Unravelling morphological disparity and faunal change across the J/K transition using geometric morphometrics: A case study of ammonites from central Chile

Changes in species diversity may be accompanied by morphospace variation at different spatial and temporal scales. Evidence indicates that shifts in species diversity and morphological disparity (morphodisparity) are prevalent among marine invertebrates across the geological scale. This study estimates the diversity and morphodisparity of ammonites across the Jurassic–Cretaceous (J/K) transition—from the Tithonian to the Valanginian stages—by examining 158 specimens representing 33 species and 21 genera from four localities in central Chile. Ammonites were photographed in lateral and frontal views to perform geometric morphometrics analyses based on landmarks and semilandmarks. The resulting shapes were evaluated using multivariate statistical approaches across geological stages. Results suggest that faunal changes in diversity during the J/K transition are associated with significant variations in ammonite morphospace. Notably, significant changes in morphodisparity were observed between the late Tithonian and early Berriasian, and again between the early and late Berriasian. This study not only documents faunal turnover in central Chile but also contributes to our understanding of how extinction pulses can drive morphological innovation or constraint during key evolutionary transitions.