Phased ecological assembly of Triassic marine reptile faunas following the Permian–Triassic mass extinction: ecospace dynamics in South China

The Permian-Triassic mass extinction triggered a fundamental reorganization of marine ecosystems, yet the community-level functional dynamics of early marine reptile radiations remain understudied. Here we quantify ecospace dynamics of Triassic marine reptile faunas across an approximately 20-million-year sequence in South China, integrating 10 functional traits of fossil taxa and modern ecological analogues. We identify five ecotypes defined by habitat use, foraging modes and trophic strategy. Statistical analyses reveal cumulative and incremental faunal assembly in South China in three phases: Early Triassic niche conservatism with high functional redundancy; Middle Triassic ecological radiation with expanded disparity and reduced redundancy; and late Middle to Late Triassic ecological transition and ecosystem maturation, culminating in pelagic specialization and megafaunal predation. Environmental amelioration from the late Early Triassic was associated with ecological radiation, while sea-level fluctuations may have contributed to the nearshore-to-offshore transition. Comparison with other Triassic records reveals spatial heterogeneity and temporal asynchrony in ecological assembly: marine reptiles did not occupy roles analogous to those of modern cetaceans in South China until the Ladinian, whereas comparable roles had likely appeared earlier elsewhere. This succession thus records one of several regionally distinct pathways along which marine reptiles acted as ecological innovators during post-extinction recovery.