Lamniform sharks are amongst the largest-bodied extant fishes and have an evolutionary history spanning ~135 million years (Ma). Fossils correlate their initial development of mega-body size ( ≥6m)with ecological radiation as marine top-predators during the later part of the mid-Cretaceous (after the late Albian, ~100 Ma). Here, we push back this earliest appearance of gigantic lamniforms by ~15 Ma (upper Aptian, ~115 Ma) with the discovery of enormous cardabiodontid shark remains from northern Australia. We compiled a comprehensive dataset of vertebral centrum diameters versus maximum body length measurements for living lamniforms to calculate length and mass estimates of extinct taxa using both intraspecific and interspecific regression models. Our results show that mega-body size is an ancient lamniform trait, with the Australian cardabiodontid being around 6–8 m and over 3 tons. This rivalled some of the largest coeval marine reptiles and suggests that lamniforms invaded top-predator niches from an early stage in their adaptive evolution.