Tyrannosaurids were large carnivorous dinosaurs that underwent major changes in skull robusticity and body proportions as they grew, suggesting they occupied different ecological niches during their lifespan. Although adults commonly fed on dinosaurian megaherbivores, the diet of juvenile tyrannosaurids is largely unknown. Here we describe a remarkable specimen of a juvenile Gorgosaurus libratus that preserves the articulated hindlimbs of two yearling caenagnathid dinosaurs inside its abdominal cavity. The prey were selectively dismembered and consumed in two separate feeding events. This predator-prey association provides direct evidence of an ontogenetic dietary shift in tyrannosaurids. Juvenile tyrannosaurids may have hunted small and young dinosaurs until they reached a size when, to satisfy energetic requirements, they transitioned to feeding on dinosaurian megaherbivores. Tyrannosaurids thus occupied both mesopredator and apex predator roles during their lifespan, a factor that may have been key to their evolutionary success.

The matrix block containing the Citipes caudal vertebrae was subjected to computed tomography (CT) on a Toshiba Aquilion medical CT scanner at the Drumheller Health Centre in Drumheller, Alberta, Canada. CT scanning was conducted at a voltage of 120 kV, an X-ray tube current of 300 mA, and with contiguous slices of a thickness of 0.5 mm.