentition collected from the Las Leñas drainage basin (100 km SSE of Santiago), representing a new species of Rhynchippus. Specimens from the Salla Beds, Bolivia, originally assigned to Rhynchippus brasiliensis are here referred to the same new species of Rhynchippus as proposed for SGOPV 3221. A phylogenetic analysis resolves a sister group relationship between Eurygenium and Argyrohippus for the first time, but “Notohippidae” remains paraphyletic. The taxa described here nest within a polytomy including the pairing of Eurygenium and Argyrohippus, Rhynchippus equinus, R. pumilus, and a clade comprising Pascualihippus boliviensis plus Toxodontidae.

The unique history of mammals in South America has long provided significant evolutionary and geological insights, but the continent's fossil record is strongly biased toward the lowlands of Argentine Patagonia. Recent collecting efforts in the Andean Main Range of Chile, near Laguna del Laja (LdL) (~37.5º S, 71º W), have yielded a stratigraphically superposed series of early to late Miocene fossil mammal assemblages, shedding light on the poorly known extra-Patagonian history of South American Neogene mammals. Several hundred specimens have been recovered from the Cura-Mallín and overlying Trapa-Trapa formations at LdL. Nearly all of these fossils were collected from a well-described stratigraphic sequence that includes 17 high precision 40Ar/39Ar ages. This robust temporal framework encompasses fossiliferous horizons spanning ~20-9 Ma. Herein I provide a detailed account of the ungulates from the Cura-Mallín Formation at LdL; these range in age from ~20-15 Ma (early to middle Miocene). Dentitions referred to Protypotherium praerutilum, a well-known but inadequately diagnosed species, motivated a long overdue (but preliminary) taxonomic revision. Occurrences of Protypotherium, Colpodon, and Astrapothericulus, together with radioisotopic ages, indicate that faunas spanning the Colhuehuapian and Santacrucian South American Land Mammal "Ages" (SALMAs) occur in direct stratigraphic superposition at LdL. These deposits have also yielded five new genera of typotherians (Notoungulata); this high level of endemism has also been noted among rodent faunas from LdL. The five new taxa described in this study are included in rigorous phylogenetic analyses of Interatheriinae and Hegetotheriidae (Typotheria, Notoungulata), providing new hypotheses of morphological evolution within these clades, and documenting independent yet apparently simultaneous radiations of interatheres and hegetotheres with ever-growing (hypselodont) dentitions. A significant majority of fossils from LdL are small-bodied taxa (e.g., rodents and typotherian notoungualtes) represented by dental and cranial elements. This size distribution is unusual compared to other well-known early Miocene faunas, and may reflect differential transport of skeletal elements in lahar-associated depositional settings. The high level of endemism across multiple SALMAs at LdL is also striking, particularly considering the geographic proximity of faunas from LdL with roughly coeval counterparts in neighboring regions of Argentina. This provinciality of the Andean faunas, relative to Argentine assemblages, likely reflects sampling of a regionally distinct and isolated paleoenvironment.

A nearly complete skull and fragmentary postcrania of a late Oligocene pinnipedimorph (SDNHM 146624) from the Pysht Formation of Clallam County, Washington, represent a new species of Pinnarctidion, a taxon previously known only from California and Oregon. This study provides a detailed anatomical description of SDNHM 146624 and offers the most comprehensive phylogenetic analysis of early-diverging pinnipedimorphs to date. Notable features of SDNHM 146624 include a posteriorly broad palate coupled with an anteriorly narrow rostrum, dorsoventrally deep zygomatic arches, and accessory cuspules on P3 and P4. The results of extensive comparisons indicate that SDNHM 146624 represents a new species of Pinnarctidion, P. iverseni, closely related to P. bishopi from California. Given the specimen’s superb preservation and relative completeness, SDNHM 146624 provides welcome new dental and cranial information for Pinnarctidion.