Grasses (Poaceae) exhibit exceptional taxonomic and ecological diversity, with grassland-dominated habitats around the world covering about 40% of Earth’s land surface. Modern grasslands occur in temperate to tropical ecosystems spanning low to high altitudes, a wide range of temperatures, and wet to arid climates. However, the ecology of early grasses and their environmental affinities remain poorly understood because of the scant fossil record of grasses before the global Oligocene-Miocene spread of grasslands. We analyze phytolith assemblages extracted from sediment samples collected in Paleocene-Eocene Rio Chico Group sites along the San Jorge Basin of Argentine Patagonia to reconstruct the ecosystems early grasses occurred in. The coastal lowland deposits of the Las Violetas up to the base of Las Flores formations of the Rio Chico Group are preliminarily dated between 60.01±0.58 Ma to 50.93±0.40 Ma (unpublished information). These deposits produced phytolith assemblages comprising among the oldest occurrences of grass phytoliths in the fossil record of South America. Preliminary results of traditional phytolith assemblage analysis and reconstructed canopy structure using epidermal phytolith shape suggest that early grasses in the Paleocene and Eocene of the San Jorge Basin occurred in low abundances in palm and woody dicot-dominated forests of moderately open canopies. These results support the hypothesis that early grasses emerged as rare understory components in non-analog shrublands dominated by palms, or forests dominated by woody angiosperms, before expanding to open habitats later in the Cenozoic, where they would become dominant elements of terrestrial landscapes during the mid-late Miocene. Continuing to build the phytolith record in the Cretaceous and early Cenozoic of South America will be necessary to fully understand the habitats that early grasses occurred in, which will, in turn, shed light on the timing and possible mechanisms that drove the global expansion of grasses and grasslands over the last 60 million years. Furthermore, the studied records capture the critical Paleocene-Eocene transition in southern South America, contributing to our understanding of the impact of this period of extreme global warming on terrestrial ecosystems in the Southern Hemisphere.