The highest trophic niches in Mesozoic oceans were filled by diverse marine reptiles, including ichthyosaurians, plesiosaurians, and thalattosuchians, dominating food webs during the Jurassic and Early Cretaceous. Yet during the mid-Cretaceous, ichthyosaurs, thalattosuchians, and pliosaurids vanished, replaced by mosasaurs, xenopsarian plesiosaurians, and new groups like sharks, fish, turtles, and birds. This shift restructured marine ecosystems. Project SEASCAPE analyzes this turnover using phylogenies and functional data, revealing selective extinctions and divergent functional landscapes between the Early and Late Cretaceous.