Through the Cenozoic, paleoclimate records show general trends of global cooling and increased aridity, and environments in North America shifted from predominantly forests to more open habitats. Paleobotanical records indicate grasses were present on the continent in the Eocene; however, paleosol and phytolith studies indicate that open habitats did not arise until the late Eocene or even later in the Oligocene. Studies of large mammalian herbivores have documented changes in ecomorphology and community structure through time, revealing that shifts in mammalian morphology occurred millions of years after the environmental changes thought to have triggered them. Smaller mammals, like rodents and lagomorphs, should more closely track climate and habitat changes due to their shorter generation times and smaller ranges, but these animals have received much less study. To examine changes in smaller mammals through time, we have assembled and analyzed an ecomorphological database of all North American rodent and lagomorph species. Analyses of these data found that rodent and lagomorph community structure changed dramatically through the Cenozoic, and shifts in diversity and ecology correspond closely with the timing of habitat changes. Cenozoic rodent and lagomorph species diversity is strongly biased by sampling of localities, but sampling-corrected diversity reveals diversity dynamics that, after an initial density-dependent diversification in the Eocene, track habitat changes and the appearance of new ecological adaptations. As habitats became more open and arid through time, rodent and lagomorph crown heights increased while burrowing, jumping, and cursorial adaptations became more prevalent. Through time, open-habitat specialists were added during periods of diversification, while closed-habitat taxa were disproportionately lost in subsequent diversity declines. While shifts among rodents and lagomorphs parallel changes in ungulate communities, they started millions of years earlier than in larger mammals. This is likely a consequence of the smaller mammal' greater sensitivity to environmental changes and more rapid evolution. These results highlight the importance of examining understudied members of vertebrate faunas for understanding the evolution of terrestrial communities through time.

Atlantic Meridional Overturning Circulation (AMOC) is a vital process that transfers heat and nutrients throughout the world's oceans, helping to regulate global climate and support marine ecosystems. The timing and nature of the shift to modern AMOC, and especially to deep-water formation in the North Atlantic, has been a topic of ongoing study, with the Eocene-Oligocene Transition (EOT, ~34 Ma) as a potential focal point of this shift. However, the role played by abrupt EOT cooling and Antarctic glaciation in North Atlantic circulation remains unclear. Improved constraints on Paleogene circulation will provide insight into the sensitivity of AMOC to perturbations in global climate, which is particularly relevant in light of contemporary climate change. To examine deep North Atlantic circulation response to the EOT we obtained grain-size data from the terrigenous fraction of the mud-dominated sediments of the Southeast Newfoundland Ridge contourite drift complex at IODP Site U1411, which is interpreted to have formed under the influence of the Deep Western Boundary Current. We analyzed 195 samples that span 150 m of stratigraphy from 36-26 Ma. The main objective was to use the 'sortable silt' fraction (10-63 µm) to generate a record of relative change in bottom-current intensity. These data are complemented with a record of the abundance and size of lithogenic sand (>63 µm). Here we present the first physical proxy record of abyssal current intensity in the North Atlantic, from late Eocene to mid Oligocene. Invigoration of North Atlantic deep circulation occurred gradually (over Myr timescales), with no significant changes linked temporally to the EOT. We infer that deep circulation in the North Atlantic was not sensitive to the abrupt global cooling and Antarctic glaciation associated with the EOT. Rather, our data suggest that changes in North Atlantic circulation were likely governed by longer-term processes related to the opening of key tectonic gateways, such as the Greenland-Scotland Ridge in the North Atlantic, and the Drake and Tasman Passages in the Southern Ocean. Additionally, we identify a significant mid-Oligocene invigoration of North Atlantic abyssal circulation, which climaxes around 27.9 Ma, and is coeval with a decrease in atmospheric CO2. / Master of Science / Atlantic Meridional Overturning Circulation (AMOC) is a vital process that transfers heat and nutrients throughout the world’s oceans, helping to regulate global climate and support marine ecosystems. However, AMOC has not always existed as we know it, and different modes of ocean circulation have operated in Earth’s past. The timing and nature of the shift to the modern form of AMOC/global ocean circulation has been a topic of ongoing study. The Eocene-Oligocene Transition (EOT, ~34 million years ago), a period of intense global cooling and Antarctic ice sheet growth, is thought to be a potential focal point of this shift to modern ocean circulation. However, the role played by abrupt EOT cooling and Antarctic glaciation in the evolution of ocean circulation remains unclear, especially in the North Atlantic. Understanding how and why ocean circulation has changed in more recent geologic time (within the past 65 million years), and ultimately evolved into its modern state, will help us understand the processes which led to, and now maintain, Earth’s modern climate state, as well as provide insight into the sensitivity of AMOC to perturbations in global climate, which is particularly relevant in light of contemporary climate change. To examine the response of ocean circulation in the North Atlantic to climate changes at the EOT, we obtained grain-size data from the mud-dominated sediments of the Southeast Newfoundland Ridge contourite drift complex at IODP Site U1411, which is interpreted to have formed under the influence of the Deep Western Boundary Current. Contourite drifts are massive sediment build-ups on the ocean floor, which are formed due to bottom-currents depositing large amounts of sediment in one area. Therefore, contourite drift deposits can hold valuable records of bottom-current activity over millions of years. We analyzed 195 samples that span 150 m of stratigraphy from 36-26 Ma (Ma = millions of years ago). The main objective was to use the ‘sortable silt’ fraction (10-63 µm) to generate a record of relative change in bottom-current intensity. These data are complemented with a record of the abundance and size of sand (>63 µm). Here we present the first physical proxy record of abyssal current intensity in the North Atlantic, from late Eocene to mid Oligocene. We find that invigoration of North Atlantic deep circulation occurred gradually (over million-year timescales), with no significant changes linked temporally to the EOT. We infer that deep circulation in the North Atlantic was not sensitive to the abrupt global cooling and Antarctic glaciation associated with the EOT. Rather, our data suggest that changes in North Atlantic circulation were likely governed by longer-term processes related to the opening of key tectonic gateways, such as the Greenland-Scotland Ridge in the North Atlantic, and the Drake and Tasman Passages in the Southern Ocean. Additionally, we identify a significant midOligocene invigoration of North Atlantic abyssal circulation, which climaxes around 27.9 Ma, and coincides with a decrease in atmospheric CO₂.