Abstract We modelled the resilience and transient dynamics of terrestrial paleocommunities from the Karoo Basin, South Africa, around the Permian-Triassic mass extinction. Using recently refined biostratigraphic data that suggest two pulses of extinction leading up to the Permian-Triassic boundary, we show that during times of low extinction, paleocommunities were no more stable than randomly assembled communities, but they became stable during the mass extinction. Modelled food webs before and after the mass extinction have lower resilience and less stable transient dynamics compared to random food webs lacking in functional structure but of equal species richness. They are, however, more stable than random food webs of equal richness but with randomized functional structure. In contrast, models become increasingly more resilient and have more stable transient dynamics, relative to the random models, as the mass extinction progressed. The increased stability of the community that resulted from the first pulse of extinction was driven by significant selective extinction against small-bodied amniotes, and significantly greater probabilities of survival of large-bodied amniotes. These results point to a positive relationship between evolved patterns of functional diversity and emergent community dynamics, with observed patterns being more stable than alternative possibilities. Significance Anthropogenic impacts on modern ecosystems have no precedents in human history. The fossil record does contain episodes of severe biodiversity crises, but incomplete preservation and low temporal resolution make it difficult to equate fossil data to modern ecological processes. We examined terrestrial paleocommunities from Earth’s most severe mass extinction, the Permian-Triassic mass extinction (PTME), and modelled their dynamic stabilities. We show that during times of low extinction, paleocommunities were no more stable than randomly assembled communities, but they became more stable during the mass extinction. Increased stability resulted mostly from selective extinction and survival based on vertebrate body size. Whether modern communities will behave similarly depends on the similarity between human drivers and those of the PTME.