Decoupled Taxonomic Diversity and Morphological Complexity During Two Early Triassic Extinctions

During mass extinction events, taxonomic diversity declines were often coupled with declining morphological complexity. Here, we studied the morphological complexity dynamics of ammonoids, estimated from their ornamentation complexity, during the late Smithian and late Spathian (Early Triassic) extinction events. The analyzed zonal scale stratigraphic dataset comprises 603 species across 272 genera from the Early–Middle Triassic and shows that ornamentation complexity did not decline during these two crises, despite significant declines in both genus and species richnesses. Overall, the evolution of ornamentation complexity strongly depends on clades and lineages, various multiple intricated environmental factors and concomitant ecological pressures (e.g., niche constraints and predatory pressures). Therefore, we propose that since environmental changes during late Smithian and late Spathian were not as severe as the Permian-Triassic mass extinction, morphological redundancy may have decreased, but overall morphological complexity was partly maintained by surviving ammonoid species. Based on the late Smithian and late Spathian case studies, it is hypothesized that changes in marine productivity caused by cooling may have also indirectly participated to this phenomenon, offering a fresh perspective and new questions for future works on the complex interplay between environmental changes and morphological evolution.