Background/Question/Methods Understanding how communities are structured is a fundamental objective of ecology. Theory predicts that similarly sized species within the same ecological guild competing for the same resource will exclude each other from local communities, but these species can co-exist if resources are partitioned along a niche axis that doesn’t correlate with size. These assembly processes are strongest within local ecological guilds, and can be studied using the structure of the body size distributions of these guilds. Terrestrial herbivores occupy a variety of body-sizes and are important components of ecosystems. They thus provide an ideal system to study how local ecological processes shape the structure of a guild. Modern ecosystems, however, may not be true representatives of what is typical. The late Quaternary megafaunal extinctions wiped out most terrestrial herbivores weighing more than 50kg everywhere except Africa and Southern Asia, thus biasing our view of guild structure. Therefore, a deep-time perspective is essential to understand the variation in guild structure through time. The fossil record of North American mammals is well-sampled, and well resolved for such a study. North America was also recently home to large co-occurring herbivores such as ground sloths, horses, bison, and mammoths. In this study, I sampled mammalian herbivores weighing over 1kg from bone-beds and individual fossil quarries dating from the mid-Holocene to the Eocene. Using allometric equations and dental and skeletal measurements, I estimated the body-size of these specie, and then examined moments of the distribution of body sizes through time. Data on fossil assemblages were sourced from the Paleobiology Database, FAUNMAP, and MIOMAP. Results/Conclusions There was a remarkable degree of similarity in the structure of the local herbivore guild during the Cenozoic. However, two general guild structures were present—one found between 55 and 30 million years ago, and one found between 30 million years ago and the modern. Older herbivore assemblages tend to be dominated by more small species, and produce right-skewed body size distributions. The younger assemblages in contrast were structured such that there were few small and large species, but with several species of mid-sized herbivores, producing gaussian body size distributions. These two guild structures correspond with warmer, closed habitats, and more seasonal open habitats respectively, and modern African herbivore communities also seem to mirror this pattern, suggesting that the way energy is proportioned stays fairly conserved through time, regardless of the taxa present in the guild.