Body Size Evolution of Marine Ostracods in the Triassic and Jurassic

The Lilliput Effect among invertebrates after the Permian–Triassic mass extinction is conspicuously mirrored by ostracods. However, precisely when this effect dissipated among ostracods remains unclear, and little is known about how their body sizes subsequently recovered. The Early Mesozoic represents a critical transitional phase in the evolution of life. This period not only witnessed the ecological recovery and radiation following the Permian–Triassic mass extinction, but also experienced, during the Triassic–Jurassic transition, a dramatic reorganization of global paleogeography and the continuous reshaping of both marine and terrestrial environments. As ostracods are crustaceans highly sensitive to environmental changes, their body size evolution provides a crucial window into understanding the co-evolution of organisms and their environments during the Early Mesozoic. Here, we systematically compile and integrate ostracod fossil body size parameters (carapace length, height, and H/L ratio) from the Triassic to Jurassic, along with contemporaneous paleogeographic, climatic, and marine environmental data. Using multivariate statistical and path analysis methods, we quantify the macroevolutionary patterns of ostracods and explore the mechanisms by which multiple environmental drivers operated during this interval. Quantitative analysis reveals a significant negative covariation between temperature fluctuations and ostracod body size. Global environmental changes indirectly promoted ostracod body size evolution, with temperature variation acting as the primary control on benthic ostracod morphogenesis during this interval. Therefore, coupling body size changes in ostracods with environmental factors helps to elucidate their macroevolutionary patterns and the mechanisms of multiple driving forces.